/usr/bin/dash: Difference between revisions

This section lists the builtin commands which are builtin 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 builtin for efficiency (e.g.  printf(1), echo(1), test(1), etc).

:

;true   

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

;file

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

;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 builtin 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 builtin 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 builtin.

;-v

:Do not execute the command but search for the command and print the absolute pathname of utilities, the name for builtins or the expansion of aliases.

;cd -

;cd [-LP] [directory]

:Switch to the specified directory (default HOME).  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, then the directories listed in CDPATH will be searched for the specified directory.  The format of CDPATH is the same as that of PATH.  If a single dash is specified as the argument, it will be replaced by the value of OLDPWD.  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 the argument is a single dash.  The -P option causes the physical directory structure to be used, that is, all symbolic links are resolved to their respective values.  The -L option turns off the effect of any preceding -P options.

;echo [-n] args...

:Print the arguments on the standard output, separated by spaces.  Unless the -n option is present, a newline is output following the arguments.

:If any of the following sequences of characters is encountered during output, the sequence is not output.  Instead, the specified action is performed:

::\b      A backspace character is output.

::\c      Subsequent output is suppressed.  This is normally used at the end of the last argument to suppress the trailing newline that echo would otherwise output.

::\e      Outputs an escape character (ESC).

::\f      Output a form feed.

::\n      Output a newline character.

::\r      Output a carriage return.

::\t      Output a (horizontal) tab character.

::\v      Output a vertical tab.

::\0digits

:::Output the character whose value is given by zero to three octal digits.  If there are zero digits, a nul character is output.

::\\      Output a backslash.

:All other backslash sequences elicit undefined behaviour.

;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 builtin 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 builtin 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.

:first

: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:

::[+]number

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

::-number

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

:string

::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.

;gtopts 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 option.

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

:The getopts builtin 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 builtin 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 white space.  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.

:After the last option getopts will return a non-zero value and 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.

:This code will accept any of the following as equivalent:

;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.

;pwd [-LP]

:builtin command remembers what the current directory is rather than recomputing it each time. This makes it faster.  However, if the current directory is renamed, the builtin version of pwd will continue to print the old name for the directory.  The -P option causes the physical value of the current working directory to be shown, that is, all symbolic links are resolved to their respective values.  The -L option turns off the effect of any preceding -P options.

;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 section on word splitting above, and the pieces are assigned to the variables in order.  At least one variable must be specified.  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 varibles are assigned the null string.  The read builtin 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 writing

;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.

;printf format [arguments ...]

:printf formats and prints its arguments, after the first, under control of the format.  The format is a character string which contains three types of objects: plain characters, which are simply copied to standard output, character escape sequences which are converted and copied to the standard output, and format specifications, each of which causes printing of the next successive argument.

:The arguments after the first are treated as strings if the corresponding format is either b, c or s; otherwise it is evaluated as a C constant, with the following extensions:

::•  A leading plus or minus sign is allowed.

::•  If the leading character is a single or double quote, the value is the ASCII code of the next character.

:The format string is reused as often as necessary to satisfy the arguments.  Any extra format specifications are evaluated with zero or the null string.

:Character escape sequences are in backslash notation as defined in ANSI X3.159-1989 (“ANSI C89”).  The characters and their meanings are as follows:

::\a      Write a <bell> character.

::\b      Write a <backspace> character.

::\e      Write an <escape> (ESC) character.

::\f      Write a <form-feed> character.

::\n      Write a <new-line> character.

::\r      Write a <carriage return> character.

::\t      Write a <tab> character.

::\v      Write a <vertical tab> character.

::\\      Write a backslash character.

::\num    Write an 8-bit character whose ASCII value is the 1-, 2-, or 3-digit octal number num.

:Each format specification is introduced by the percent character (``%'').  The remainder of the format specification includes, in the following order:

:Zero or more of the following flags:

::#      A `#' character specifying that the value should be printed in an ``alternative form''.  For b, c, d, and s formats, this option has no effect.  For the o format the precision of the number is increased to force the first character of the output string to a zero.  For the x (X) format, a non-zero result has the string 0x (0X) prepended to it.  For e, E, f, g, and G formats, the result will always contain a decimal point, even if no digits follow the point (normally, a decimal point only appears in the results of those formats if a digit follows the decimal point).  For g and G formats, trailing zeros are not removed from the result as they would otherwise be.

::-      A minus sign `-' which specifies left adjustment of the output in the indicated field;

::+      A `+' character specifying that there should always be a sign placed before the number when using signed formats.

::' ’    A space specifying that a blank should be left before a positive number for a signed format.  A `+' overrides a space if both are used;

::0      A zero `0' character indicating that zero-padding should be used rather than blank-padding.  A `-' overrides a `0' if both are used;

:Field Width:

::An optional digit string specifying a field width; if the output string has fewer characters than the field width it will be blank-padded on the left (or right, if the left-adjustment indicator has been given) to make up the field width (note that a leading zero is a flag, but an embedded zero is part of a field width);

:Precision:

::An optional period, ‘.’, followed by an optional digit string giving a precision which specifies the number of digits to appear after the decimal point, for e and f formats, or the maximum number of bytes to be printed from a string (b and s formats); if the digit string is missing, the precision is treated as zero;

:Format:

::A character which indicates the type of format to use (one of diouxXfwEgGbcs).

:A field width or precision may be ‘*’ instead of a digit string.  In this case an argument supplies the field width or precision.

:The format characters and their meanings are:

::diouXx      The argument is printed as a signed decimal (d or i), unsigned octal, unsigned decimal, or unsigned hexadecimal (X or x), respectively.

::f          The argument is printed in the style [-]ddd.ddd where the number of d's after the decimal point is equal to the precision specification for the argument.  If the precision is missing, 6 digits are given; if the precision is explicitly 0, no digits and no decimal point are printed.

::eE          The argument is printed in the style [-]d.ddde±dd where there is one digit before the decimal point and the number after is equal to the precision specification for the argument; when the precision is missing, 6 digits are produced.  An upper-case E is used for an `E' format.

::gG          The argument is printed in style f or in style e (E) whichever gives full precision in minimum space.

::b          Characters from the string argument are printed with backslash-escape sequences expanded. The following additional backslash-escape sequences are supported:

::\c      Causes dash to ignore any remaining characters in the string operand containing it, any remaining string operands, and any additional characters in the format operand.

::\0num  Write an 8-bit character whose ASCII value is the 1-, 2-, or 3-digit octal number num.

::c          The first character of argument is printed.

::s          Characters from the string argument are printed until the end is reached or until the number of bytes indicated by the precision specification is reached; if the precision is omitted, all characters in the string are printed.

::%          Print a `%'; no argument is used.

:In no case does a non-existent or small field width cause truncation of a field; padding takes place only if the specified field width exceeds the actual width.

;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 section called Argument List Processing.  As a special case, if the option is -o or +o and no argument is supplied, the shell prints the settings of all its options.  If the option is -o, the settings are printed in a human-readable format; if the option is +o, the settings are printed in a format suitable for reinput to the shell to affect the same option settings.

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

;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 n is greater than the number of positional parameters, shift will issue an error message, and exit with return status 2.

;test expression

;[ expression ]

:The test utility evaluates the expression and, if it evaluates to true, returns a zero (true) exit status; otherwise it returns 1 (false).  If there is no expression, test also returns 1 (false).

:All operators and flags are separate arguments to the test utility.

:The following primaries are used to construct expression:

::-b file      True if file exists and is a block special file.

::-c file      True if file exists and is a character special file.

::-d file      True if file exists and is a directory.

::-e file      True if file exists (regardless of type).

::-f file      True if file exists and is a regular file.

::-g file      True if file exists and its set group ID flag is set.

::-h file      True if file exists and is a symbolic link.

::-k file      True if file exists and its sticky bit is set.

::-n string    True if the length of string is nonzero.

::-p file      True if file is a named pipe (FIFO).

::-r file      True if file exists and is readable.

::-s file      True if file exists and has a size greater than zero.

::-t file_descriptor

:::True if the file whose file descriptor number is file_descriptor is open and is associated with a terminal.

::-u file      True if file exists and its set user ID flag is set.

::-w file      True if file exists and is writable.  True indicates only that the write flag is on.  The file is not writable on a read-only file system even if this test indicates true.

::-x file      True if file exists and is executable.  True indicates only that the execute flag is on.  If file is a directory, true indicates that file can be searched.

::-z string    True if the length of string is zero.

::-L file      True if file exists and is a symbolic link.  This operator is retained for compatibility with previous versions of this program.  Do not rely on its existence; use -h instead.

::-O file      True if file exists and its owner matches the effective user id of this process.

::-G file      True if file exists and its group matches the effective group id of this process.

::-S file      True if file exists and is a socket.

::file1 -nt file2

:::True if file1 and file2 exist and file1 is newer than file2.

::file1 -ot file2

:::True if file1 and file2 exist and file1 is older than file2.

::file1 -ef file2

:::True if file1 and file2 exist and refer to the same file.

::string        True if string is not the null string.

::s1 = s2      True if the strings s1 and s2 are identical.

::s1 != s2      True if the strings s1 and s2 are not identical.

::s1 < s2      True if string s1 comes before s2 based on the ASCII value of their characters.

::s1 > s2      True if string s1 comes after s2 based on the ASCII value of their characters.

::n1 -eq n2    True if the integers n1 and n2 are algebraically equal.

::n1 -ne n2    True if the integers n1 and n2 are not algebraically equal.

::n1 -gt n2    True if the integer n1 is algebraically greater than the integer n2.

::n1 -ge n2    True if the integer n1 is algebraically greater than or equal to the integer n2.

::n1 -lt n2    True if the integer n1 is algebraically less than the integer n2.

::n1 -le n2    True if the integer n1 is algebraically less than or equal to the integer n2.

:These primaries can be combined with the following operators:

::! expression  True if expression is false.

::expression1 -a expression2

:::True if both expression1 and expression2 are true.

::expression1 -o expression2

:::True if either expression1 or expression2 are true.

::(expression)  True if expression is true.

:The -a operator has higher precedence than the -o operator.

:times  Print the accumulated user and system times for the shell and for processes run from the shell. The return status is 0.

;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 EXIT, the action is executed when the shell exits.  action may be empty (''), which causes 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.  The trap command has no effect on signals that were ignored on entry to the shell.  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.

::Examples:

:::trap

::List trapped signals and their corresponding action

:::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 builtin, 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 | -tfdscmlpnv [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 wins.

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

::-a          show all the current limits

::-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 kilo‐bytes)

::-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 files a process can have open at once

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

::-r          show or set the limit on the real-time scheduling priority of a process

: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 [-fv] name ...

:The specified variables and functions are unset and unexported.  If -f or -v is specified, the corresponding function or variable is unset, respectively.  If a given name corresponds to both a variable and a function, and no options are given, only the variable is 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 return an exit status of zero.