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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 (4.3 will come soon), I consider it to be "standard". This page is not maintained anymore and is left here to keep your links working. See the [[scripting:bashchanges]] page for new stuff introduced.</note>
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Besides many bugfixes since Bash 3.2, Bash 4 will bring some interesting new
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features for shell users and scripters. See also [[scripting:bashchanges]] for a small general overview with 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 component
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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 your
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Bash 4 distribution.
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<WRAP center round download 60%>
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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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===== 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 other
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shells. The basic concept is simple: It will start any command in the background
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|
and set up an array that is populated with accessible files that represent the
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|
filedescriptors of the started process.
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In other words: It lets you start a process in background and communicate with
|
|
its input and output data streams.
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See [[syntax:keywords:coproc | The coproc keyword]]
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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 an
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array. This avoids having to fill an array yourself using a loop. It enables you to define
|
|
the range of lines to read, and optionally call a callback, for example to
|
|
display a progress bar.
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See: [[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 list
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(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 [[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 variables (or
|
|
functions, when used with ''-f''). The output is fully re-usable as input.
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|
The new option ''-l'' declares a variable in a way that the content is
|
|
converted to lowercase on assignment. For uppercase, the same applies to
|
|
''-u''. The option ''-c'' causes the content to be capitalized before
|
|
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 now
|
|
accepts fractional values and the special value 0 (zero). When ''-t 0'' is
|
|
specified, ''read'' immediately returns with an exit status indicating if
|
|
there's data waiting or not. However, when a timeout is given, and the ''read''
|
|
builtin times out, any partial data recieved up to the timeout is stored in the
|
|
given variable, rather than lost. When a timeout is hit, ''read'' exits with a
|
|
code greater than 128.
|
|
|
|
A new option, ''-i'', was introduced to be able to preload the input buffer with
|
|
some text (when Readline is used, with ''-e''). The user is able to change the
|
|
text, or press return to accept it.
|
|
|
|
See [[commands:builtin:read]]
|
|
|
|
==== Changes to the "help" builtin ====
|
|
|
|
The builtin itself didn't change much, but the data displayed is more structured
|
|
now. The help texts are in a better format, much easier to read.
|
|
|
|
There are two new options: ''-d'' displays the summary of a help text, ''-m''
|
|
displays a manpage-like format.
|
|
|
|
==== Changes to the "ulimit" builtin ====
|
|
|
|
Besides the use of the 512 bytes blocksize everywhere in POSIX mode, ''ulimit''
|
|
supports two new limits: ''-b'' for max socket buffer size and ''-T'' for max
|
|
number of threads.
|
|
|
|
===== Expansions =====
|
|
|
|
==== Brace Expansion ====
|
|
|
|
The brace expansion was tuned to provide expansion results with leading zeros
|
|
when requesting a row of numbers.
|
|
|
|
See [[syntax:expansion:brace]]
|
|
|
|
==== Parameter Expansion ===
|
|
|
|
Methods to modify the case on expansion time have been added.
|
|
|
|
On expansion time you can modify the syntax by adding operators to the parameter
|
|
name.
|
|
|
|
See [[syntax:pe#case_modification | Case modification on parameter expansion]]
|
|
|
|
==== Substring expansion ====
|
|
When using substring expansion on the positional parameters, a starting
|
|
index of 0 now causes $0 to be prepended to the list (if the positional parameters are used). Before, this expansion
|
|
started with $1:
|
|
<code>
|
|
# this should display $0 on Bash v4, $1 on Bash v3
|
|
echo ${@:0:1}
|
|
</code>
|
|
|
|
==== Globbing ====
|
|
|
|
There's a new shell option ''[[internals:shell_options#globstar | globstar]]''. When enabled, Bash will perform
|
|
recursive globbing on ''<nowiki>**</nowiki>'' -- this means it matches all directories and files
|
|
from the current position in the filesystem, rather than only the current level.
|
|
|
|
The new shell option ''[[internals:shell_options#dirspell | dirspell]]'' enables spelling corrections on directory
|
|
names during globbing.
|
|
|
|
See [[syntax:expansion:globs]]
|
|
|
|
===== Associative Arrays =====
|
|
|
|
Besides the classic method of integer indexed arrays, Bash 4 supports associative arrays.
|
|
|
|
An associative array is an array indexed by an arbitrary string, something like
|
|
<code>
|
|
declare -A ASSOC
|
|
|
|
ASSOC[First]="first element"
|
|
ASSOC[Hello]="second element"
|
|
ASSOC[Peter Pan]="A weird guy"
|
|
</code>
|
|
|
|
See [[syntax:arrays]]
|
|
|
|
===== Redirection =====
|
|
|
|
There is a new ''<nowiki>&>></nowiki>'' redirection operator, which appends the standard output
|
|
and standard error to the named file. This is the same as the good old ''<nowiki>>>FILE 2>&1</nowiki>''
|
|
notation.
|
|
|
|
The parser now understands ''|&'' as a synonym for ''2>&1 |'', which redirects
|
|
the standard error for a command through a pipe.
|
|
|
|
See [[syntax:redirection]]
|
|
|
|
===== Interesting new shell variables =====
|
|
^Variable ^Description ^
|
|
|[[syntax:shellvars#BASHPID|BASHPID]] | contains the PID of the current shell (this is different than what ''$$'' does!) |
|
|
|[[syntax:shellvars#PROMPT_DIRTRIM|PROMPT_DIRTRIM]] | specifies the max. level of unshortened pathname elements in the prompt |
|
|
|[[syntax:shellvars#FUNCNEST|FUNCNEST]] | control the maximum number of shell function recursions |
|
|
|
|
See [[syntax:shellvars]]
|
|
|
|
===== Interesting new Shell Options =====
|
|
|
|
The mentioned shell options are **off by default** unless otherwise mentioned.
|
|
|
|
^Option ^Description ^
|
|
|''[[internals:shell_options#checkjobs | checkjobs]]'' | check for and report any running jobs at shell exit |
|
|
|''[[internals:shell_options#compat31 | compat*]]'' | set compatiblity modes for older shell versions (influences regular expression matching in ''<nowiki>[[ ... ]]</nowiki>'' |
|
|
|''[[internals:shell_options#dirspell | dirspell]]'' | enables spelling corrections on directory names during globbing |
|
|
|''[[internals:shell_options#globstar | globstar]]'' | enables recursive globbing with ''<nowiki>**</nowiki>'' |
|
|
|''[[internals:shell_options#lastpipe | lastpipe]]'' | (4.2) to execute the last command in a pipeline in the current environment |
|
|
|
|
|
|
See [[internals:shell_options]]
|
|
|
|
===== Misc =====
|
|
|
|
* If a command is not found, the shell attempts to execute a shell function named ''command_not_found_handle'', supplying the command words as the function arguments. This can be used to display userfriendly messages or perform different command searches
|
|
* The behaviour of the ''set -e'' (''errexit'') mode was changed, it now acts more intuitive (and is better documented in the manpage).
|
|
* The output target for the ''xtrace'' (''set -x''/''set +x'') feature is configurable **since Bash 4.1** (previously, it was fixed to ''stderr''): a variable named [[syntax:shellvars#BASH_XTRACEFD|BASH_XTRACEFD]] can be set to the filedescriptor that should get the output
|
|
* Bash 4.1 is able to log the history to syslog (only to be enabled at compile time in ''config-top.h'')
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<span class="docInfo">
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<ul class="list-inline"><li><span class="iconify text-muted" data-icon="mdi:file-document-outline"></span> <span title="bash4.txt">bash4.txt</span></li><li><span class="iconify text-muted" data-icon="mdi:calendar"></span> Last modified: <span title="2018/02/03 07:52">2018/02/03 07:52</span></li><li class="text-muted">by <bdi>narutowindy</bdi></li></ul> </span>
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