Bash Concepts

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Metacharacters are characters that, when unquoted, separate words. These characters have a special meaning to bash and are interpreted and possibly modified before the modified strings are sent to the command as arguments. Also see regular expressions metacharacters. For more details on how bash processes a command line, see "bash Command Line Expansion"

Also see


For bash, a space is a metacharacter that separates a command from its arguments, and arguments from each other.

Single Quotes

Enclosing a string in single quotes tells the shell NOT to interpret any shell metacharacters in the text between quotes: single quotes suppress all types of expansion.

Double Quotes

Double quotes permit parameter and variable expansion but suppress other types of expansion.

Dollar Sign ($)

Exclamation Mark (!)


'|' is the "pipe" character, and it is interpreted by bash as a request to set up a pipeline.




$(...) and (...)


Executed the enclosed commands in the same shell. There must be a space between '{' and the command and a space between the command and '}'. Also, the last command must be terminated with ';':

{ set a=1; echo ${a}; }



Redirection Operator (>)




Pathname Expansion

Control Operators

A control operator is a sequence of characters that performs a control function. It is one of the following characters or sequences:


See Pipelines.


See Pipelines.







$(...) and (...)

The New Line

Other Operators



+= and -=

+= and -=

Login Shell

A login shell is a shell whose first character of its argument zero is a -, or it was started with the --login option.

Interactive Shell

An interactive shell is a shell started:

  • without the -c option
  • without non-option arguments
  • with the -i option

The interactive shell's standard input and error are both connected to terminals.

$- includes i if the shell is interactive, allowing a script or a startup file to test this state.

Non-interactive Shell

A shell started with -c option.

bash Startup Files

When bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first executes commands from /etc/profile, if exists.

Them it looks for ~/.bash_profile, ~/.bash_login and ~/.profile in that order. It executes commands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior.

When a login shell exits, bash executes commands from ~/.bash_logout, if it exists.

When an interactive shell that is not a login shell is started, bash executes commands from ~/.bashrc, if it exists. This may be inhibited by using the --norc option. The --rcfile file option will force bash to execute commands from the specified file instead of ~/.bashrc.

When bash is started in non-interactive mode, to run a shell script, for example, it looks for the variable BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as the name of a file execute. Bash behaves as if the following command were executed:

if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi

but the value of the PATH variable is not used to search for the file name.

If any of the files exist but cannot be read, bash reports an error.

Parameters and Variables

bash Parameters and Variables

Command Substitution $(...)

Command Substitution

Command Line Expansion

Command Line Expansion




Foreground Process

Unless otherwise specified, a process launched by the shell will run in foreground, blocking use of the shell for the duration of the process, because the shells stdin, stdout and stderr are redirected to the process.

A foreground process can be typically stopped by sending SIGINT signal into it, and the shell sends SIGINT when Ctrl-C is pressed.

A foreground process can be typically suspended by sending SIGTSTP signal into it. The shell sends SIGTSTP when Ctrl-Z is pressed. A suspended process can be resumed by executing


in the shell that suspended it.

Background Process

A background process is associated with the specific terminal that started it (the shell process is the parent of the background process), but does not block access to the shell. Instead, it executes in the background, leaving the user able to interact with the system while the command runs. Many background processes can be started by the same shell and can run at the same time.

A process can be launched in background by appending "&" at the end of the command line.The stdout, stderr of the background process are directed to the shell's stdout/stderr, unless they are explicitly redirected.

All background and stopped processes can be listed with:



A pipeline is a sequence of commands separated by one of the control operators '|' or '|&'

When '|' is used, the stdout of the command is connected via a pipe to the stdin of the next command, and stderr goes to the controlling terminal.

When '|&' is used, both stdout and stderr of the command is connected via a pipe to the stdin of the next command, and nothing goes to the controlling terminal. The effect is equivalent with:

2>&1 |

When the pipeline execution status is evaluated, the default behavior is to consider only the exit code of the last command of the pipeline. To change this behavior to use the exit status of the rightmost command that exits with a non-zero status, execute set -o pipefail.

The exit status of individual components of the pipeline can be accessed with:


where the command-index is zero based, the left-most command has the index 0:


Tests [...] [[...]]

[...] is part of the shell built-in command test, and it is a synonym for it. It tests the expression between [ ]. The [[...]] construct is the more versatile version of [...], also called the extended test command. For more details see


A subshell is a child process launched by a shell - a separate instance of the command processor.

A subshell is launched with:


Command substitution executes the command in a subshell and replaces it with its stdout:


A subshell's PID is provided by the BASHPID built-in variable. The subshell level is indicated by the BASH_SUBSHELL built-in variable.

External Commands, Built-ins and Subshells

In general, an external command in a script forks off a subprocess, whereas a bash builtin does not. For this reason, builtins execute more quickly and use fewer system resources than their external command equivalents.


A builtin is a command contained within the bash tool set, literally built in. This is either for performance reasons, as builtins execute faster than external commands, which usually require forking off a separate process, or because a particular builtin needs direct access to the shell internals.

List of Builtins:
Built-In Commands




$((...)) and Arithmetic in bash


An alias is a textual substitution performed by the shell on a command. It is defined as such:

alias something='ls -l'

Aliases cannot be recursive, so it can be self-referencing without ambiguity:

alias ls='ls -F'

The already defined aliases can be displayed with the alias command:


An alias can be removed by using the unalias command followed by the name of the alias. Aliases are local to the shell. When you start a new shell, aliases must be recreated. Thus, if you create an alias and start a new shell, that new shell does not have the new alias. They are more like local variables than environment variables, and there is no command for aliases that is equivalent to export.

In bash, aliases are not expanded by default in shell scripts. alias expansion must be enabled by using shopt at the beginning of the script:

shopt -s expand_aliases