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.
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.
.
. filenameRead and execute commands from the filename argument in the current shell context.
break
break [n]Exit from a
for
, while
, until
, or select
loop.
If n is supplied, the nth enclosing loop is exited.
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; symlinks are followed by default or with the
`-L' option.
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.
eval
eval [arguments]The arguments are concatenated together into a single command, which is then read and executed.
exec
exec [-cl] [-a name] [command] [arguments]If command is supplied, it replaces the shell. 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.
exit
exit [n]Exit the shell, returning a status of n to the shell's parent.
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. 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.
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.
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 illegal options or missing option arguments are
encountered.
If the variable OPTERR
is set to 0, no error message will be displayed, even if the first
character of optstring
is not a colon.
If an illegal 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.
getopts
normally parses the positional parameters, but if more arguments are
given in args, getopts
parses those instead.
hash
hash [-r] [-p filename] [name]Remember the full filenames of commands specified as 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.
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.
readonly
readonly [-apf] [name] ...Mark each name as unchangable. 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.
return
return [n]Cause a shell function to exit with value n. This may also be used to terminate execution of a script being executed with the
.
builtin.
shift
shift [n]Shift positional parameters to the left by n. The positional parameters from n+1 ... are renamed to
$1
... .
Parameters represented by the numbers
$#
to n+1 are unset. n
must be a non-negative number less than or equal to $#
.
test
[
times
timesPrint out the user and system times used by the shell and its children.
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.
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.
umask
umask [-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.
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. Read-only variables and functions may not be unset.
Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable.
IFS
PATH
HOME
cd
builtin
command.
CDPATH
cd
command.
MAILPATH
$_
stands for the name of the current mailfile.
MAIL
MAILPATH
variable
is not set, Bash informs the user of the arrival of mail in
the specified file.
PS1
PS2
OPTIND
getopts
builtin.
OPTARG
getopts
builtin.
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 the POSIX standard; this section quickly details the differences of significance. A number of these differences are explained in greater depth in subsequent sections.
Bash is POSIX-conformant, even where the POSIX specification
differs from traditional sh
behavior.
Bash has multi-character invocation options (see section Invoking Bash).
Bash has command-line editing (see section Command Line Editing) and
the bind
builtin.
Bash has command history (see section Bash History Facilities) and the
history
and fc
builtins to manipulate it.
Bash implements csh
-like history expansion (see section Interactive History Expansion).
Bash has one-dimensional array variables (see section Arrays), and the appropriate variable expansions and assignment syntax to use them. Some of the Bash builtins take options to act on arrays. Bash provides some built-in array variables.
Bash implements the !
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.
Bash includes the select
compound command, which allows the
generation of simple menus (see section Korn Shell Constructs).
Bash includes brace expansion (see section Brace Expansion) and tilde expansion (see section Tilde Expansion).
Bash implements command aliases and the alias
and unalias
builtins (see section Aliases).
Bash provides shell arithmetic and arithmetic expansion (see section Shell Arithmetic).
The POSIX and ksh
-style $()
form of command substitution
is implemented (see section Command Substitution),
and preferred to the Bourne shell's "
(which
is also implemented for backwards compatibility).
Variables present in the shell's initial environment are automatically
exported to child processes. The Bourne shell does not normally do
this unless the variables are explicitly marked using the export
command.
Bash includes the POSIX and ksh
-style pattern removal
`%', `#', `%%' and `##' constructs to remove
leading or trailing substrings from variable values
(see section Shell Parameter Expansion).
The expansion ${#xx}
, which returns the length of $xx
,
is supported (see section Shell Parameter Expansion).
The $'...'
quoting syntax, which expands ANSI-C
backslash-escaped characters in the text between the single quotes,
is supported (see section ANSI-C Quoting).
Bash supports the $"..."
quoting syntax to do
locale-specific translation of the characters between the double
quotes. The `-D' and `--dump-strings' invocation options
list the translatable strings found in a script
(see section Locale-Specific Translation).
The expansion ${var:
offset[:
length]}
,
which expands to the substring of var
's value of length
length, optionally beginning at offset, is present
(see section Shell Parameter Expansion).
The expansion
${var/[/]
pattern[/
replacement]}
,
which matches pattern and replaces it with replacement in
the value of var
, is available (see section Shell Parameter Expansion).
Bash has indirect variable expansion using ${!word}
(see section Shell Parameter Expansion).
Bash can expand positional parameters beyond $9
using
${num}
.
Bash has process substitution (see section Process Substitution).
Bash automatically assigns variables that provide information about the
current user (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.
The 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.
It is possible to have a variable and a function with the same name;
sh
does not separate the two name spaces.
Bash functions are permitted to have local variables using the
local
builtin, and thus useful recursive functions may be written.
Variable assignments preceding commands affect only that command, even
builtins and functions (see section Environment).
In sh
, all variable assignments
preceding commands are global unless the command is executed from the
file system.
Bash performs filename expansion on filenames specified as operands to output redirection operators.
Bash contains the `<>' redirection operator, allowing a file to be opened for both reading and writing, and the `&>' redirection operator, for directing standard output and standard error to the same file (see section Redirections).
The 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
.
Bash interprets special backslash-escaped characters in the prompt strings when interactive (see section Controlling the Prompt).
Bash allows you to write a function to override a builtin, and provides
access to that builtin's functionality within the function via the
builtin
and command
builtins (see section Bash Builtin Commands).
The command
builtin allows selective disabling of functions
when command lookup is performed (see section Bash Builtin Commands).
Individual builtins may be enabled or disabled using the enable
builtin (see section Bash Builtin Commands).
The Bash 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'.
Shell functions may be exported to children via the environment (see section Shell Functions).
Bash includes a help
builtin for quick reference to shell
facilities (see section Bash Builtin Commands).
The Bash 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.
Bash includes the shopt
builtin, for finer control of shell
optional capabilities (see section Bash Builtin Commands).
Bash has much more optional behavior controllable with the set
builtin (see section The Set Builtin).
The disown
builtin can remove a job from the internal shell
job table (see section Job Control Builtins).
The return
builtin may be used to abort execution of scripts
executed with the .
or source
builtins
(see section Bourne Shell Builtins).
The test
builtin (see section Bourne Shell Builtins)
is slightly different, as it implements the
POSIX 1003.2 algorithm, which specifies the behavior based on the
number of arguments.
The 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.
The Bash 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.
The Bash cd
and pwd
builtins (see section Bourne Shell Builtins)
each take `-L' and `-P' builtins to switch between logical and
physical modes.
The Bash type
builtin is more extensive and gives more information
about the names it finds (see section Bash Builtin Commands).
Bash implements a csh
-like directory stack, and provides the
pushd
, popd
, and dirs
builtins to manipulate it
(see section C Shell Builtins).
Bash also makes the directory stack visible as the value of the
DIRSTACK
shell variable.
The Bash restricted mode is more useful (see section The Restricted Shell); the SVR4.2 shell restricted mode is too limited.
Bash has the time
reserved word and command timing (see section Pipelines).
The display of the timing statistics may be controlled with the
TIMEFORMAT
variable.
The SVR4.2 shell has two privilege-related builtins
(mldmode
and priv
) not present in Bash.
Bash does not have the stop
or newgrp
builtins.
Bash does not use the SHACCT
variable or perform shell accounting.
The SVR4.2 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), the shell misbehaves badly.
SIGALRM
or
SIGCHLD
.
MAILCHECK
variable 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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