The GNU project (GNU's Not UNIX) provides tools for UNIX-like system administration which are free software and comply to UNIX standards.
Bash is an sh-compatible shell that incorporates useful features from the Korn shell (ksh) and C shell (csh). It is intended to conform to the IEEE POSIX P1003.2/ISO 9945.2 Shell and Tools standard. It offers functional improvements over sh for both programming and interactive use; these include command line editing, unlimited size command history, job control, shell functions and aliases, indexed arrays of unlimited size, and integer arithmetic in any base from two to sixty-four. Bash can run most sh scripts without modification.
Like the other GNU projects, the bash initiative was started to preserve, protect and promote the freedom to use, study, copy, modify and redistribute software. It is generally known that such conditions stimulate creativity. This was also the case with the bash program, which has a lot of extra features that other shells can't offer.
In addition to the single-character shell command line options which can generally be configured using the set shell built-in command, there are several multi-character options that you can use. We will come across a couple of the more popular options in this and the following chapters; the complete list can be found in the Bash info pages,-> .
Interactive means you can enter commands. The shell is not running because a script has been activated. A login shell means that you got the shell after authenticating to the system, usually by giving your user name and password.
Error messages are printed if configuration files exist but are not readable. If a file does not exist, bash searches for the next.
A non-login shell means that you did not have to authenticate to the system. For instance, when you open a terminal using an icon, or a menu item, that is a non-login shell.
This file is usually referred to in ~/.bash_profile:
if [ -f ~/.bashrc ]; then . ~/.bashrc; fi
See for more information on the if construct.
All scripts use non-interactive shells. They are programmed to do certain tasks and cannot be instructed to do other jobs than those for which they are programmed.
PATH is not used to search for this file, so if you want to use it, best refer to it by giving the full path and file name.
Bash tries to behave as the historical Bourne sh program while conforming to the POSIX standard as well.
When invoked interactively, the ENV variable can point to extra startup information.
This option is enabled either using the set built-in:
set -o posix
or by calling the bash program with the --posix option. Bash will then try to behave as compliant as possible to the POSIX standard for shells. Setting the POSIXLY_CORRECT variable does the same.
Files read when invoked by rshd:
An interactive shell generally reads from, and writes to, a user's terminal: input and output are connected to a terminal. Bash interactive behavior is started when the bash command is called upon without non-option arguments, except when the option is a string to read from or when the shell is invoked to read from standard input, which allows for positional parameters to be set (see ).
Test by looking at the content of the special parameter -, it contains an 'i' when the shell is interactive:
eddy:~> echo $- himBH
In non-interactive shells, the prompt, PS1, is unset.
Differences in interactive mode:
Conditional expressions are used by the [[ compound command and by the test and [ built-in commands.
Expressions may be unary or binary. Unary expressions are often used to examine the status of a file. You only need one object, for instance a file, to do the operation on.
There are string operators and numeric comparison operators as well; these are binary operators, requiring two objects to do the operation on. If the FILE argument to one of the primaries is in the form /dev/fd/N, then file descriptor N is checked. If the FILE argument to one of the primaries is one of /dev/stdin, /dev/stdout or /dev/stderr, then file descriptor 0, 1 or 2 respectively is checked.
Conditionals are discussed in detail in .
More information about the file descriptors in .
The shell allows arithmetic expressions to be evaluated, as one of the shell expansions or by the let built-in.
Evaluation is done in fixed-width integers with no check for overflow, though division by 0 is trapped and flagged as an error. The operators and their precedence and associativity are the same as in the C language, see .
Aliases allow a string to be substituted for a word when it is used as the first word of a simple command. The shell maintains a list of aliases that may be set and unset with the alias and unalias commands.
Bash always reads at least one complete line of input before executing any of the commands on that line. Aliases are expanded when a command is read, not when it is executed. Therefore, an alias definition appearing on the same line as another command does not take effect until the next line of input is read. The commands following the alias definition on that line are not affected by the new alias.
Aliases are expanded when a function definition is read, not when the function is executed, because a function definition is itself a compound command. As a consequence, aliases defined in a function are not available until after that function is executed.
We will discuss aliases in detail in .
Bash provides one-dimensional array variables. Any variable may be used as an array; the declare built-in will explicitly declare an array. There is no maximum limit on the size of an array, nor any requirement that members be indexed or assigned contiguously. Arrays are zero-based. See .
The directory stack is a list of recently-visited directories. The pushd built-in adds directories to the stack as it changes the current directory, and the popd built-in removes specified directories from the stack and changes the current directory to the directory removed.
Content can be displayed issuing the dirs command or by checking the content of the DIRSTACK variable.
More information about the workings of this mechanism can be found in the Bash info pages.
Bash makes playing with the prompt even more fun. See the section Controlling the Prompt in the Bash info pages.
When invoked as rbash or with the --restricted or -r option, the following happens:
When a command that is found to be a shell script is executed, rbash turns off any restrictions in the shell spawned to execute the script.
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