Documentation forBashis available online, asis documentation for most GNU software. You mayalso find information aboutBashby running info bashorman bash, or by looking at/usr/share/doc/bash/, /usr/local/share/doc/bash/,or similar directories on your system. A brief summary is available byrunning bash --help.
Bash handles several filenames specially when they are used inredirections, as described in the following table.If the operating system on which Bash is running provides thesespecial files, bash will use them; otherwise it will emulate theminternally with the behavior described below.
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If set, Bashquotes all shell metacharacters in filenames and directory names whenperforming completion.If not set, Bashremoves metacharacters such as the dollar sign from the set ofcharacters that will be quoted in completed filenameswhen these metacharacters appear in shell variable references in words to becompleted.This means that dollar signs in variable names that expand to directorieswill 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 completedfilenames.This variable is set by default, which is the default Bash behavior inversions through 4.2.
An array variable whose values are the number of parameters in eachframe of the current bash execution call stack. The number ofparameters to the current subroutine (shell function or script executedwith . or source) is at the top of the stack. When asubroutine is executed, the number of parameters passed is pushed ontoBASH_ARGC.The shell sets BASH_ARGC only when in extended debugging mode(see The Shopt Builtinfor a description of the extdebug option to the shoptbuiltin).Setting extdebug after the shell has started to execute a script,or referencing this variable when extdebug is not set,may result in inconsistent values.
An array variable containing all of the parameters in the current bashexecution call stack. The final parameter of the last subroutine callis at the top of the stack; the first parameter of the initial call isat the bottom. When a subroutine is executed, the parameters suppliedare pushed onto BASH_ARGV.The shell sets BASH_ARGV only when in extended debugging mode(see The Shopt Builtinfor a description of the extdebug option to the shoptbuiltin).Setting extdebug after the shell has started to execute a script,or referencing this variable when extdebug is not set,may result in inconsistent values.
When Bash is invoked as an interactive login shell, or as anon-interactive shell with the --login option, it first reads andexecutes commands from the file /etc/profile, if that file exists.After reading that file, it looks for ~/.bash_profile,~/.bash_login, and ~/.profile, in that order, and readsand executes commands from the first one that exists and is readable.The --noprofile option may be used when the shell is started toinhibit this behavior.
When an interactive shell that is not a login shell is started, Bashreads and executes commands from ~/.bashrc, if that file exists.This may be inhibited by using the --norc option.The --rcfile file option will force Bash to read andexecute commands from file instead of ~/.bashrc.
Bash attempts to determine when it is being run with its standard inputconnected to a network connection, as when executed bythe historical remote shell daemon, usually rshd,or the secure shell daemon sshd.If Bashdetermines it is being run non-interactively in this fashion,it reads and executes commands from ~/.bashrc, if thatfile exists and is readable.It will not do this if invoked as sh.The --norc option may be used to inhibit this behavior, and the--rcfile option may be used to force another file to be read, butneither rshd nor sshd generally invoke the shell with thoseoptions or allow them to be specified.
Referencing an array variable without a subscript is equivalent toreferencing with a subscript of 0.Any reference to a variable using a valid subscript is legal, andbash will create an array if necessary.
If Bash is started with the name rbash, or the--restrictedor-roption is supplied at invocation, the shell becomes restricted.A restricted shell is used toset up an environment more controlled than the standard shell.A restricted shell behaves identically to bashwith the exception that the following are disallowed or not performed:
This section does not mention behavior that is standard for a particularversion (e.g., setting compat32 means that quoting the rhs of the regexpmatching operator quotes special regexp characters in the word, which isdefault behavior in bash-3.2 and subsequent versions).
If a user enables, say, compat32, it may affect the behavior of othercompatibility levels up to and including the current compatibility level.The idea is that each compatibility level controls behavior that changedin that version of Bash,but that behavior may have been present in earlier versions.For instance, the change to use locale-based comparisons with the [[command came in bash-4.1, and earlier versions used ASCII-based comparisons,so enabling compat32 will enable ASCII-based comparisons as well.That granularity may not be sufficient forall uses, and as a result users should employ compatibility levels carefully.Read the documentation for a particular feature to find out thecurrent behavior.
The following table describes the behavior changes controlled by eachcompatibility level setting.The compatNN tag is used as shorthand for setting thecompatibility levelto NN using one of the following mechanisms.For versions prior to bash-5.0, the compatibility level may be set usingthe corresponding compatNN shopt option.For bash-4.3 and later versions, the BASH_COMPAT variable is preferred,and it is required for bash-5.1 and later versions.
When the shell starts up, the history is initialized from thefile named by the HISTFILE variable (default ~/.bash_history).The file named by the value of HISTFILE is truncated, ifnecessary, to contain no more than the number of lines specified bythe value of the HISTFILESIZE variable.When a shell with history enabled exits, the last$HISTSIZE lines are copied from the history list to the filenamed by $HISTFILE.If the histappend shell option is set (see Bash Builtin Commands),the lines are appended to the history file,otherwise the history file is overwritten.If HISTFILEis unset, or if the history file is unwritable, the history is not saved.After saving the history, the history file is truncatedto contain no more than $HISTFILESIZE lines.If HISTFILESIZE is unset, or set to null, a non-numeric value, ora numeric value less than zero, the history file is not truncated.
If you need to do unusual things to compile Bash, pleasetry to figure out how configure could check whether or notto do them, and mail diffs or instructions tobash-maintainers@gnu.org so they can beconsidered for the next release.
will install bash into /fs1/bash-install/usr/local/bin/bash,the documentation into directories within/fs1/bash-install/usr/local/share, the example loadable builtins into/fs1/bash-install/usr/local/lib/bash, and so on.You can use the usual exec_prefix and prefix variables to alterthe directory paths beneath the value of DESTDIR.
This builds bash using an alternate implementation of arrays(see Arrays) that provides faster access at the expense of usingmore memory (sometimes many times more, depending on how sparse an array is).
When Bash starts, it executes the commands in a variety of dot files. Unlike Bash shell scripts, dot files do not typically have execute permission enabled nor an interpreter directive like #!/bin/bash.
The skeleton ~/.bash_profile below is compatible with the Bourne shell and gives semantics similar to csh for the ~/.bashrc and ~/.bash_login. The [ -r filename ] && cmd is a short-circuit evaluation that tests if filename exists and is readable, skipping the part after the && if it is not.
Invoking Bash with the --posix option or stating set -o posix in a script causes Bash to conform very closely to the POSIX 1003.2 standard.[57] Bash shell scripts intended for portability should take into account at least the POSIX shell standard. Some bash features not found in POSIX are:[57][58]
An external command called bashbug reports Bash shell bugs. When the command is invoked, it brings up the user's default editor with a form to fill in. The form is mailed to the Bash maintainers (or optionally to other email addresses).[65][66]
So I'm filling out an application for a new job and you get asked for experience of different programming languages and then there is this box at the bottom other experiences - I started typing python 2.7, powershell, bas... Wait! bash isn't a programming language - it's a console that can execute shell scripts... so... eh.... oh my god I have no idea!
According to man bash, Bash is a "sh-compatible command language". Then, we can say a "command language" is "a programming language through which a user communicates with the operating system or an application".
The problem with Bash, shells in general is, that it lacks a lot of base functionality, thus when writing scripts for them, you often in fact call external programs to perform the desired work. But that's only taking a shortcut. E.g. if you'd need floating point functionality in a shell, you could actually implement it. It would be possible to write a full IEEE 754 standard implementation in everything that is Turing-complete. In practice such an implementation would be huge, require tons of memory and be horribly slow, so one better calls bc for that. But even implementing bc entirely in bash would be possible.
Here's a bash script I've once written that draws a Mandelbrot set to console. You better be prepared to get some cups of coffee if you want to see the final result, it's going to be a very long night:
About Turing consideration, yes, you could... I personally wrote a lot of libraries around bash (around monitoring, backups, sysadmin, networking, etc.), but clearly for writing a program,you have to use a real programming language. e24fc04721
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