Editing Bash (Unix shell) (section)

=== List of Short Descriptions ===
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As a [[Command-line interface|command processor]], Bash can operate in two modes: an interactive and non-interactive mode. In the interactive mode a [[Terminal emulator|text window]] where users input commands to execute various tasks. The second mode, is execution of commands from files, known as [[shell script]]s, facilitating [[automation]]. In keeping with [[Unix shell]] conventions, Bash incorporates a rich set of features, including:

* A [https://www.gnu.org/software/bash/manual/index.html User Manual] provided by the GNU Project, also available at <code>info bash</code>, and a technical manual available at <code>man bash</code>.
* [[Function (computer programming)|Invocation]] as a...
** [[Interactivity|Interactive]] shell,
** Non-interactive shell, or
** Login shell;
* A [[command-line interface]];
* [[Exit status]] codes;
* [[Control flow|Control structures]] for 
** [[Conditional (computer programming)|Condition testing]],
*** <code>if</code>, <code>case</code>, <code>select</code>,
*** logical AND (<code>&&</code>) and OR (<code>||</code>), and
** [[Iteration]]:
*** <code>for</code>, <code>while</code>, <code>until</code> loops, and
*** Arithmetic C-style loop: <code>for ((</code>;
* Syntaxes for Boolean testing of file attributes, string and integer values, etc.: 
** Traditional <code>test</code> command,
** Traditional single bracket test: <code>[</code>, 
** Modern double bracket test: <code> &#91;&#91; ... &#93;&#93;</code>, which includes advanced features:
*** Extended [[regular expression]] and extglob matching
*** Lexicographic sorting with <code><</code> and <code>></code>;
* [[Unix|UNIX]]-style [[Pipeline (Unix)|pipelines]]: <code>|</code>;
* [[Child process|Subshells]]: <code>$( ... )</code>;
* [[Signal (IPC)|Signaling]] as a means of [[inter-process communication]] using the <code>trap</code> builtin;
* Asynchronous execution, i.e., [[Job (computing)|Jobs]] and [[Job control (Unix)|job control]]: 
** <code>job_spec &</code> where <code>job_spec</code> can be one of:
*** A full commandline: <code><command_name> <options> <operands> <arguments> &</code>, or
*** A job control identifier as denoted by a leading percent symbol: <code>%1 &</code>;
* A shell [[Porting|portability]] mode where command lines can be interpreted in conformance with the [[POSIX terminal interface|POSIX]] standard;
* Command parsing:
** Comments are ignored:
*** Bourne-style <code>#</code> hashtag comments, and
*** Thompson-style <code>:</code> colon comments;
** Commands are parsed one line at a time:
*** Control structures are honored, and
*** Backslash <code>\</code> escapes are also honored at the ends of lines;
** Split into words (i.e., [[Text segmentation|word splitting]]) according to [[String literal|quoting]] rules, 
*** Including ANSI-C quoting <code>$'...'</code>;
** Seven kinds of expansions are performed in the following order on the resulting string:
*** (Step 1) Brace expansion <code>kernel{-headers}</code>,
*** (Step 2) Tilde expansion <code>~</code>,
*** (Step 3) In a left-to-right fashion: 
**** [[Parameter (computer programming)|Parameter]] and [[Variable (computer science)|variable]] expansion <code>$foo</code> or <code>${bar}</code>, including
***** [[Scope (computer science)#Dynamic scoping|Dynamically scoped]] variables, 
***** Indexed [[Array (data type)|arrays]] of unlimited size, 
***** [[Associative array]]s via <code>declare -A</code>, and 
***** Expansion syntaxes which can perform some tasks more quickly than external utilities, including, among others:
****** Pattern Substitution
******* <code>${foo//x/y}</code> for <code>sed 's/x/y/g'</code>,
****** Remove Matching Prefix or Suffix Pattern
******* <code>${bar##[a-zA-Z0-9]*}</code> for <code>cut -c8-</code>,
****** Print Array Keys
******* <code>${!array[@]}</code>, and
****** Display Error if Null or Unset
******* <code>${var:?error message}</code>,
**** [[Command substitution]]: <code>$( ... )</code>,
**** [[Process substitution]], <code><()</code> or <code>>()</code>, when a system supports it:
**** Arithmetic expansion, <code>(( ... ))</code> or <code>$(( ... ))</code>, including
***** Integer [[arithmetic]] in any [[Radix|base]] from two to sixty-four, although
***** [[Floating-point arithmetic]] is not available from within the shell itself (for this functionality, see current versions of [[Bc (programming language)|<code>bc</code>]] and [[AWK|<code>awk</code>]], among others),
*** (Step 4) [[Text segmentation|Word splitting]] (again), 
*** (Step 5) Pathname expansion, i.e., shell-style [[Glob (programming)|globbing]] and [[pattern matching]] using <code>*</code>, <code>?</code>, <code>[...]</code>, and
**** (Although they can be used in conjunction, the use of brackets in pattern matching, <code>[...]</code>, and the use of brackets in the testing commands, <code>[</code> and <code> &#91;&#91; ... &#93;&#93;</code>, are each one different things.)
*** [[String literal|Quote]] removal;
** [[Redirection (computing)|Redirections]] of Standard Input, Standard Output and Standard Error [[Standard streams|data streams]] are performed, including
*** File writing, <code>></code>, and appending, <code>>></code>,
*** [[Here document]]s, <code><<</code>,
*** Here strings, <code><<<</code>, which allow parameters to be used as input, and
*** A redirection operator, <code>>|</code>, which can force overwriting of a file when a shell's "noclobber" setting is enabled; 
** Command name lookup is performed, in the following order:
*** [[Command (computing)|Commands]] internal to the [[Shell (computing)|shell]]: 
**** [[Alias (command)|Shell aliases]], 
**** Shell reserved words,
**** [[Function (computer programming)|Shell functions]], and
**** [[Shell builtin|Shell built-in commands]];
*** Commands external to the shell: 
**** Separate [[Unix|UNIX]]-style programs such as [[ls|<code>ls</code>]] or [[Ln (Unix)|<code>ln</code>]], and
**** [[Shell script]]s, which are [[File system|files]] containing executable commands. (Shell scripts do not require compilation before execution and, when certain requirements are met, can be invoked as commands by using their filename.)
** The resulting string is executed as a command.

Bash also offers...

* Configurable execution environment(s):<ref>{{Cite web |title=Command Execution Environment (Bash Reference Manual)     |url=https://www.gnu.org/software/bash/manual/html_node/Command-Execution-Environment.html     |website=www.gnu.org}}/</ref>
** Shell and [[Session (computer science)|session]] startup files such as <code>~/.bashrc</code> and <code>~/.profile</code> (i.e., [[Hidden file and hidden directory|dotfiles]]);
** Settings (<code>set</code> [[Shell builtin|built-in]]) and shell options (<code>shopt</code> [[Shell builtin|built-in]]) which alter shell behavior;
* Support for [[Unicode]];
* With interactive invocation only,
** Unlimited size [[command history]], 
** A directory stack (see <code>pushd</code> and <code>popd</code> built-ins),
** [[Command-line completion|Tab completion]],
** Configurable [[Command-line interface#Command prompt|prompts]], and
** Command line editing with [[GNU Readline|GNU readline]];
* Lightweight [[Logging (computing)|logging]] for [[debugging]] purposes (xtrace), and other lightweight debugging options (errexit, noexec, nounset, pipefail, etc.);
* Shell [[compatibility mode]]s: bash 5.1 can operate as if it were bash 4.2, etc.;
* Various Built-In Commands:
** <code>cd</code>
** <code>pwd</code>
* Documentation:
** A built-in <code>help</code> command.
** A [[man page]], and 
** An [[Info (Unix)|info page]] which is the same as the GNU manual;
* Informal avenues of support via:
** IRC at libera.chat #bash 
** Mailing lists at [https://www.gnu.org/software/bash/ Bash - GNU Project - Free Software Foundation]