@subsubsection ANSI-C Quoting
@cindex quoting, ANSI
-Character sequences of the form $'@var{string}' are treated as a special
-kind of single quotes.
+Character sequences of the form @code{$'@var{string}'} are treated as
+a special kind of single quotes.
The sequence expands to @var{string}, with backslash-escaped characters
in @var{string} replaced as specified by the ANSI C standard.
Backslash escape sequences, if present, are decoded as follows:
When the shell is in @sc{posix} mode (@pxref{Bash POSIX Mode}), @code{time}
may be followed by a newline. In this case, the shell displays the
total user and system time consumed by the shell and its children.
-The @env{TIMEFORMAT} variable may be used to specify the format of
-the time information.
+The @env{TIMEFORMAT} variable specifies the format of the time information.
If the pipeline is not executed asynchronously (@pxref{Lists}), the
shell waits for all commands in the pipeline to complete.
After these expansions are performed, quote characters present in the
original word are removed unless they have been quoted themselves
-(@dfn{quote removal}).
+(@dfn{quote removal}). @xref{Quote Removal} for more details.
Only brace expansion, word splitting, and filename expansion
can increase the number of words of the expansion; other expansions
@code{"$@{@var{name}[@@]@}"} and @code{$@{@var{name}[*]@}}
(@pxref{Arrays}).
-After all expansions, @code{quote removal} (@pxref{Quote Removal})
-is performed.
-
@node Brace Expansion
@subsection Brace Expansion
@cindex brace expansion
$ v=123
$ echo $@{v-unset@}
123
+$ echo $@{v:-unset-or-null@}
+123
+$ unset v
+$ echo $@{v-unset@}
+unset
+$ v=
+$ echo $@{v:-unset-or-null@}
+unset-or-null
@end example
@item $@{@var{parameter}:=@var{word}@}
Command substitution allows the output of a command to replace
the command itself.
-Command substitution occurs when a command is enclosed as follows:
+The standard form of command substitution occurs when a command is
+enclosed as follows:
@example
$(@var{command})
@end example
@noindent
-or
+or (deprecated)
@example
-`@var{command}`
+`@var{command}`.
@end example
@noindent
-Bash performs the expansion by executing @var{command} in a subshell environment
-and replacing the command substitution with the standard output of the
-command, with any trailing newlines deleted.
+Bash performs command substitution by executing @var{command} in a subshell
+environment and replacing the command substitution with the standard
+output of the command, with any trailing newlines deleted.
Embedded newlines are not deleted, but they may be removed during
word splitting.
The command substitution @code{$(cat @var{file})} can be
replaced by the equivalent but faster @code{$(< @var{file})}.
-When the old-style backquote form of substitution is used,
+With the old-style backquote form of substitution,
backslash retains its literal meaning except when followed by
@samp{$}, @samp{`}, or @samp{\}.
The first backquote not preceded by a backslash terminates the
When using the @code{$(@var{command})} form, all characters between
the parentheses make up the command; none are treated specially.
+There is an alternate form of command substitution:
+
+@example
+$@{@var{c} @var{command}; @}
+@end example
+
+@noindent
+which executes @var{command} in the current execution environment
+and captures its output, again with trailing newlines removed.
+
+The character @var{c} following the open brace must be a space, tab,
+newline, or @samp{|}, and the close brace must be in a position
+where a reserved word may appear (i.e., preceded by a command terminator
+such as semicolon).
+Bash allows the close brace to be joined to the remaining characters in
+the word without being followed by a shell metacharacter as a reserved
+word would usually require.
+
+Any side effects of @var{command} take effect immediately
+in the current execution environment and persist in the current
+environment after the command completes (e.g., the @code{exit} builtin
+will exit the shell).
+
+This type of command substitution superficially resembles executing an
+unnamed shell function: local variables are created as when a shell
+function is executing, and the @code{return} builtin forces
+@var{command} to complete;
+however, the rest of the execution environment,
+including the positional parameters, is shared with the caller.
+
+If the first character following the open brace
+is a @samp{|}, the construct expands to the
+value of the @code{REPLY} shell variable after @var{command} executes,
+without removing any trailing newlines,
+and the standard output of @var{command} remains the same as in the
+calling shell.
+Bash creates @code{REPLY} as an initially-unset local variable when
+@var{command} executes, and restores @code{REPLY} to the value it had
+before the command substitution after @var{command} completes,
+as with any local variable.
+
+For example, this construct expands to @samp{12345}, and leaves the
+shell variable @code{X} unchanged in the current execution environment:
+
+@example
+
+$@{ local X=12345 ; echo $X; @}
+@end example
+
+@noindent
+(not declaring @code{X} as local would modify its value in the current
+environment, as with normal shell function execution),
+while this construct does not require any output to expand to
+@samp{12345}:
+
+@example
+$@{| REPLY=12345; @}
+@end example
+
+@noindent
+and restores @code{REPLY} to the value it had before the command substitution.
+
Command substitutions may be nested. To nest when using the backquoted
form, escape the inner backquotes with backslashes.
-If the substitution appears within double quotes, word splitting and
-filename expansion are not performed on the results.
+If the substitution appears within double quotes, Bash does not perform
+word splitting and filename expansion on the results.
@node Arithmetic Expansion
@subsection Arithmetic Expansion
The @code{dotglob} option is disabled when @env{GLOBIGNORE}
is unset.
+After the pattern is expanded and matched against filenames, the value of the
+@env{GLOBSORT} variable controls how the results are sorted, as described
+below (@pxref{Bash Variables}).
+
@node Pattern Matching
@subsubsection Pattern Matching
@cindex pattern matching
Bash handles several filenames specially when they are used in
redirections, as described in the following table.
If the operating system on which Bash is running provides these
-special files, bash will use them; otherwise it will emulate them
+special files, Bash will use them; otherwise it will emulate them
internally with the behavior described below.
@table @code
@subsection Here Documents
This type of redirection instructs the shell to read input from the
-current source until a line containing only @var{word}
-(with no trailing blanks) is seen. All of
-the lines read up to that point are then used as the standard
+current source until a line containing only @var{delimiter}
+(with no trailing blanks) is seen.
+All of the lines read up to that point are then used as the standard
input (or file descriptor @var{n} if @var{n} is specified) for a command.
The format of here-documents is:
No parameter and variable expansion, command substitution,
arithmetic expansion, or filename expansion is performed on
-@var{word}. If any part of @var{word} is quoted, the
+@var{word}.
+
+If any part of @var{word} is quoted, the
@var{delimiter} is the result of quote removal on @var{word},
and the lines in the here-document are not expanded.
If @var{word} is unquoted,
+@var{delimiter} is @var{word} itself,
all lines of the here-document are subjected to
parameter expansion, command substitution, and arithmetic expansion,
the character sequence @code{\newline} is ignored, and @samp{\}
@item cd
@btindex cd
@example
-cd [-L|[-P [-e]] [-@@] [@var{directory}]
+cd [-L|[-P [-e]]] [-@@] [@var{directory}]
@end example
Change the current working directory to @var{directory}.
the names is not a valid shell variable name, or @option{-f} is supplied
with a name that is not a shell function.
+@item false
+@btindex false
+@example
+false
+@end example
+
+Does nothing, returns a non-zero status.
+
@item getopts
@btindex getopts
@example
hash [-r] [-p @var{filename}] [-dt] [@var{name}]
@end example
-Each time @code{hash} is invoked, it remembers the full pathnames of the
+Each time @code{hash} is invoked, it remembers the full filenames of the
commands specified as @var{name} arguments,
so they need not be searched for on subsequent invocations.
The commands are found by searching through the directories listed in
@env{$PATH}.
-Any previously-remembered pathname is discarded.
+Any previously-remembered filename is discarded.
The @option{-p} option inhibits the path search, and @var{filename} is
used as the location of @var{name}.
The @option{-r} option causes the shell to forget all remembered locations.
+Assigning to the @env{PATH} variable also clears all hashed filenames.
The @option{-d} option causes the shell to forget the remembered location
of each @var{name}.
If the @option{-t} option is supplied, the full pathname to which each
using the rules listed above.
@end table
-When used with @code{test} or @samp{[}, the @samp{<} and @samp{>}
+If the shell is not in @sc{posix} mode,
+when used with @code{test} or @samp{[}, the @samp{<} and @samp{>}
operators sort lexicographically using ASCII ordering.
+If the shell is in @sc{posix} mode, these operators use the current locale.
+
+The historical operator-precedence parsing with 4 or more arguments can
+lead to ambiguities when it encounters strings that look like primaries.
+The @sc{posix} standard has deprecated the @option{-a} and @option{-o}
+primaries and enclosing expressions within parentheses.
+Scripts should no longer use them.
+It's much more reliable to restrict test invocations to a single primary,
+and to replace uses of @option{-a} and @option{-o} with the shell's
+@code{&&} and @code{||} list operators. For example, use
+
+@example
+test -n string1 && test -n string2
+@end example
+
+@noindent
+instead of
+
+@example
+test -n string1 -a -n string2
+@end example
@item times
@btindex times
The return status is zero unless a @var{sigspec} does not specify a
valid signal.
+@item true
+@btindex true
+@example
+true
+@end example
+
+Does nothing, returns a 0 status.
+
@item umask
@btindex umask
@example
@example
set [-abefhkmnptuvxBCEHPT] [-o @var{option-name}] [--] [-] [@var{argument} @dots{}]
set [+abefhkmnptuvxBCEHPT] [+o @var{option-name}] [--] [-] [@var{argument} @dots{}]
+set -o
+set +o
@end example
If no options or arguments are supplied, @code{set} displays the names
@item -o @var{option-name}
-Set the option corresponding to @var{option-name}:
+Set the option corresponding to @var{option-name}.
+If @option{-o} is supplied with no @var{option-name},
+@code{set} prints the current shell options settings.
+If @option{+o} is supplied with no @var{option-name},
+@code{set} prints a series of
+@code{set}
+commands to recreate the current option settings
+on the standard output.
+Valid option names are:
@table @code
@item allexport
The list of @code{shopt} options is:
@table @code
-@item assoc_expand_once
-If set, the shell suppresses multiple evaluation of associative array
-subscripts during arithmetic expression evaluation, while executing
+@item array_expand_once
+If set, the shell suppresses multiple evaluation of
+associative and indexed array subscripts
+during arithmetic expression evaluation, while executing
builtins that can perform variable assignments,
and while executing builtins that perform array dereferencing.
+@item assoc_expand_once
+Deprecated; a synonym for @code{array_expand_once}.
+
@item autocd
If set, a command name that is the name of a directory is executed as if
it were the argument to the @code{cd} command.
This means that dollar signs in variable names that expand to directories
will not be quoted;
however, any dollar signs appearing in filenames will not be quoted, either.
-This is active only when bash is using backslashes to quote completed
+This is active only when Bash is using backslashes to quote completed
filenames.
This variable is set by default, which is the default Bash behavior in
versions through 4.2.
@item BASH_ARGC
An array variable whose values are the number of parameters in each
-frame of the current bash execution call stack. The number of
+frame of the current Bash execution call stack. The number of
parameters to the current subroutine (shell function or script executed
with @code{.} or @code{source}) is at the top of the stack. When a
subroutine is executed, the number of parameters passed is pushed onto
Setting @code{extdebug} after the shell has started to execute a script,
or referencing this variable when @code{extdebug} is not set,
may result in inconsistent values.
+Assignments to @env{BASH_ARGC} have no effect, and it may not be unset.
@item BASH_ARGV
-An array variable containing all of the parameters in the current bash
+An array variable containing all of the parameters in the current Bash
execution call stack. The final parameter of the last subroutine call
is at the top of the stack; the first parameter of the initial call is
at the bottom. When a subroutine is executed, the parameters supplied
Setting @code{extdebug} after the shell has started to execute a script,
or referencing this variable when @code{extdebug} is not set,
may result in inconsistent values.
+Assignments to @env{BASH_ARGV} have no effect, and it may not be unset.
@item BASH_ARGV0
When referenced, this variable expands to the name of the shell or shell
@code{$@{FUNCNAME[$i]@}} was called (or @code{$@{BASH_LINENO[$i-1]@}} if
referenced within another shell function).
Use @code{LINENO} to obtain the current line number.
+Assignments to @env{BASH_LINENO} have no effect, and it may not be unset.
@item BASH_LOADABLES_PATH
A colon-separated list of directories in which the shell looks for
dynamically loadable builtins specified by the
@code{enable} command.
+@item BASH_MONOSECONDS
+Each time this variable is referenced, it expands to the value returned
+by the system's monotonic clock, if one is available.
+If there is no monotonic clock, this is equivalent to @env{EPOCHSECONDS}.
+If @env{BASH_MONOSECONDS}
+is unset, it loses its special properties, even if it is
+subsequently reset.
+
@item BASH_REMATCH
An array variable whose members are assigned by the @samp{=~} binary
operator to the @code{[[} conditional command
variable are defined.
The shell function @code{$@{FUNCNAME[$i]@}} is defined in the file
@code{$@{BASH_SOURCE[$i]@}} and called from @code{$@{BASH_SOURCE[$i+1]@}}
+Assignments to @env{BASH_SOURCE} have no effect, and it may not be unset.
@item BASH_SUBSHELL
Incremented by one within each subshell or subshell environment when
is unset, it loses its special properties, even if it is
subsequently reset.
+@item BASH_TRAPSIG
+Set to the signal number corresponding to the trap action being executed
+during its execution.
+See the description of @code{trap}
+(@pxref{Bourne Shell Builtins})
+for information about signal numbers and trap execution.
+
@item BASH_VERSINFO
A readonly array variable (@pxref{Arrays})
whose members hold version information for this instance of Bash.
The pattern matching honors the setting of the @code{extglob} shell
option.
+@item GLOBSORT
+Control how the results of filename expansion are sorted.
+The value of this variable specifies the sort criteria and sort order for
+the results of filename expansion.
+If this variable is unset or set to the null string, filename expansion
+uses the historial behavior of sorting by name.
+If set, a valid value begins with an optional @samp{+}, which is ignored,
+or @samp{-}, which reverses the sort order from ascending to descending,
+followed by a sort specifier.
+The valid sort specifiers are
+@samp{name},
+@samp{size},
+@samp{mtime},
+@samp{atime},
+@samp{ctime},
+and
+@samp{blocks},
+which sort the files on name, file size, modification time, access time,
+inode change time, and number of blocks, respectively.
+
+For example, a value of @code{-mtime} sorts the results in descending
+order by modification time (newest first).
+
+A sort specifier of @samp{nosort} disables sorting completely; the results
+are returned in the order they are read from the file system,.
+
+If the sort specifier is missing, it defaults to @var{name},
+so a value of @samp{+} is equivalent to the null string,
+and a value of @samp{-} sorts by name in descending order.
+
+Any invalid value restores the historical sorting behavior.
+
@item GROUPS
An array variable containing the list of groups of which the current
user is a member.
The optional @var{p} is a digit specifying the precision, the number of
fractional digits after a decimal point.
A value of 0 causes no decimal point or fraction to be output.
-At most three places after the decimal point may be specified; values
-of @var{p} greater than 3 are changed to 3.
+At most six places after the decimal point may be specified;
+values of @var{p} greater than 6 are changed to 6.
If @var{p} is not specified, the value 3 is used.
The optional @code{l} specifies a longer format, including minutes, of
The levels are listed in order of decreasing precedence.
@table @code
+@cindex arithmetic operators
+@cindex unary arithmetic operators
+@cindex binary arithmetic operators
+@cindex conditional arithmetic operator
+@cindex bitwise arithmetic operators
@item @var{id}++ @var{id}--
variable post-increment and post-decrement
@item ||
logical OR
-@item expr ? expr : expr
+@item expr ? if-true-expr : if-false-expr
conditional operator
@item = *= /= %= += -= <<= >>= &= ^= |=
otherwise the index of the element assigned is the last index assigned
to by the statement plus one. Indexing starts at zero.
-Each @var{value} in the list undergoes all the shell expansions
-described above (@pxref{Shell Expansions}).
+Each @var{value} in the list undergoes the shell expansions
+described above (@pxref{Shell Expansions}),
+but @var{value}s that are valid variable assignments
+including the brackets and subscript do not undergo
+brace expansion and word splitting, as with individual
+variable assignments.
When assigning to an associative array, the words in a compound assignment
may be either assignment statements, for which the subscript is required,
@node Bash POSIX Mode
-@section Bash POSIX Mode
+@section Bash and POSIX
+
+@subsection What is POSIX?
+@cindex POSIX description
+
+@sc{posix} is the name for a family of standards based on Unix.
+A number of Unix services, tools, and functions are part of the standard,
+ranging from the basic system calls and C library functions to common
+applications and tools to system administration and management.
+
+The @sc{posix} Shell and Utilities standard was originally developed by
+IEEE Working Group 1003.2 (POSIX.2).
+The first edition of the 1003.2 standard was published in 1992.
+It was merged with the original IEEE 1003.1 Working Group and is
+currently maintained by the Austin Group (a joint working group of the
+IEEE, The Open Group and ISO/IEC SC22/WG15).
+Today the Shell and Utilities are a volume within the set of documents that
+make up IEEE Std 1003.1-2017, and thus the former POSIX.2 (from 1992)
+is now part of the current unified @sc{posix} standard.
+
+The Shell and Utilities volume concentrates on the command
+interpreter interface and utility programs commonly executed from
+the command line or by other programs.
+The standard is freely available on the web at
+@url{https://pubs.opengroup.org/onlinepubs/9699919799/utilities/contents.html}.
+
+Bash is concerned with the aspects of the shell's behavior defined
+by the @sc{posix} Shell and Utilities volume. The shell command
+language has of course been standardized, including the basic flow
+control and program execution constructs, I/O redirection and
+pipelines, argument handling, variable expansion, and quoting.
+
+The @i{special} builtins, which must be implemented as part of the
+shell to provide the desired functionality, are specified as
+being part of the shell; examples of these are @code{eval} and
+@code{export}.
+Other utilities appear in the sections of POSIX not
+devoted to the shell which are commonly (and in some cases must
+be) implemented as builtin commands, such as
+@code{read} and @code{test}.
+POSIX also specifies aspects of the shell's interactive
+behavior, including job control and command
+line editing.
+Only vi-style line editing commands have been
+standardized; emacs editing commands were left out due to
+objections.
+
+@subsection Bash POSIX Mode
@cindex POSIX Mode
+Although Bash is an implementation of the @sc{posix} shell
+specification, there are areas where the Bash default behavior
+differs from the specification.
+The Bash @dfn{posix mode} changes the Bash
+behavior in these areas so that it conforms to the standard more closely.
+
Starting Bash with the @option{--posix} command-line option or executing
@samp{set -o posix} while Bash is running will cause Bash to conform more
closely to the @sc{posix} standard by changing the behavior to
That means, for example, that a backslash preceding a double quote
character will escape it and the backslash will be removed.
+@item
+The @code{test} builtin compares strings using the current locale when
+processing the @samp{<} and @samp{>} binary operators.
+
+@item
+The @code{test} builtin's @option{-t} unary primary requires an argument.
+Historical versions of @code{test} made the argument optional in certain
+cases, and bash attempts to accommodate those for backwards compatibility.
+
+@item
+Command substitutions don't set the @samp{?} special parameter. The exit
+status of a simple command without a command word is still the exit status
+of the last command substitution that occurred while evaluating the variable
+assignments and redirections in that command, but that does not happen until
+after all of the assignments and redirections.
+
@end enumerate
There is other @sc{posix} behavior that Bash does not implement by
entries if @code{FCEDIT} is unset, rather than defaulting directly to
@code{ed}. @code{fc} uses @code{ed} if @code{EDITOR} is unset.
+@item
+A non-interactive shell does not exit if a variable assignment preceding
+the @code{command} builtin or another non-special builtin fails.
+
@item
As noted above, Bash requires the @code{xpg_echo} option to be enabled for
the @code{echo} builtin to be fully conformant.
has no special effect
@end itemize
-@item compat32
-@itemize @bullet
-@item
-interrupting a command list such as "a ; b ; c" causes the execution
-of the next command in the list (in bash-4.0 and later versions,
-the shell acts as if it received the interrupt, so
-interrupting one command in a list aborts the execution of the
-entire list)
-@end itemize
-
@item compat40
@itemize @bullet
@item
If you want to build Bash in a directory separate from the source
directory -- to build for multiple architectures, for example --
just use the full path to the configure script. The following commands
-will build bash in a directory under @file{/usr/local/build} from
+will build Bash in a directory under @file{/usr/local/build} from
the source code in @file{/usr/local/src/bash-4.4}:
@example
variable when running @samp{make install}
(e.g., @samp{make install prefix=@var{PATH}}).
The @env{prefix} variable provides a default for @env{exec_prefix} and
-other variables used when installing bash.
+other variables used when installing Bash.
You can specify separate installation prefixes for
architecture-specific files and architecture-independent files.
@samp{make install exec_prefix=/} will install @code{bash} and
@code{bashbug} into @file{/bin} instead of the default @file{/usr/local/bin}.
-If you want to see the files bash will install and where it will install
+If you want to see the files Bash will install and where it will install
them without changing anything on your system, specify the variable
@env{DESTDIR} as an argument to @code{make}. Its value should be the
absolute directory path you'd like to use as the root of your sample
@table @code
@item --enable-largefile
-Enable support for @uref{http://www.unix.org/version2/whatsnew/lfs20mar.html,
+Enable support for @url{http://www.unix.org/version2/whatsnew/lfs20mar.html,
large files} if the operating system requires special compiler options
to build programs which can access large files. This is enabled by
default, if the operating system provides large file support.
builtins (@pxref{Aliases}).
@item --enable-alt-array-implementation
-This builds bash using an alternate implementation of arrays
+This builds Bash using an alternate implementation of arrays
(@pxref{Arrays}) that provides faster access at the expense of using
more memory (sometimes many times more, depending on how sparse an array is).
(@pxref{Pipelines}).
@item --enable-debugger
-Include support for the bash debugger (distributed separately).
+Include support for the Bash debugger (distributed separately).
@item --enable-dev-fd-stat-broken
If calling @code{stat} on /dev/fd/@var{N} returns different results than
Once you have determined that a bug actually exists, use the
@code{bashbug} command to submit a bug report or use the form at the
-<a href="https://savannah.gnu.org/projects/bash/">Bash project page</a>.
+@uref{https://savannah.gnu.org/projects/bash/,Bash project page}.
If you have a fix, you are encouraged to submit that as well!
Suggestions and `philosophical' bug reports may be mailed
to @email{bug-bash@@gnu.org} or @email{help-bash@@gnu.org}.
projects / thirdparty / bash.git / blobdiff