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# Bash 4 - a rough overview
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:V4:
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\<note\>Attention: Since Bash 4 has been around for quite some time now
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(4.3 will come soon), I consider it to be \"standard\". This page is not
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maintained anymore and is left here to keep your links working. See the
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[bashchanges](/scripting/bashchanges) page for new stuff
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introduced.\</note\>
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Besides many bugfixes since Bash 3.2, Bash 4 will bring some interesting
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new features for shell users and scripters. See also
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[bashchanges](/scripting/bashchanges) for a small general overview with
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more details.
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Not all of the changes and news are included here, just the biggest or
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most interesting ones. The changes to completion, and the readline
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component are not covered. **Though, if you\'re familiar with these
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parts of Bash (and Bash 4), feel free to write a chapter here.**
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The complete list of fixes and changes is in the CHANGES or NEWS file of
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your Bash 4 distribution.
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\<WRAP center round download 60%\> The current available **stable**
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version is 4.4.18 release (February 03, 2018):
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- <ftp://ftp.cwru.edu/pub/bash/bash-4.4.18.tar.gz>
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- <ftp://ftp.gnu.org/pub/gnu/bash/bash-4.4.18.tar.gz>
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\</WRAP\>
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## New or changed commands and keywords
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### The new \"coproc\" keyword
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Bash 4 introduces the concepts of coprocesses, a well known feature of
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other shells. The basic concept is simple: It will start any command in
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the background and set up an array that is populated with accessible
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files that represent the filedescriptors of the started process.
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In other words: It lets you start a process in background and
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communicate with its input and output data streams.
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See [The coproc keyword](/syntax/keywords/coproc)
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### The new \"mapfile\" builtin
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The `mapfile` builtin is able to map the lines of a file directly into
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an array. This avoids having to fill an array yourself using a loop. It
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enables you to define the range of lines to read, and optionally call a
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callback, for example to display a progress bar.
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See: [mapfile](/commands/builtin/mapfile)
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### Changes to the \"case\" keyword
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The `case` construct understands two new action list terminators:
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The `;&` terminator causes execution to continue with the next action
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list (rather than terminate the `case` construct).
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The `;;&` terminator causes the `case` construct to test the next given
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pattern instead of terminating the whole execution.
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See [case](/syntax/ccmd/case)
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### Changes to the \"declare\" builtin
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The `-p` option now prints all attributes and values of declared
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variables (or functions, when used with `-f`). The output is fully
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re-usable as input.
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The new option `-l` declares a variable in a way that the content is
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converted to lowercase on assignment. For uppercase, the same applies to
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`-u`. The option `-c` causes the content to be capitalized before
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assignment.
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`declare -A` declares associative arrays (see below).
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### Changes to the \"read\" builtin
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The `read` builtin command has some interesting new features.
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The `-t` option to specify a timeout value has been slightly tuned. It
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now accepts fractional values and the special value 0 (zero). When
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`-t 0` is specified, `read` immediately returns with an exit status
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indicating if there\'s data waiting or not. However, when a timeout is
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given, and the `read` builtin times out, any partial data recieved up to
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the timeout is stored in the given variable, rather than lost. When a
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timeout is hit, `read` exits with a code greater than 128.
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A new option, `-i`, was introduced to be able to preload the input
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buffer with some text (when Readline is used, with `-e`). The user is
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able to change the text, or press return to accept it.
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See [read](/commands/builtin/read)
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### Changes to the \"help\" builtin
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The builtin itself didn\'t change much, but the data displayed is more
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structured now. The help texts are in a better format, much easier to
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read.
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There are two new options: `-d` displays the summary of a help text,
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`-m` displays a manpage-like format.
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### Changes to the \"ulimit\" builtin
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Besides the use of the 512 bytes blocksize everywhere in POSIX mode,
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`ulimit` supports two new limits: `-b` for max socket buffer size and
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`-T` for max number of threads.
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## Expansions
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### Brace Expansion
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The brace expansion was tuned to provide expansion results with leading
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zeros when requesting a row of numbers.
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See [brace](/syntax/expansion/brace)
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### Parameter Expansion
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Methods to modify the case on expansion time have been added.
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On expansion time you can modify the syntax by adding operators to the
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parameter name.
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See [Case modification on parameter
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expansion](/syntax/pe#case_modification)
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### Substring expansion
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When using substring expansion on the positional parameters, a starting
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index of 0 now causes \$0 to be prepended to the list (if the positional
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parameters are used). Before, this expansion started with \$1:
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# this should display $0 on Bash v4, $1 on Bash v3
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echo ${@:0:1}
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### Globbing
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There\'s a new shell option `globstar`. When enabled, Bash will perform
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recursive globbing on `**` \-- this means it matches all directories and
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files from the current position in the filesystem, rather than only the
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current level.
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The new shell option `dirspell` enables spelling corrections on
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directory names during globbing.
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See [globs](/syntax/expansion/globs)
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## Associative Arrays
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Besides the classic method of integer indexed arrays, Bash 4 supports
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associative arrays.
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An associative array is an array indexed by an arbitrary string,
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something like
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declare -A ASSOC
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ASSOC[First]="first element"
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ASSOC[Hello]="second element"
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ASSOC[Peter Pan]="A weird guy"
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See [arrays](/syntax/arrays)
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## Redirection
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There is a new `&>>` redirection operator, which appends the standard
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output and standard error to the named file. This is the same as the
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good old `>>FILE 2>&1` notation.
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The parser now understands `|&` as a synonym for `2>&1 |`, which
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redirects the standard error for a command through a pipe.
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See [redirection](/syntax/redirection)
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## Interesting new shell variables
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Variable Description
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---------------------------------------------------- --------------------------------------------------------------------------------
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[BASHPID](/syntax/shellvars#BASHPID) contains the PID of the current shell (this is different than what `$$` does!)
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[PROMPT_DIRTRIM](/syntax/shellvars#PROMPT_DIRTRIM) specifies the max. level of unshortened pathname elements in the prompt
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[FUNCNEST](/syntax/shellvars#FUNCNEST) control the maximum number of shell function recursions
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See [shellvars](/syntax/shellvars)
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## Interesting new Shell Options
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The mentioned shell options are **off by default** unless otherwise
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mentioned.
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Option Description
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------------- --------------------------------------------------------------------------------------------------------
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`checkjobs` check for and report any running jobs at shell exit
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`compat*` set compatiblity modes for older shell versions (influences regular expression matching in `[[ ... ]]`
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`dirspell` enables spelling corrections on directory names during globbing
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`globstar` enables recursive globbing with `**`
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`lastpipe` (4.2) to execute the last command in a pipeline in the current environment
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See [shell_options](/internals/shell_options)
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## Misc
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- If a command is not found, the shell attempts to execute a shell
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function named `command_not_found_handle`, supplying the command
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words as the function arguments. This can be used to display
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userfriendly messages or perform different command searches
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- The behaviour of the `set -e` (`errexit`) mode was changed, it now
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acts more intuitive (and is better documented in the manpage).
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- The output target for the `xtrace` (`set -x`/`set +x`) feature is
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configurable **since Bash 4.1** (previously, it was fixed to
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`stderr`): a variable named
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[BASH_XTRACEFD](/syntax/shellvars#BASH_XTRACEFD) can be set to the
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filedescriptor that should get the output
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- Bash 4.1 is able to log the history to syslog (only to be enabled at
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compile time in `config-top.h`)
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