This section briefly summarizes things which Bash inherits from the Bourne Shell: builtins, variables, and other features. It also lists the significant differences between Bash and the Bourne Shell. Many of the builtins have been extended by POSIX or Bash.
The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the POSIX 1003.2 standard.
:
: [arguments]Do nothing beyond expanding arguments and performing redirections. The return status is zero.
.
. filenameRead and execute commands from the filename argument in the current shell context. If filename does not contain a slash, the
$PATH variable is used to find
filename. The current directory is searched if filename
is not found in $PATH.
The return status is the exit status of the last command executed, or
zero if no commands are executed. If filename is not found, or
cannot be read, the return status is non-zero.
break
break [n]Exit from a
for, while, until, or select loop.
If n is supplied, the nth enclosing loop is exited.
n must be greater than or equal to 1.
The return status is zero unless n is not greater than or equal to 1.
cd
cd [-LP] [directory]Change the current working directory to directory. If directory is not given, the value of the
HOME shell variable is used. If the
shell variable CDPATH exists, it is used as a search path. If
directory begins with a slash, CDPATH is not used.
The `-P' option means
to not follow symbolic links; symbolic links are followed by default
or with the `-L' option.
If directory is `-', it is equivalent to $OLDPWD.
The return status is zero if the directory is successfully changed,
non-zero otherwise.
continue
continue [n]Resume the next iteration of an enclosing
for, while,
until, or select loop.
If n is supplied, the execution of the nth enclosing loop
is resumed.
n must be greater than or equal to 1.
The return status is zero unless n is not greater than or equal to 1.
eval
eval [arguments]The arguments are concatenated together into a single command, which is then read and executed, and its exit status returned as the exit status of
eval.
If there are no arguments or only empty arguments, the return status is
zero.
exec
exec [-cl] [-a name] [command [arguments]]If command is supplied, it replaces the shell without creating a new process. If the `-l' option is supplied, the shell places a dash in the zeroth arg passed to command. This is what the
login program does.
The `-c' option causes command to be executed with an empty
environment.
If `-a' is supplied, the shell passes name as the zeroth
argument to command.
If no command is specified, redirections may be used to affect
the current shell environment. If there are no redirection errors, the
return status is zero; otherwise the return status is non-zero.
exit
exit [n]Exit the shell, returning a status of n to the shell's parent. Any trap on
EXIT is executed before the shell terminates.
export
export [-fn] [-p] [name[=value]]Mark each name to be passed to child processes in the environment. If the `-f' option is supplied, the names refer to shell functions; otherwise the names refer to shell variables. The `-n' option means to no longer mark each name for export. If no names are supplied, or if the `-p' option is given, a list of exported names is displayed. The `-p' option displays output in a form that may be reused as input. The return status is zero unless an invalid option is supplied, one of the names is not a valid shell variable name, or `-f' is supplied with a name that is not a shell function.
getopts
getopts optstring name [args]
getopts is used by shell scripts to parse positional parameters.
optstring contains the option letters to be recognized; if a letter
is followed by a colon, the option is expected to have an
argument, which should be separated from it by white space.
Each time it is invoked, getopts
places the next option in the shell variable name, initializing
name if it does not exist,
and the index of the next argument to be processed into the
variable OPTIND.
OPTIND is initialized to 1 each time the shell or a shell script
is invoked.
When an option requires an argument,
getopts places that argument into the variable OPTARG.
The shell does not reset OPTIND automatically; it must be manually
reset between multiple calls to getopts within the same shell
invocation if a new set of parameters is to be used.
When the end of options is encountered, getopts exits with a
return value greater than zero.
OPTIND is set to the index of the first non-option argument,
and name is set to `?'.
getopts
normally parses the positional parameters, but if more arguments are
given in args, getopts parses those instead.
getopts can report errors in two ways. If the first character of
optstring is a colon, silent
error reporting is used. In normal operation diagnostic messages
are printed when invalid options or missing option arguments are
encountered.
If the variable OPTERR
is set to 0, no error messages will be displayed, even if the first
character of optstring is not a colon.
If an invalid option is seen,
getopts places `?' into name and, if not silent,
prints an error message and unsets OPTARG.
If getopts is silent, the option character found is placed in
OPTARG and no diagnostic message is printed.
If a required argument is not found, and getopts
is not silent, a question mark (`?') is placed in name,
OPTARG is unset, and a diagnostic message is printed.
If getopts is silent, then a colon (`:') is placed in
name and OPTARG is set to the option character found.
hash
hash [-r] [-p filename] [name]Remember the full pathnames of commands specified as name arguments, so they need not be searched for on subsequent invocations. The commands are found by searching through the directories listed in
$PATH.
The `-p' option inhibits the path search, and filename is
used as the location of name.
The `-r' option causes the shell to forget all remembered locations.
If no arguments are given, information about remembered commands is printed.
The return status is zero unless a name is not found or an invalid
option is supplied.
pwd
pwd [-LP]Print the current working directory. If the `-P' option is supplied, the path printed will not contain symbolic links. If the `-L' option is supplied, the path printed may contain symbolic links. The return status is zero unless an error is encountered while determining the name of the current directory or an invalid option is supplied.
readonly
readonly [-apf] [name] ...Mark each name as readonly. The values of these names may not be changed by subsequent assignment. If the `-f' option is supplied, each name refers to a shell function. The `-a' option means each name refers to an array variable. If no name arguments are given, or if the `-p' option is supplied, a list of all readonly names is printed. The `-p' option causes output to be displayed in a format that may be reused as input. The return status is zero unless an invalid option is supplied, one of the name arguments is not a valid shell variable or function name, or the `-f' option is supplied with a name that is not a shell function.
return
return [n]Cause a shell function to exit with the return value n. This may also be used to terminate execution of a script being executed with the
. builtin, returning either n or the exit status
of the last command executed within the script as the exit status of the
script.
The return status is false if return is used outside a function
and not during the execution of a script by `.'.
shift
shift [n]Shift the positional parameters to the left by n. The positional parameters from n+1 ...
$# are
renamed to $1 ... $#-n+1.
Parameters represented by the numbers $# to n+1 are unset.
n must be a non-negative number less than or equal to $#.
If n is zero or greater than $#, the positional parameters
are not changed.
The return status is zero unless n is greater than $# or
less than zero, non-zero otherwise.
test
[
! expr
( expr )
expr1 -a expr2
expr1 -o expr2
test and [ builtins evaluate conditional
expressions using a set of rules based on the number of arguments.
times
timesPrint out the user and system times used by the shell and its children. The return status is zero.
trap
trap [-lp] [arg] [sigspec ...]The commands in arg are to be read and executed when the shell receives signal sigspec. If arg is absent or equal to `-', all specified signals are reset to the values they had when the shell was started. If arg is the null string, then the signal specified by each sigspec is ignored by the shell and commands it invokes. If arg is `-p', the shell displays the trap commands associated with each sigspec. If no arguments are supplied, or only `-p' is given,
trap prints the list of commands
associated with each signal number in a form that may be reused as
shell input.
Each sigspec is either a signal name such as SIGINT (with
or without the SIG prefix) or a signal number.
If a sigspec
is 0 or EXIT, arg is executed when the shell exits.
If a sigspec is DEBUG, the command arg is executed
after every simple command.
The `-l' option causes the shell to print a list of signal names
and their corresponding numbers.
Signals ignored upon entry to the shell cannot be trapped or reset.
Trapped signals are reset to their original values in a child
process when it is created.
The return status is zero unless a sigspec does not specify a
valid signal.
umask
umask [-p] [-S] [mode]Set the shell process's file creation mask to mode. If mode begins with a digit, it is interpreted as an octal number; if not, it is interpreted as a symbolic mode mask similar to that accepted by the
chmod command. If mode is
omitted, the current value of the mask is printed. If the `-S'
option is supplied without a mode argument, the mask is printed
in a symbolic format.
If the `-p' option is supplied, and mode
is omitted, the output is in a form that may be reused as input.
The return status is zero if the mode is successfully changed or if
no mode argument is supplied, and non-zero otherwise.
unset
unset [-fv] [name]Each variable or function name is removed. If no options are supplied, or the `-v' option is given, each name refers to a shell variable. If the `-f' option is given, the names refer to shell functions, and the function definition is removed. Readonly variables and functions may not be unset. The return status is zero unless a name does not exist or is readonly.
Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable.
CDPATH
cd builtin command.
HOME
cd builtin
command.
The value of this variable is also used by tilde expansion
(see section Tilde Expansion).
IFS
MAIL
MAILPATH variable
is not set, Bash informs the user of the arrival of mail in
the specified file.
MAILPATH
$_ expands to the name of
the current mail file.
OPTARG
getopts builtin.
OPTIND
getopts builtin.
PATH
PS1
PS2
Bash implements essentially the same grammar, parameter and variable expansion, redirection, and quoting as the Bourne Shell. Bash uses the POSIX 1003.2 standard as the specification of how these features are to be implemented. There are some differences between the traditional Bourne shell and Bash; this section quickly details the differences of significance. A number of these differences are explained in greater depth in subsequent sections.
sh behavior.
bind builtin.
history and fc builtins to manipulate it.
csh-like history expansion
(see section History Expansion).
$'...' quoting syntax, which expands ANSI-C
backslash-escaped characters in the text between the single quotes,
is supported (see section ANSI-C Quoting).
$"..." quoting syntax to do
locale-specific translation of the characters between the double
quotes. The `-D', `--dump-strings', and `--dump-po-strings'
invocation options list the translatable strings found in a script
(see section Locale-Specific Translation).
! keyword to negate the return value of
a pipeline (see section Pipelines).
Very useful when an if statement needs to act only if a test fails.
time reserved word and command timing (see section Pipelines).
The display of the timing statistics may be controlled with the
TIMEFORMAT variable.
select compound command, which allows the
generation of simple menus (see section Conditional Constructs).
[[ compound command, which makes conditional
testing part of the shell grammar (see section Conditional Constructs).
alias and unalias
builtins (see section Aliases).
(( compound command
(see section Conditional Constructs),
and arithmetic expansion (see section Shell Arithmetic).
export
command.
${#xx}, which returns the length of ${xx},
is supported (see section Shell Parameter Expansion).
${var:offset[:length]},
which expands to the substring of var's value of length
length, beginning at offset, is present
(see section Shell Parameter Expansion).
${var/[/]pattern[/replacement]},
which matches pattern and replaces it with replacement in
the value of var, is available (see section Shell Parameter Expansion).
${!word}
(see section Shell Parameter Expansion).
$9 using
${num}.
$() form of command substitution
is implemented (see section Command Substitution),
and preferred to the Bourne shell's " (which
is also implemented for backwards compatibility).
UID, EUID, and GROUPS), the current host
(HOSTTYPE, OSTYPE, MACHTYPE, and HOSTNAME),
and the instance of Bash that is running (BASH,
BASH_VERSION, and BASH_VERSINFO). See section Bash Variables,
for details.
IFS variable is used to split only the results of expansion,
not all words (see section Word Splitting).
This closes a longstanding shell security hole.
extglob
shell option is enabled (see section Pattern Matching).
sh does not separate the two name spaces.
local builtin, and thus useful recursive functions may be written.
sh, all variable assignments
preceding commands are global unless the command is executed from the
file system.
noclobber option is available to avoid overwriting existing
files with output redirection (see section The Set Builtin).
The `>|' redirection operator may be used to override noclobber.
cd and pwd builtins (see section Bourne Shell Builtins)
each take `-L' and `-P' builtins to switch between logical and
physical modes.
builtin and command builtins (see section Bash Builtin Commands).
command builtin allows selective disabling of functions
when command lookup is performed (see section Bash Builtin Commands).
enable
builtin (see section Bash Builtin Commands).
exec builtin takes additional options that allow users
to control the contents of the environment passed to the executed
command, and what the zeroth argument to the command is to be
(see section Bourne Shell Builtins).
export -f (see section Shell Functions).
export, readonly, and declare builtins can
take a `-f' option to act on shell functions, a `-p' option to
display variables with various attributes set in a format that can be
used as shell input, a `-n' option to remove various variable
attributes, and `name=value' arguments to set variable attributes
and values simultaneously.
hash builtin allows a name to be associated with
an arbitrary filename, even when that filename cannot be found by
searching the $PATH, using `hash -p'
(see section Bourne Shell Builtins).
help builtin for quick reference to shell
facilities (see section Bash Builtin Commands).
printf builtin is available to display formatted output
(see section Bash Builtin Commands).
read builtin (see section Bash Builtin Commands)
will read a line ending in `\' with
the `-r' option, and will use the REPLY variable as a
default if no arguments are supplied. The Bash read builtin
also accepts a prompt string with the `-p' option and will use
Readline to obtain the line when given the `-e' option.
return builtin may be used to abort execution of scripts
executed with the . or source builtins
(see section Bourne Shell Builtins).
shopt builtin, for finer control of shell
optional capabilities (see section Bash Builtin Commands).
set
builtin (see section The Set Builtin).
test builtin (see section Bourne Shell Builtins)
is slightly different, as it implements the POSIX algorithm,
which specifies the behavior based on the number of arguments.
trap builtin (see section Bourne Shell Builtins)
allows a DEBUG pseudo-signal specification,
similar to EXIT. Commands specified with a DEBUG trap are
executed after every simple command. The DEBUG trap is not
inherited by shell functions.
type builtin is more extensive and gives more information
about the names it finds (see section Bash Builtin Commands).
umask builtin permits a `-p' option to cause
the output to be displayed in the form of a umask command
that may be reused as input (see section Bourne Shell Builtins).
csh-like directory stack, and provides the
pushd, popd, and dirs builtins to manipulate it
(see section The Directory Stack).
Bash also makes the directory stack visible as the value of the
DIRSTACK shell variable.
disown builtin can remove a job from the internal shell
job table (see section Job Control Builtins) or suppress the sending
of SIGHUP to a job when the shell exits as the result of a
SIGHUP.
mldmode and priv) not present in Bash.
stop or newgrp builtins.
SHACCT variable or perform shell accounting.
sh uses a TIMEOUT variable like Bash uses
TMOUT.
More features unique to Bash may be found in section Bash Features.
Since Bash is a completely new implementation, it does not suffer from many of the limitations of the SVR4.2 shell. For instance:
if or while
statement.
EOF under certain circumstances.
This can be the cause of some hard-to-find errors.
SIGSEGV. If the shell is started from a process with
SIGSEGV blocked (e.g., by using the system() C library
function call), it misbehaves badly.
SIGSEGV,
SIGALRM, or SIGCHLD.
IFS, MAILCHECK,
PATH, PS1, or PS2 variables to be unset.
-x -v);
the SVR4.2 shell allows only one option argument (-xv). In
fact, some versions of the shell dump core if the second argument begins
with a `-'.
jsh
(it turns on job control).
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