dash
NAME
SYNOPSIS
k -words
k -words
k -words
k -words
k -words
k -words
k -words
k -words
k -words
DESCRIPTION
is the standard command interpreter for the system.
The current version of
is in the process of being changed to conform with the
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
plus a few Berkeley extensions, are being incorporated into this shell.
We expect
conformance by the time 4.4 BSD is released.
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 args are present and if the standard input of the shell
is connected to a terminal (or if the
flag is set),
and the
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
and
if they exist.
If the environment variable
is set on entry to an interactive shell, or is set in the
of a login shell, the shell next reads
commands from the file named in
Therefore, a user should place commands that are to be executed only at
login time in the
file, and commands that are executed for every interactive shell inside the
file.
To set the
variable to some file, place the following line in your
of your home directory
substituting for
any filename you wish.
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.
All of the single letter options have a corresponding name that can be
used as an argument to the
option.
The set
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
c set
builtin (described later).
- allexport Export all variables assigned to.
- c Read commands from the operand instead of from the standard input. Special parameter 0 will be set from the operand and the positional parameters ($1, $2, etc.) set from the remaining argument operands.
- noclobber Don't overwrite existing files with
- 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 c if , c elif , c while , or c until ; or if the command is the left hand operand of an or operator.
- noglob Disable pathname expansion.
- noexec If not interactive, read commands but do not execute them. This is useful for checking the syntax of shell scripts.
- 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.
- verbose The shell writes its input to standard error as it is read. Useful for debugging.
- xtrace Write each command to standard error (preceded by a before it is executed. Useful for debugging.
- ignoreeof Ignore EOF's from input when interactive.
- interactive Force the shell to behave interactively.
- monitor Turn on job control (set automatically when interactive).
- 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 c set ) .
- vi Enable the built-in vi(1) command line editor (disables if it has been set).
- emacs Enable the built-in emacs(1) command line editor (disables if it has been set).
- notify Enable asynchronous notification of background job completion. (UNIMPLEMENTED for 4.4alpha)
- operators:
- operators:
backquote
and backslash
The backslash inside double quotes is historically weird, and serves to
quote only the following characters:
Otherwise it remains literal.
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:
- case
- }
- esac
Their meaning is discussed later.
An alias is a name and corresponding value set using the
alias(1)
builtin 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
with the value
then the input:
would become
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 discouraged.
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
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.
If a simple command has been recognized, the shell performs
the following actions:
- t
Leading words of the form
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.
t
The remaining words are expanded as described in
the section called
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
variable assignments recognized in item 1 affect the current shell.
t
Redirections are performed as described in the next section.
where
is one of the redirection operators mentioned previously.
Following is a list of the possible redirections.
The
q n
is an optional number, as in
(not
that refers to a file descriptor.
- file Redirect standard output (or n) to file.
- file Same, but override the option.
- file Append standard output (or n) to file.
- file Redirect standard input (or n) from file.
- n2 Duplicate standard input (or n1) from file descriptor n2.
- <&- Close standard input (or n).
- n2 Duplicate standard output (or n1) to n2.
- >&- Close standard output (or n).
- file Open file for reading and writing on standard input (or n).
The following redirection is often called a
- t
All the text on successive lines up to the delimiter 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 section on
If the operator is
instead of
then leading tabs in the here-doc-text are stripped.
There are three types of commands: shell functions, builtin 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 builtins are executed internally to the shell, without spawning a
new process.
Otherwise, if the command name doesn't match a function or builtin, 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
representation is "#!", so
execve(2)
returns
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".
When locating a command, the shell first looks to see if it has a shell
function by that name.
Then it looks for a builtin command by that name.
If a builtin command is not found, one of two things happen:
- t
Command names containing a slash are simply executed without performing
any searches.
t
The shell searches each entry in
in turn for the command.
The value of the
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.
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 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:
- t
simple command
t
pipeline
t
list or compound-list
t
compound command
t
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:
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
zero.
Because pipeline assignment of standard input or standard output or both
takes place before redirection, it can be modified by redirection.
For example:
sends both the standard output and standard error of command1
to the standard input of command2.
A ; or
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 builtin, in which case
it executes in the current shell -- but any effect it has on the
environment is wiped).
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:
If the shell is not interactive, the standard input of an asynchronous
command is set to
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.
and
are AND-OR list operators.
executes the first command, and then executes the second command iff the
exit status of the first command is zero.
is similar, but executes the second command iff the exit status of the first
command is nonzero.
and
both have the same priority.
The syntax of the if command is
if list then list [ elif list then list ] ... [ else list ] fi
The syntax of the while command is
while list do list done
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 done
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 builtin commands.
The syntax of the case command is
case word in pattern) list ;; ... esac
The pattern can actually be one or more patterns (see
described later), separated by
characters.
Commands may be grouped by writing either
or
The first of these executes the commands in a subshell.
Builtin 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:
{ printf \*q hello \*q ; printf \*q world\n" ; } > 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
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 is implemented as a builtin command.
When a variable is made local, it inherits the initial value and exported
and readonly 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 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
It terminates the currently executing function.
Return is implemented as a builtin command.
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.
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
c set
builtin can also be used to set or reset them.
A special parameter is a parameter denoted by one of the following special
characters.
The value of the parameter is listed next to its character.
- * 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 variable, or by a if 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 and $2 is then expands to the two arguments:
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:
- t
Tilde Expansion, Parameter Expansion, Command Substitution,
Arithmetic Expansion (these all occur at the same time).
t
Field Splitting is performed on fields
generated by step (1) unless the
variable is null.
t
Pathname Expansion (unless set
is in effect).
t
Quote Removal.
The $ character is used to introduce parameter expansion, command
substitution, or arithmetic evaluation.
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
the tilde is replaced with the value of the
variable (the current user's home directory).
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:
- t
Pathname expansion is not performed on the results of the expansion.
t
Field splitting is not performed on the results of the
expansion, with the exception of @.
In addition, a parameter expansion can be modified by using one of the
following formats.
- ${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.
In the parameter expansions shown previously, use of the colon in the
format results in a test for a parameter that is unset or null; omission
of the colon results in a test for a parameter that is only unset.
- ${#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
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 effect.
- ${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.
or
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
at the end of the substitution.
(Embedded
before
the end of the output are not removed; however, during field splitting,
they may be translated into
depending on the value of
and quoting that is in effect.)
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.
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 result.
The shell treats each character of the
as a delimiter and uses the delimiters to split the results of parameter
expansion and command substitution into fields.
Unless the
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
c case
command.
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 back quotes 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-characters.
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
if the
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 point 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 minus sign, make it the first or last character listed.
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.
- ... If is specified, the shell defines the alias with value If just is specified, the value of the alias is printed. With no arguments, the c alias builtin prints the names and values of all defined aliases (see c unalias ) .
- ... Continue the specified jobs (or the current job if no jobs are given) in the background.
- command 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 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.
- -
- LP Switch to the specified directory (default If an entry for appears in the environment of the c cd command or the shell variable is set and the directory name does not begin with a slash, then the directories listed in will be searched for the specified directory. The format of is the same as that of If a single dash is specified as the argument, it will be replaced by the value of The c 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 mechanism was used or because the argument is a single dash. The option causes the physical directory structure to be used, that is, all symbolic links are resolved to their respective values. The option turns off the effect of any preceding options.
- n Print the arguments on the standard output, separated by spaces. Unless the 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 c echo would otherwise output.
- \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.
- digits 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.
...
Concatenate all the arguments with spaces.
Then re-parse and execute the command.
...
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
c exec
command are marked as permanent, so that they are not undone when the
c exec
command finishes.
exitstatus
Terminate the shell process.
If
is given it is used as the exit status of the shell; otherwise the
exit status of the preceding command is used.
...
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
With no arguments the export command lists the names of all exported variables.
With the
option specified the output will be formatted suitably for non-interactive use.
editor
l
old=new
The
c fc
builtin lists, or edits and re-executes, commands previously entered
to an interactive shell.
editor
Use the editor named by editor to edit the commands.
The editor string is a command name, subject to search via the
variable.
The value in the
variable is used as a default when
is not specified.
If
is null or unset, the value of the
variable is used.
If
is null or unset,
ed(1)
is used as the editor.
(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
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
or edited (with neither
nor
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
variable.
The value of first or last or both are one of the following:
string
A string indicating the most recently entered command that begins with
that string.
If the old=new operand is not also specified with
the string form of the first operand cannot contain an embedded equal sign.
- [+]number A positive number representing a command number; command numbers can be displayed with the option.
- number A negative decimal number representing the command that was executed number of commands previously. For example, -1 is the immediately previous command.
The following environment variables affect the execution of fc:
job
Move the specified job or the current job to the foreground.
var
The
c getopts
command, not to be confused with the
-derived
getopt(1)
- FCEDIT Name of the editor to use.
- HISTSIZE The number of previous commands that are accessible.
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
c getopts
command deprecates the older
getopt(1)
utility due to its handling of arguments containing whitespace.
The
c getopts
builtin may be used to obtain options and their arguments
from a list of parameters.
When invoked,
c getopts
places the value of the next option from the option string in the list in
the shell variable specified by
and its index in the shell variable
When the shell is invoked,
is initialized to 1.
For each option that requires an argument, the
c getopts
builtin will place it in the shell variable
If an option is not allowed for in the
then
will be unset.
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,
c getopts
will set the variable
to a
c getopts
will then unset
and write output to standard error.
By specifying a colon as the first character of
all errors will be ignored.
A nonzero value is returned when the last option is reached.
If there are no remaining arguments,
c getopts
will set
to the special option,
otherwise, it will set
to
The following code fragment shows how one might process the arguments
for a command that can take the options
and
and the option
which requires an argument.
while getopts abc: f do case $f in a | b) flag=$f;; c) carg=$OPTARG;; \?) echo $USAGE; exit 1;; esac done 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
With arguments, the
c hash
command removes the specified commands from the hash table (unless
they are functions) and then locates them.
With the
option, hash prints the locations of the commands as it finds them.
The
option causes the hash command to delete all the entries in the hash table
except for functions.
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
c pwd
will continue to print the old name for the directory.
The
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
option turns off the effect of any preceding
options.
prompt
The prompt is printed if the
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
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
c read
builtin will indicate success unless EOF is encountered on input, in
which case failure is returned.
By default, unless the
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.
...
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
With no arguments the readonly command lists the names of all read only
variables.
With the
option specified the output will be formatted suitably for non-interactive use.
The
after the first are treated as strings if the corresponding format is
either
or
otherwise it is evaluated as a C constant, with the following extensions:
- t
A leading plus or minus sign is allowed.
t
If the leading character is a single or double quote, the value is the
code of the next character.
The format string is reused as often as necessary to satisfy the
Any extra format specifications are evaluated with zero or the null
string.
Character escape sequences are in backslash notation as defined in
The characters and their meanings are as follows:
- \a Write a <bell> character.
- \b Write a <backspace> 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 value is the 1-, 2-, or 3-digit octal number
Each format specification is introduced by the percent character
(``%'').
The remainder of the format specification includes,
in the following order:
- flags:
- # A `#' character specifying that the value should be printed in an ``alternative form''. For and formats, this option has no effect. For the format the precision of the number is increased to force the first character of the output string to a zero. For the
format, a non-zero result has the string
prepended to it.
For
and
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
and
formats, trailing zeros are not removed from the result as they
would otherwise be.
-
A minus sign `-' which specifies
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;
Width:
An optional digit string specifying a
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);
:
An optional period,
followed by an optional digit string giving a
which specifies the number of digits to appear after the decimal point,
for
and
formats, or the maximum number of characters to be printed
from a string
and
formats); if the digit string is missing, the precision is treated
as zero;
:
A character which indicates the type of format to use (one of
A field width or precision may be
instead of a digit string.
In this case an
supplies the field width or precision.
The format characters and their meanings are:
- diouXx The is printed as a signed decimal (d or i), unsigned octal, unsigned decimal, or unsigned hexadecimal (X or x), respectively.
- f The is printed in the style
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
is printed in the style
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
is printed in style
or in style
whichever gives full precision in minimum space.
b
Characters from the string
are printed with backslash-escape sequences expanded.
The following additional backslash-escape sequences are supported:
\c
Causes
to ignore any remaining characters in the string operand containing it,
any remaining string operands, and any additional characters in
the format operand.
num
Write an 8-bit character whose
value is the 1-, 2-, or 3-digit
octal number
c
The first character of
is printed.
s
Characters from the string
are printed until the end is reached or until the number of characters
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.
The following additional backslash-escape sequences are supported:
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
The
c 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
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
n
Shift the positional parameters n times.
A
c shift
sets the value of
to the value of
the value of
to the value of
and so on, decreasing
the value of
by one.
If n is greater than the number of positional parameters,
c shift
will issue an error message, and exit with return status 2.
expression
]
The
c 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
c test
utility.
The following primaries are used to construct expression:
- file True if exists and is a block special file.
- file True if exists and is a character special file.
- file True if exists and is a directory.
- file True if exists (regardless of type).
- file True if exists and is a regular file.
- file True if exists and its set group ID flag is set.
- file True if exists and is a symbolic link.
- file True if exists and its sticky bit is set.
- string True if the length of is nonzero.
- file True if is a named pipe
These primaries can be combined with the following operators:
- expression True if is false.
- expression2 True if both and are true.
- expression2 True if either or are true.
- ) True if expression is true.
The
operator has higher precedence than the
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
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
is
the action is executed when the shell exits.
may be null, which cause the specified signals to be ignored.
With
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
c trap
command has no effect on signals that were
ignored on entry to the shell.
c 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:
List trapped signals and their corresponding action
Ignore signals INT QUIT TSTP USR1
Print date upon receiving signal INT
...
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.
Xo
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
nor
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 kilobytes)
- 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
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.
EXIT STATUS
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 builtin is used with a numeric argument, it will return the
argument.
ENVIRONMENT
- HOME Set automatically by login(1) from the user's login directory in the password file
This environment variable also functions as the default argument for the
cd builtin.
PATH
The default search path for executables.
See the above section
CDPATH
The search path used with the cd builtin.
MAIL
The name of a mail file, that will be checked for the arrival of new mail.
Overridden by
MAILCHECK
The frequency in seconds that the shell checks for the arrival of mail
in the files specified by the
or the
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
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
and
See the
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 modes.
HISTSIZE
The number of lines in the history buffer for the shell.
PWD
The logical value of the current working directory. This is set by the
c cd
command.
OLDPWD
The previous logical value of the current working directory. This is set by
the
c cd
command.
PPID
The process ID of the parent process of the shell.
FILES
- t
t
SEE ALSO
HISTORY
A
command appeared in
It was, however, unmaintainable so we wrote this one.
BUGS
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 displayed.
