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docs/bash4.md
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docs/bash4.md
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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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!!! warning "Attention"
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Since Bash 4 has been around for quite some time now (4.3 will come soon), I consider it to be "standard".
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This page is not maintained anymore and is left here to keep your links working.
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See the [bashchanges](/scripting/bashchanges) page for new stuff introduced.
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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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@ -15,23 +12,19 @@ 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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component are not covered. **Though, if you're familiar with these 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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The current available **stable** 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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- <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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## New or changed commands and keywords
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### The new \"coproc\" keyword
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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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@ -43,7 +36,7 @@ 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 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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@ -52,7 +45,7 @@ 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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### Changes to the "case" keyword
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The `case` construct understands two new action list terminators:
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@ -64,7 +57,7 @@ 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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### 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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@ -77,7 +70,7 @@ 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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### Changes to the "read" builtin
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The `read` builtin command has some interesting new features.
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@ -95,7 +88,7 @@ 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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### 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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@ -104,7 +97,7 @@ 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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### 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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@ -135,13 +128,15 @@ 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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```bash
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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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```
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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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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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@ -158,11 +153,13 @@ 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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```bash
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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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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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```
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See [arrays](/syntax/arrays)
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@ -179,11 +176,11 @@ 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 `$$` does!)
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[PROMPT_DIRTRIM](/syntax/shellvars#PROMPT_DIRTRIM) specifies the max. level of unshortened pathname elements in the prompt
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[FUNCNEST](/syntax/shellvars#FUNCNEST) control the maximum number of shell function recursions
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| Variable | Description |
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| -------------------------------------------------- | ------------------------------------------------------------------------------- |
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| [BASHPID](/syntax/shellvars#BASHPID) | contains the PID of the current shell (this is different than what `$$` 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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@ -192,28 +189,28 @@ See [shellvars](/syntax/shellvars)
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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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| 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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- 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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