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SH(1)                     BSD General Commands Manual                    SH(1)

     sh -- command interpreter (shell)

     sh [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
        [+o option_name] [command_file [argument ...]]
     sh -c [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
        [+o option_name] command_string [command_name [argument ...]]
     sh -s [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
        [+o option_name] [argument ...]

     sh is the standard command interpreter for the system.  The current
     version of sh is in the process of being changed to conform with the
     POSIX 1003.2 and 1003.2a specifications for the shell.  This version has
     many features which make it appear similar in some respects to the Korn
     shell, but it is not a Korn shell clone (see ksh(1)).  Only features
     designated by POSIX, plus a few Berkeley extensions, are being
     incorporated into this shell.  This man page is not intended to be a
     tutorial or a complete specification of the shell.

     The shell is a command that reads lines from either a file or the
     terminal, interprets them, and generally executes other commands.  It is
     the program that is running when a user logs into the system (although a
     user can select a different shell with the chsh(1) command).  The shell
     implements a language that has flow control constructs, a macro facility
     that provides a variety of features in addition to data storage, along
     with built in history and line editing capabilities.  It incorporates
     many features to aid interactive use and has the advantage that the
     interpretative language is common to both interactive and non-interactive
     use (shell scripts).  That is, commands can be typed directly to the
     running shell or can be put into a file and the file can be executed
     directly by the shell.

     If no arguments are present and if the standard input of the shell is
     connected to a terminal (or if the -i flag is set), and the -c option is
     not present, the shell is considered an interactive shell.  An
     interactive shell generally prompts before each command and handles
     programming and command errors differently (as described below).  When
     first starting, the shell inspects argument 0, and if it begins with a
     dash '-', the shell is also considered a login shell.  This is normally
     done automatically by the system when the user first logs in.  A login
     shell first reads commands from the files /etc/profile and .profile if
     they exist.  If the environment variable ENV is set on entry to a shell,
     or is set in the .profile of a login shell, the shell next reads commands
     from the file named in ENV.  Therefore, a user should place commands that
     are to be executed only at login time in the .profile file, and commands
     that are executed for every shell inside the ENV file.  To set the ENV
     variable to some file, place the following line in your .profile of your
     home directory

           ENV=$HOME/.shinit; export ENV

     substituting for ".shinit" any filename you wish.  Since the ENV file is
     read for every invocation of the shell, including shell scripts and non-
     interactive shells, the following paradigm is useful for restricting
     commands in the ENV file to interactive invocations.  Place commands
     within the "case" and "esac" below (these commands are described later):

           case $- in *i*)
                 # commands for interactive use only

     If command line arguments besides the options have been specified, then
     the shell treats the first argument as the name of a file from which to
     read commands (a shell script), and the remaining arguments are set as
     the positional parameters of the shell ($1, $2, etc).  Otherwise, the
     shell reads commands from its standard input.

   Argument List Processing
     All of the single letter options have a corresponding name that can be
     used as an argument to the -o option.  The set -o name is provided next
     to the single letter option in the description below.  Specifying a dash
     "-" turns the option on, while using a plus "+" disables the option.  The
     following options can be set from the command line or with the set built-
     in (described later).

           -a allexport     Export all variables assigned to.

           -c               Read commands from the command_string operand
                            instead of from the standard input.  Special
                            parameter 0 will be set from the command_name
                            operand and the positional parameters ($1, $2,
                            etc.)  set from the remaining argument operands.

           -C noclobber     Don't overwrite existing files with ">".

           -e errexit       If not interactive, exit immediately if any
                            untested command fails.  The exit status of a
                            command is considered to be explicitly tested if
                            the command is used to control an if, elif, while,
                            or until, or if the command is the left hand
                            operand of an "&&" or "||" operator.

           -f noglob        Disable pathname expansion.

           -n noexec        If not interactive, read commands but do not
                            execute them.  This is useful for checking the
                            syntax of shell scripts.

           -u nounset       Write a message to standard error when attempting
                            to expand a variable that is not set, and if the
                            shell is not interactive, exit immediately.

           -v verbose       The shell writes its input to standard error as it
                            is read.  Useful for debugging.

           -x xtrace        Write each command to standard error (preceded by
                            a '+ ') before it is executed.  Useful for

           -q quietprofile  If the -v or -x options have been set, do not
                            apply them when reading initialization files,
                            these being /etc/profile, .profile, and the file
                            specified by the ENV environment variable.

           -I ignoreeof     Ignore EOFs from input when interactive.

           -i interactive   Force the shell to behave interactively.

           -m monitor       Turn on job control (set automatically when

           -s stdin         Read commands from standard input (set
                            automatically if no file arguments are present).
                            This option has no effect when set after the shell
                            has already started running (i.e. with set).

           -V vi            Enable the built-in vi(1) command line editor
                            (disables -E if it has been set).  (See the
                            Command Line Editing section below.)

           -E emacs         Enable the built-in emacs style command line
                            editor (disables -V if it has been set).  (See the
                            Command Line Editing section below.)

           -b notify        Enable asynchronous notification of background job
                            completion.  (Not implemented.)

              cdprint       Make an interactive shell always print the new
                            directory name when changed by the cd command.

              tabcomplete   Enables filename completion in the command line
                            editor.  Typing a tab character will extend the
                            current input word to match a filename.  If more
                            than one filename matches it is only extended to
                            be the common prefix.  Typing a second tab
                            character will list all the matching names.  One
                            of the editing modes, either -E or -V, must be
                            enabled for this to work.

   Lexical Structure
     The shell reads input in terms of lines from a file and breaks it up into
     words at whitespace (blanks and tabs), and at certain sequences of
     characters that are special to the shell called "operators".  There are
     two types of operators: control operators and redirection operators
     (their meaning is discussed later).  Following is a list of operators:

           Control operators:
                 & && ( ) ; ;; | || <newline>

           Redirection operators:
                 < > >| << >> <& >& <<- <>

     Quoting is used to remove the special meaning of certain characters or
     words to the shell, such as operators, whitespace, or keywords.  There
     are three types of quoting: matched single quotes, matched double quotes,
     and backslash.

     A backslash preserves the literal meaning of the following character,
     with the exception of <newline>.  A backslash preceding a <newline> is
     treated as a line continuation.

   Single Quotes
     Enclosing characters in single quotes preserves the literal meaning of
     all the characters (except single quotes, making it impossible to put
     single quotes in a single-quoted string).

   Double Quotes
     Enclosing characters within double quotes preserves the literal meaning
     of all characters except dollar sign ($), backquote (`), and backslash
     (\).  The backslash inside double quotes is historically weird, and
     serves to quote only the following characters:
           $ ` " \ <newline>.
     Otherwise it remains literal.

   Reserved Words
     Reserved words are words that have special meaning to the shell and are
     recognized at the beginning of a line and after a control operator.  The
     following are reserved words:

           !       elif    fi      while   case
           else    for     then    {       }
           do      done    until   if      esac

     Their meaning is discussed later.

     An alias is a name and corresponding value set using the alias built-in
     command.  Whenever a reserved word may occur (see above), and after
     checking for reserved words, the shell checks the word to see if it
     matches an alias.  If it does, it replaces it in the input stream with
     its value.  For example, if there is an alias called "lf" with the value
     "ls -F", then the input:

           lf foobar <return>

     would become

           ls -F foobar <return>

     Aliases provide a convenient way for naive users to create shorthands for
     commands without having to learn how to create functions with arguments.
     They can also be used to create lexically obscure code.  This use is

     The shell interprets the words it reads according to a language, the
     specification of which is outside the scope of this man page (refer to
     the BNF in the POSIX 1003.2 document).  Essentially though, a line is
     read and if the first word of the line (or after a control operator) is
     not a reserved word, then the shell has recognized a simple command.
     Otherwise, a complex command or some other special construct may have
     been recognized.

   Simple Commands
     If a simple command has been recognized, the shell performs the following

           1.   Leading words of the form "name=value" are stripped off and
                assigned to the environment of the simple command.
                Redirection operators and their arguments (as described below)
                are stripped off and saved for processing.

           2.   The remaining words are expanded as described in the Word
                Expansions section below, and the first remaining word is
                considered the command name and the command is located.  The
                remaining words are considered the arguments of the command.
                If no command name resulted, then the "name=value" variable
                assignments recognized in item 1 affect the current shell.

           3.   Redirections are performed as described in the next section.

     Redirections are used to change where a command reads its input or sends
     its output.  In general, redirections open, close, or duplicate an
     existing reference to a file.  The overall format used for redirection

           [n] redir-op file

     where redir-op is one of the redirection operators mentioned previously.
     Following is a list of the possible redirections.  The [n] is an optional
     number, as in '3' (not '[3]'), that refers to a file descriptor.

           [n]> file   Redirect standard output (or n) to file.

           [n]>| file  Same, but override the -C option.

           [n]>> file  Append standard output (or n) to file.

           [n]< file   Redirect standard input (or n) from file.

           [n1]<&n2    Duplicate standard input (or n1) from file descriptor

           [n]<&-      Close standard input (or n).

           [n1]>&n2    Duplicate standard output (or n1) to n2.

           [n]>&-      Close standard output (or n).

           [n]<> file  Open file for reading and writing on standard input (or

     The following redirection is often called a "here-document".

           [n]<< delimiter
                 here-doc-text ...

     All the text on successive lines up to the delimiter, or to an EOF, is
     saved away and made available to the command on standard input, or file
     descriptor n if it is specified.  If the delimiter as specified on the
     initial line is quoted, then the here-doc-text is treated literally;
     otherwise, the text is subjected to parameter expansion, command
     substitution, and arithmetic expansion as described in the Word
     Expansions section below.  If the operator is "<<-" instead of "<<", then
     leading tabs in the here-doc-text are stripped.

   Search and Execution
     There are three types of commands: shell functions, built-in commands,
     and normal programs -- and the command is searched for (by name) in that
     order.  They each are executed in a different way.

     When a shell function is executed, all of the shell positional parameters
     (except $0, which remains unchanged) are set to the arguments of the
     shell function.  The variables which are explicitly placed in the
     environment of the command (by placing assignments to them before the
     function name) are made local to the function and are set to the values
     given.  Then the command given in the function definition is executed.
     The positional parameters are restored to their original values when the
     command completes.  This all occurs within the current shell.

     Shell built-ins are executed internally to the shell, without spawning a
     new process.

     Otherwise, if the command name doesn't match a function or built-in, the
     command is searched for as a normal program in the file system (as
     described in the next section).  When a normal program is executed, the
     shell runs the program, passing the arguments and the environment to the
     program.  If the program is not a normal executable file (i.e., if it
     does not begin with the "magic number" whose ASCII representation is
     "#!", so execve(2) returns ENOEXEC then) the shell will interpret the
     program in a subshell.  The child shell will reinitialize itself in this
     case, so that the effect will be as if a new shell had been invoked to
     handle the ad-hoc shell script, except that the location of hashed
     commands located in the parent shell will be remembered by the child.

     Note that previous versions of this document and the source code itself
     misleadingly and sporadically refer to a shell script without a magic
     number as a "shell procedure".

   Path Search
     When locating a command, the shell first looks to see if it has a shell
     function by that name.  Then it looks for a built-in command by that
     name.  If a built-in command is not found, one of two things happen:

     1.   Command names containing a slash are simply executed without
          performing any searches.

     2.   The shell searches each entry in PATH in turn for the command.  The
          value of the PATH variable should be a series of entries separated
          by colons.  Each entry consists of a directory name.  The current
          directory may be indicated implicitly by an empty directory name, or
          explicitly by a single period.

   Command Exit Status
     Each command has an exit status that can influence the behavior of other
     shell commands.  The paradigm is that a command exits with zero for
     normal or success, and non-zero for failure, error, or a false
     indication.  The man page for each command should indicate the various
     exit codes and what they mean.  Additionally, the built-in commands
     return exit codes, as does an executed shell function.

     If a command consists entirely of variable assignments then the exit
     status of the command is that of the last command substitution if any,
     otherwise 0.

   Complex Commands
     Complex commands are combinations of simple commands with control
     operators or reserved words, together creating a larger complex command.
     More generally, a command is one of the following:

     +o   simple command

     +o   pipeline

     +o   list or compound-list

     +o   compound command

     +o   function definition

     Unless otherwise stated, the exit status of a command is that of the last
     simple command executed by the command.

     A pipeline is a sequence of one or more commands separated by the control
     operator |.  The standard output of all but the last command is connected
     to the standard input of the next command.  The standard output of the
     last command is inherited from the shell, as usual.

     The format for a pipeline is:

           [!] command1 [| command2 ...]

     The standard output of command1 is connected to the standard input of
     command2.  The standard input, standard output, or both of a command is
     considered to be assigned by the pipeline before any redirection
     specified by redirection operators that are part of the command.

     If the pipeline is not in the background (discussed later), the shell
     waits for all commands to complete.

     If the reserved word ! does not precede the pipeline, the exit status is
     the exit status of the last command specified in the pipeline.
     Otherwise, the exit status is the logical NOT of the exit status of the
     last command.  That is, if the last command returns zero, the exit status
     is 1; if the last command returns greater than zero, the exit status is

     Because pipeline assignment of standard input or standard output or both
     takes place before redirection, it can be modified by redirection.  For

           $ command1 2>&1 | command2

     sends both the standard output and standard error of command1 to the
     standard input of command2.

     A ; or <newline> terminator causes the preceding AND-OR-list (described
     next) to be executed sequentially; a & causes asynchronous execution of
     the preceding AND-OR-list.

     Note that unlike some other shells, each process in the pipeline is a
     child of the invoking shell (unless it is a shell built-in, in which case
     it executes in the current shell -- but any effect it has on the
     environment is wiped).

   Background Commands -- &
     If a command is terminated by the control operator ampersand (&), the
     shell executes the command asynchronously -- that is, the shell does not
     wait for the command to finish before executing the next command.

     The format for running a command in background is:

           command1 & [command2 & ...]

     If the shell is not interactive, the standard input of an asynchronous
     command is set to /dev/null.

   Lists -- Generally Speaking
     A list is a sequence of zero or more commands separated by newlines,
     semicolons, or ampersands, and optionally terminated by one of these
     three characters.  The commands in a list are executed in the order they
     are written.  If command is followed by an ampersand, the shell starts
     the command and immediately proceed onto the next command; otherwise it
     waits for the command to terminate before proceeding to the next one.

   Short-Circuit List Operators
     "&&" and "||" are AND-OR list operators.  "&&" executes the first
     command, and then executes the second command if and only if the exit
     status of the first command is zero.  "||" is similar, but executes the
     second command if and only if the exit status of the first command is
     nonzero.  "&&" and "||" both have the same priority.  Note that these
     operators are left-associative, so "true || echo bar && echo baz" writes
     "baz" and nothing else.  This is not the way it works in C.  Also, if you
     forget the left-hand side (for example when continuing lines but
     forgetting to use a backslash) it defaults to a true statement.  This
     behavior is not useful and should not be relied upon.

   Flow-Control Constructs -- if, while, for, case
     The syntax of the if command is

           if list
           then list
           [ elif list
           then    list ] ...
           [ else list ]

     The syntax of the while command is

           while list
           do   list

     The two lists are executed repeatedly while the exit status of the first
     list is zero.  The until command is similar, but has the word until in
     place of while, which causes it to repeat until the exit status of the
     first list is zero.

     The syntax of the for command is

           for variable in word ...
           do   list

     The words are expanded, and then the list is executed repeatedly with the
     variable set to each word in turn.  do and done may be replaced with "{"
     and "}".

     The syntax of the break and continue command is

           break [ num ]
           continue [ num ]

     Break terminates the num innermost for or while loops.  Continue
     continues with the next iteration of the innermost loop.  These are
     implemented as built-in commands.

     The syntax of the case command is

           case word in
           pattern) list ;;

     The pattern can actually be one or more patterns (see Shell Patterns
     described later), separated by "|" characters.

   Grouping Commands Together
     Commands may be grouped by writing either



           { list; }

     The first of these executes the commands in a subshell.  Built-in
     commands grouped into a (list) will not affect the current shell.  The
     second form does not fork another shell so is slightly more efficient.
     Grouping commands together this way allows you to redirect their output
     as though they were one program:

           { echo -n " hello " ; echo " world" ; } > greeting

     Note that "}" must follow a control operator (here, ";") so that it is
     recognized as a reserved word and not as another command argument.

     The syntax of a function definition is

           name () command

     A function definition is an executable statement; when executed it
     installs a function named name and returns an exit status of zero.  The
     command is normally a list enclosed between "{" and "}".

     Variables may be declared to be local to a function by using a local
     command.  This should appear as the first statement of a function, and
     the syntax is

           local [variable | -] ...

     "Local" is implemented as a built-in command.

     When a variable is made local, it inherits the initial value and exported
     and read-only flags from the variable with the same name in the
     surrounding scope, if there is one.  Otherwise, the variable is initially
     unset.  The shell uses dynamic scoping, so that if you make the variable
     x local to function f, which then calls function g, references to the
     variable x made inside g will refer to the variable x declared inside f,
     not to the global variable named x.

     The only special parameter that can be made local is "-".  Making "-"
     local causes any shell options that are changed via the set command
     inside the function to be restored to their original values when the
     function returns.

     The syntax of the return command is

           return [exitstatus]

     It terminates the currently executing function.  Return is implemented as
     a built-in command.

   Variables and Parameters
     The shell maintains a set of parameters.  A parameter denoted by a name
     is called a variable.  When starting up, the shell turns all the
     environment variables into shell variables.  New variables can be set
     using the form


     Variables set by the user must have a name consisting solely of
     alphabetics, numerics, and underscores - the first of which must not be
     numeric.  A parameter can also be denoted by a number or a special
     character as explained below.

   Positional Parameters
     A positional parameter is a parameter denoted by a number (n > 0).  The
     shell sets these initially to the values of its command line arguments
     that follow the name of the shell script.  The set built-in can also be
     used to set or reset them.

   Special Parameters
     A special parameter is a parameter denoted by one of the following
     special characters.  The value of the parameter is listed next to its

     *            Expands to the positional parameters, starting from one.
                  When the expansion occurs within a double-quoted string it
                  expands to a single field with the value of each parameter
                  separated by the first character of the IFS variable, or by
                  a <space> if IFS is unset.

     @            Expands to the positional parameters, starting from one.
                  When the expansion occurs within double quotes, each
                  positional parameter expands as a separate argument.  If
                  there are no positional parameters, the expansion of @
                  generates zero arguments, even when @ is double-quoted.
                  What this basically means, for example, is if $1 is "abc"
                  and $2 is "def ghi", then "$@" expands to the two arguments:

                        "abc" "def ghi"

     #            Expands to the number of positional parameters.

     ?            Expands to the exit status of the most recent pipeline.

     - (Hyphen.)  Expands to the current option flags (the single-letter
                  option names concatenated into a string) as specified on
                  invocation, by the set built-in command, or implicitly by
                  the shell.

     $            Expands to the process ID of the invoked shell.  A subshell
                  retains the same value of $ as its parent.

     !            Expands to the process ID of the most recent background
                  command executed from the current shell.  For a pipeline,
                  the process ID is that of the last command in the pipeline.

     0 (Zero.)    Expands to the name of the shell or shell script.

   Word Expansions
     This section describes the various expansions that are performed on
     words.  Not all expansions are performed on every word, as explained

     Tilde expansions, parameter expansions, command substitutions, arithmetic
     expansions, and quote removals that occur within a single word expand to
     a single field.  It is only field splitting or pathname expansion that
     can create multiple fields from a single word.  The single exception to
     this rule is the expansion of the special parameter @ within double
     quotes, as was described above.

     The order of word expansion is:

     1.   Tilde Expansion, Parameter Expansion, Command Substitution,
          Arithmetic Expansion (these all occur at the same time).

     2.   Field Splitting is performed on fields generated by step (1) unless
          the IFS variable is null.

     3.   Pathname Expansion (unless set -f is in effect).

     4.   Quote Removal.

     The $ character is used to introduce parameter expansion, command
     substitution, or arithmetic evaluation.

   Tilde Expansion (substituting a user's home directory)
     A word beginning with an unquoted tilde character (~) is subjected to
     tilde expansion.  All the characters up to a slash (/) or the end of the
     word are treated as a username and are replaced with the user's home
     directory.  If the username is missing (as in ~/foobar), the tilde is
     replaced with the value of the HOME variable (the current user's home

   Parameter Expansion
     The format for parameter expansion is as follows:


     where expression consists of all characters until the matching "}".  Any
     "}" escaped by a backslash or within a quoted string, and characters in
     embedded arithmetic expansions, command substitutions, and variable
     expansions, are not examined in determining the matching "}".

     The simplest form for parameter expansion is:


     The value, if any, of parameter is substituted.

     The parameter name or symbol can be enclosed in braces, which are
     optional except for positional parameters with more than one digit or
     when parameter is followed by a character that could be interpreted as
     part of the name.  If a parameter expansion occurs inside double quotes:

     1.   Pathname expansion is not performed on the results of the expansion.

     2.   Field splitting is not performed on the results of the expansion,
          with the exception of the special rules for @.

     In addition, a parameter expansion can be modified by using one of the
     following formats.  If the ":" is omitted in the following modifiers,
     then the expansion is applied only to unset parameters, not null ones.

     ${parameter:-word}    Use Default Values.  If parameter is unset or null,
                           the expansion of word is substituted; otherwise,
                           the value of parameter is substituted.

     ${parameter:=word}    Assign Default Values.  If parameter is unset or
                           null, the expansion of word is assigned to
                           parameter.  In all cases, the final value of
                           parameter is substituted.  Only variables, not
                           positional parameters or special parameters, can be
                           assigned in this way.

     ${parameter:?[word]}  Indicate Error if Null or Unset.  If parameter is
                           unset or null, the expansion of word (or a message
                           indicating it is unset if word is omitted) is
                           written to standard error and the shell exits with
                           a nonzero exit status.  Otherwise, the value of
                           parameter is substituted.  An interactive shell
                           need not exit.

     ${parameter:+word}    Use Alternative Value.  If parameter is unset or
                           null, null is substituted; otherwise, the expansion
                           of word is substituted.

     ${#parameter}         String Length.  The length in characters of the
                           value of parameter.

     The following four varieties of parameter expansion provide for substring
     processing.  In each case, pattern matching notation (see Shell
     Patterns), rather than regular expression notation, is used to evaluate
     the patterns.  If parameter is * or @, the result of the expansion is
     unspecified.  Enclosing the full parameter expansion string in double
     quotes does not cause the following four varieties of pattern characters
     to be quoted, whereas quoting characters within the braces has this

     ${parameter%word}     Remove Smallest Suffix Pattern.  The word is
                           expanded to produce a pattern.  The parameter
                           expansion then results in parameter, with the
                           smallest portion of the suffix matched by the
                           pattern deleted.

     ${parameter%%word}    Remove Largest Suffix Pattern.  The word is
                           expanded to produce a pattern.  The parameter
                           expansion then results in parameter, with the
                           largest portion of the suffix matched by the
                           pattern deleted.

     ${parameter#word}     Remove Smallest Prefix Pattern.  The word is
                           expanded to produce a pattern.  The parameter
                           expansion then results in parameter, with the
                           smallest portion of the prefix matched by the
                           pattern deleted.

     ${parameter##word}    Remove Largest Prefix Pattern.  The word is
                           expanded to produce a pattern.  The parameter
                           expansion then results in parameter, with the
                           largest portion of the prefix matched by the
                           pattern deleted.

   Command Substitution
     Command substitution allows the output of a command to be substituted in
     place of the command name itself.  Command substitution occurs when the
     command is enclosed as follows:


     or ("backquoted" version):


     The shell expands the command substitution by executing command in a
     subshell environment and replacing the command substitution with the
     standard output of the command, removing sequences of one or more
     <newline>s at the end of the substitution.  (Embedded <newline>s before
     the end of the output are not removed; however, during field splitting,
     they may be translated into <space>s, depending on the value of IFS and
     quoting that is in effect.)

   Arithmetic Expansion
     Arithmetic expansion provides a mechanism for evaluating an arithmetic
     expression and substituting its value.  The format for arithmetic
     expansion is as follows:


     The expression is treated as if it were in double quotes, except that a
     double quote inside the expression is not treated specially.  The shell
     expands all tokens in the expression for parameter expansion, command
     substitution, and quote removal.

     Next, the shell treats this as an arithmetic expression and substitutes
     the value of the expression.

     Arithmetic expressions use a syntax similar to that of the C language,
     and are evaluated using the 'intmax_t' data type (this is an extension to
     POSIX, which requires only 'long' arithmetic).  Shell variables may be
     referenced by name inside an arithmetic expression, without needing a "$"

   White Space Splitting (Field Splitting)
     After parameter expansion, command substitution, and arithmetic expansion
     the shell scans the results of expansions and substitutions that did not
     occur in double quotes for field splitting and multiple fields can

     The shell treats each character of the IFS as a delimiter and use the
     delimiters to split the results of parameter expansion and command
     substitution into fields.

     Non-whitespace characters in IFS are treated strictly as parameter
     terminators.  So adjacent non-whitespace IFS characters will produce
     empty parameters.

     If IFS is unset it is assumed to contain space, tab, and newline.

   Pathname Expansion (File Name Generation)
     Unless the -f flag is set, file name generation is performed after word
     splitting is complete.  Each word is viewed as a series of patterns,
     separated by slashes.  The process of expansion replaces the word with
     the names of all existing files whose names can be formed by replacing
     each pattern with a string that matches the specified pattern.  There are
     two restrictions on this: first, a pattern cannot match a string
     containing a slash, and second, a pattern cannot match a string starting
     with a period unless the first character of the pattern is a period.  The
     next section describes the patterns used for both Pathname Expansion and
     the case command.

   Shell Patterns
     A pattern consists of normal characters, which match themselves, and
     meta-characters.  The meta-characters are "!", "*", "?", and "[".  These
     characters lose their special meanings if they are quoted.  When command
     or variable substitution is performed and the dollar sign or backquotes
     are not double-quoted, the value of the variable or the output of the
     command is scanned for these characters and they are turned into meta-

     An asterisk ("*") matches any string of characters.  A question mark
     ("?") matches any single character.  A left bracket ("[") introduces a
     character class.  The end of the character class is indicated by a right
     bracket ("]"); if this "]" is missing then the "[" matches a "[" rather
     than introducing a character class.  A character class matches any of the
     characters between the square brackets.  A range of characters may be
     specified using a minus sign ("-").  The character class may be
     complemented by making an exclamation mark ("!") the first character of
     the character class.

     To include a "]" in a character class, make it the first character listed
     (after the "!", if any).  To include a "-", make it the first or last
     character listed.

     This section lists the built-in commands which are built-in because they
     need to perform some operation that can't be performed by a separate
     process.  In addition to these, there are several other commands that may
     be built in for efficiency (e.g.  printf(1), echo(1), test(1), etc).

     :      A null command that returns a 0 (true) exit value.

     . file
            The commands in the specified file are read and executed by the

     alias [name[=string ...]]
            If name=string is specified, the shell defines the alias name with
            value string.  If just name is specified, the value of the alias
            name is printed.  With no arguments, the alias built-in prints the
            names and values of all defined aliases (see unalias).

     bg [job] ...
            Continue the specified jobs (or the current job if no jobs are
            given) in the background.

     command [-p] [-v] [-V] command [arg ...]
            Execute the specified command but ignore shell functions when
            searching for it.  (This is useful when you have a shell function
            with the same name as a built-in command.)

            -p     search for command using a PATH that guarantees to find all
                   the standard utilities.

            -V     Do not execute the command but search for the command and
                   print the resolution of the command search.  This is the
                   same as the type built-in.

            -v     Do not execute the command but search for the command and
                   print the absolute pathname of utilities, the name for
                   built-ins or the expansion of aliases.

     cd [-P] [directory [replace]]
            Switch to the specified directory (default $HOME).  If replace is
            specified, then the new directory name is generated by replacing
            the first occurrence of directory in the current directory name
            with replace.  If directory is '-', then the current working
            directory is changed to the previous current working directory as
            set in OLDPWD.  Otherwise if an entry for CDPATH appears in the
            environment of the cd command or the shell variable CDPATH is set
            and the directory name does not begin with a slash, or its first
            (or only) component isn't dot or dot dot, then the directories
            listed in CDPATH will be searched for the specified directory.
            The format of CDPATH is the same as that of PATH.

            The -P option instructs the shell to update PWD with the specified
            physical directory path and change to that directory.  This is the

            When the directory changes, the variable OLDPWD is set to the
            working directory before the change.

            Some shells also support a -L option, which instructs the shell to
            update PWD with the logical path and to change the current
            directory accordingly.  This is not supported.

            In an interactive shell, the cd command will print out the name of
            the directory that it actually switched to if this is different
            from the name that the user gave.  These may be different either
            because the CDPATH mechanism was used or because a symbolic link
            was crossed.

     eval string ...
            Concatenate all the arguments with spaces.  Then re-parse and
            execute the command.

     exec [command arg ...]
            Unless command is omitted, the shell process is replaced with the
            specified program (which must be a real program, not a shell
            built-in or function).  Any redirections on the exec command are
            marked as permanent, so that they are not undone when the exec
            command finishes.

     exit [exitstatus]
            Terminate the shell process.  If exitstatus is given it is used as
            the exit status of the shell; otherwise the exit status of the
            preceding command is used.

     export name ...

     export -p
            The specified names are exported so that they will appear in the
            environment of subsequent commands.  The only way to un-export a
            variable is to unset it.  The shell allows the value of a variable
            to be set at the same time it is exported by writing

                  export name=value

            With no arguments the export command lists the names of all
            exported variables.  With the -p option specified the output will
            be formatted suitably for non-interactive use.

     fc [-e editor] [first [last]]

     fc -l [-nr] [first [last]]

     fc -s [old=new] [first]
            The fc built-in lists, or edits and re-executes, commands
            previously entered to an interactive shell.

            -e editor
                   Use the editor named by editor to edit the commands.  The
                   editor string is a command name, subject to search via the
                   PATH variable.  The value in the FCEDIT variable is used as
                   a default when -e is not specified.  If FCEDIT is null or
                   unset, the value of the EDITOR variable is used.  If EDITOR
                   is null or unset, ed(1) is used as the editor.

            -l (ell)
                   List the commands rather than invoking an editor on them.
                   The commands are written in the sequence indicated by the
                   first and last operands, as affected by -r, with each
                   command preceded by the command number.

            -n     Suppress command numbers when listing with -l.

            -r     Reverse the order of the commands listed (with -l) or
                   edited (with neither -l nor -s).

            -s     Re-execute the command without invoking an editor.


            last   Select the commands to list or edit.  The number of
                   previous commands that can be accessed are determined by
                   the value of the HISTSIZE variable.  The value of first or
                   last or both are one of the following:

                          A positive number representing a command number;
                          command numbers can be displayed with the -l option.

                          A negative decimal number representing the command
                          that was executed number of commands previously.
                          For example, -1 is the immediately previous command.

                   A string indicating the most recently entered command that
                   begins with that string.  If the old=new operand is not
                   also specified with -s, the string form of the first
                   operand cannot contain an embedded equal sign.

            The following environment variables affect the execution of fc:

            FCEDIT    Name of the editor to use.

            HISTSIZE  The number of previous commands that are accessible.

     fg [job]
            Move the specified job or the current job to the foreground.

     getopts optstring var
            The POSIX getopts command, not to be confused with the Bell Labs
            -derived getopt(1).

            The first argument should be a series of letters, each of which
            may be optionally followed by a colon to indicate that the option
            requires an argument.  The variable specified is set to the parsed

            The getopts command deprecates the older getopt(1) utility due to
            its handling of arguments containing whitespace.

            The getopts built-in may be used to obtain options and their
            arguments from a list of parameters.  When invoked, getopts places
            the value of the next option from the option string in the list in
            the shell variable specified by var and its index in the shell
            variable OPTIND.  When the shell is invoked, OPTIND is initialized
            to 1.  For each option that requires an argument, the getopts
            built-in will place it in the shell variable OPTARG.  If an option
            is not allowed for in the optstring, then OPTARG will be unset.

            optstring is a string of recognized option letters (see
            getopt(3)).  If a letter is followed by a colon, the option is
            expected to have an argument which may or may not be separated
            from it by whitespace.  If an option character is not found where
            expected, getopts will set the variable var to a "?"; getopts will
            then unset OPTARG and write output to standard error.  By
            specifying a colon as the first character of optstring all errors
            will be ignored.

            A nonzero value is returned when the last option is reached.  If
            there are no remaining arguments, getopts will set var to the
            special option, "--", otherwise, it will set var to "?".

            The following code fragment shows how one might process the
            arguments for a command that can take the options [a] and [b], and
            the option [c], which requires an argument.

                  while getopts abc: f
                          case $f in
                          a | b)  flag=$f;;
                          c)      carg=$OPTARG;;
                          \?)     echo $USAGE; exit 1;;
                  shift $(expr $OPTIND - 1)

            This code will accept any of the following as equivalent:

                  cmd -acarg file file
                  cmd -a -c arg file file
                  cmd -carg -a file file
                  cmd -a -carg -- file file

     hash -rv command ...
            The shell maintains a hash table which remembers the locations of
            commands.  With no arguments whatsoever, the hash command prints
            out the contents of this table.  Entries which have not been
            looked at since the last cd command are marked with an asterisk;
            it is possible for these entries to be invalid.

            With arguments, the hash command removes the specified commands
            from the hash table (unless they are functions) and then locates
            them.  With the -v option, hash prints the locations of the
            commands as it finds them.  The -r option causes the hash command
            to delete all the entries in the hash table except for functions.

     inputrc file
            Read the file to set keybindings as defined by editrc(5).

     jobid [job]
            Print the process id's of the processes in the job.  If the job
            argument is omitted, the current job is used.

     jobs   This command lists out all the background processes which are
            children of the current shell process.

     pwd [-LP]
            Print the current directory.  If -L is specified the cached value
            (initially set from PWD) is checked to see if it refers to the
            current directory; if it does the value is printed.  Otherwise the
            current directory name is found using getcwd(3).  The environment
            variable PWD is set to the printed value.

            The default is pwd -L, but note that the built-in cd command
            doesn't currently support the -L option and will cache (almost)
            the absolute path.  If cd is changed, pwd may be changed to
            default to pwd -P.

            If the current directory is renamed and replaced by a symlink to
            the same directory, or the initial PWD value followed a symbolic
            link, then the cached value may not be the absolute path.

            The built-in command may differ from the program of the same name
            because the program will use PWD and the built-in uses a
            separately cached value.

     read [-p prompt] [-r] variable [...]
            The prompt is printed if the -p option is specified and the
            standard input is a terminal.  Then a line is read from the
            standard input.  The trailing newline is deleted from the line and
            the line is split as described in the Word Expansions section
            above, and the pieces are assigned to the variables in order.  If
            there are more pieces than variables, the remaining pieces (along
            with the characters in IFS that separated them) are assigned to
            the last variable.  If there are more variables than pieces, the
            remaining variables are assigned the null string.  The read built-
            in will indicate success unless EOF is encountered on input, in
            which case failure is returned.

            By default, unless the -r option is specified, the backslash "\"
            acts as an escape character, causing the following character to be
            treated literally.  If a backslash is followed by a newline, the
            backslash and the newline will be deleted.

     readonly name ...

     readonly -p
            The specified names are marked as read only, so that they cannot
            be subsequently modified or unset.  The shell allows the value of
            a variable to be set at the same time it is marked read only by

                  readonly name=value

            With no arguments the readonly command lists the names of all read
            only variables.  With the -p option specified the output will be
            formatted suitably for non-interactive use.

     set [{ -options | +options | -- }] arg ...
            The set command performs three different functions.

            With no arguments, it lists the values of all shell variables.

            If options are given, it sets the specified option flags, or
            clears them as described in the Argument List Processing section.

            The third use of the set command is to set the values of the
            shell's positional parameters to the specified arguments.  To
            change the positional parameters without changing any options, use
            "--" as the first argument to set.  If no arguments are present,
            the set command will clear all the positional parameters
            (equivalent to executing "shift $#".)

     setvar variable value
            Assigns value to variable.  (In general it is better to write
            variable=value rather than using setvar.  setvar is intended to be
            used in functions that assign values to variables whose names are
            passed as parameters.)

     shift [n]
            Shift the positional parameters n times.  A shift sets the value
            of $1 to the value of $2, the value of $2 to the value of $3, and
            so on, decreasing the value of $# by one.  If there are zero
            positional parameters, shift does nothing.

     trap [-l]

     trap [action] signal ...
            Cause the shell to parse and execute action when any of the
            specified signals are received.  The signals are specified by
            signal number or as the name of the signal.  If signal is 0 or its
            equivalent, EXIT, the action is executed when the shell exits.
            action may be null, which cause the specified signals to be
            ignored.  With action omitted or set to '-' the specified signals
            are set to their default action.  When the shell forks off a
            subshell, it resets trapped (but not ignored) signals to the
            default action.  On non-interactive shells, the trap command has
            no effect on signals that were ignored on entry to the shell.  On
            interactive shells, the trap command will catch or reset signals
            ignored on entry.  Issuing trap with option -l will print a list
            of valid signal names.  trap without any arguments cause it to
            write a list of signals and their associated action to the
            standard output in a format that is suitable as an input to the
            shell that achieves the same trapping results.



            List trapped signals and their corresponding action

                  trap -l

            Print a list of valid signals

                  trap '' INT QUIT tstp 30

            Ignore signals INT QUIT TSTP USR1

                  trap date INT

            Print date upon receiving signal INT

     type [name ...]
            Interpret each name as a command and print the resolution of the
            command search.  Possible resolutions are: shell keyword, alias,
            shell built-in, command, tracked alias and not found.  For aliases
            the alias expansion is printed; for commands and tracked aliases
            the complete pathname of the command is printed.

     ulimit [-H | -S] [-a | -btfdscmlrpnv [value]]
            Inquire about or set the hard or soft limits on processes or set
            new limits.  The choice between hard limit (which no process is
            allowed to violate, and which may not be raised once it has been
            lowered) and soft limit (which causes processes to be signaled but
            not necessarily killed, and which may be raised) is made with
            these flags:

            -H          set or inquire about hard limits

            -S          set or inquire about soft limits.  If neither -H nor
                        -S is specified, the soft limit is displayed or both
                        limits are set.  If both are specified, the last one

            The limit to be interrogated or set, then, is chosen by specifying
            any one of these flags:

            -a          show all the current limits

            -b          show or set the limit on the socket buffer size of a
                        process (in bytes)

            -t          show or set the limit on CPU time (in seconds)

            -f          show or set the limit on the largest file that can be
                        created (in 512-byte blocks)

            -d          show or set the limit on the data segment size of a
                        process (in kilobytes)

            -s          show or set the limit on the stack size of a process
                        (in kilobytes)

            -c          show or set the limit on the largest core dump size
                        that can be produced (in 512-byte blocks)

            -m          show or set the limit on the total physical memory
                        that can be in use by a process (in kilobytes)

            -l          show or set the limit on how much memory a process can
                        lock with mlock(2) (in kilobytes)

            -r          show or set the limit on the number of threads this
                        user can have at one time

            -p          show or set the limit on the number of processes this
                        user can have at one time

            -n          show or set the limit on the number of files a process
                        can have open at once

            -v          show or set the limit on how large a process address
                        space can be

            If none of these is specified, it is the limit on file size that
            is shown or set.  If value is specified, the limit is set to that
            number; otherwise the current limit is displayed.

            Limits of an arbitrary process can be displayed or set using the
            sysctl(8) utility.

     umask [mask]
            Set the value of umask (see umask(2)) to the specified octal
            value.  If the argument is omitted, the umask value is printed.

     unalias [-a] [name]
            If name is specified, the shell removes that alias.  If -a is
            specified, all aliases are removed.

     unset name ...
            The specified variables and functions are unset and unexported.
            If a given name corresponds to both a variable and a function,
            both the variable and the function are unset.

     wait [job]
            Wait for the specified job to complete and return the exit status
            of the last process in the job.  If the argument is omitted, wait
            for all jobs to complete and then return an exit status of zero.

   Command Line Editing
     When sh is being used interactively from a terminal, the current command
     and the command history (see fc in the Built-ins section) can be edited
     using emacs-mode or vi-mode command-line editing.  The command 'set -o
     emacs' enables emacs-mode editing.  The command 'set -o vi' enables vi-
     mode editing and places the current shell process into vi insert mode.
     (See the Argument List Processing section above.)

     The vi mode uses commands similar to a subset of those described in the
     vi(1) man page.  With vi-mode enabled, sh can be switched between insert
     mode and command mode.  It's similar to vi(1): pressing the <ESC> key
     will throw you into command VI command mode.  Pressing the <return> key
     while in command mode will pass the line to the shell.

     The emacs mode uses commands similar to a subset available in the
     emacs(1) editor.  With emacs-mode enabled, special keys can be used to
     modify the text in the buffer using the control key.

     sh uses the editline(3) library.

     HOME       Set automatically by login(1) from the user's login directory
                in the password file (passwd(5)).  This environment variable
                also functions as the default argument for the cd built-in.

     PATH       The default search path for executables.  See the Path Search
                section above.

     CDPATH     The search path used with the cd built-in.

     LINENO     The current line number in the script or function.

     LANG       The string used to specify localization information that
                allows users to work with different culture-specific and
                language conventions.  See nls(7).

     MAIL       The name of a mail file, that will be checked for the arrival
                of new mail.  Overridden by MAILPATH.

     MAILCHECK  The frequency in seconds that the shell checks for the arrival
                of mail in the files specified by the MAILPATH or the MAIL
                file.  If set to 0, the check will occur at each prompt.

     MAILPATH   A colon ":" separated list of file names, for the shell to
                check for incoming mail.  This environment setting overrides
                the MAIL setting.  There is a maximum of 10 mailboxes that can
                be monitored at once.

     PS1        The primary prompt string, which defaults to "$  ", unless you
                are the superuser, in which case it defaults to "#  ".

     PS2        The secondary prompt string, which defaults to ">  ".

     PS4        Output before each line when execution trace (set -x) is
                enabled, defaults to "+  ".

     IFS        Input Field Separators.  This is normally set to <space>,
                <tab>, and <newline>.  See the White Space Splitting section
                for more details.

     TERM       The default terminal setting for the shell.  This is inherited
                by children of the shell, and is used in the history editing

     HISTSIZE   The number of lines in the history buffer for the shell.



     Errors that are detected by the shell, such as a syntax error, will cause
     the shell to exit with a non-zero exit status.  If the shell is not an
     interactive shell, the execution of the shell file will be aborted.
     Otherwise the shell will return the exit status of the last command
     executed, or if the exit built-in is used with a numeric argument, it
     will return the argument.

     csh(1), echo(1), getopt(1), ksh(1), login(1), printf(1), test(1),
     editline(3), getopt(3), editrc(5), passwd(5), environ(7), nls(7),

     A sh command appeared in Version 1 AT&T UNIX.  It was, however,
     unmaintainable so we wrote this one.

     Setuid shell scripts should be avoided at all costs, as they are a
     significant security risk.

     PS1, PS2, and PS4 should be subject to parameter expansion before being

     The characters generated by filename completion should probably be quoted
     to ensure that the filename is still valid after the input line has been

BSD                             October 2, 2013                            BSD