Bash

GNU Bash (Bourne-again shell) Article description::the most popular [[shell program found on Linux systems.]] Typically the shell is the first program started after a user logs in at a terminal. Which user uses what shell is defined in the file. It enables users to easier interact with the system and start additional programs. A lot of this information also applies to other shells such as.

Installation
is part of the @system set and comes installed on every Gentoo system. It is also used by Portage, Gentoo's default package manager. It is highly recommended to not uninstall (or any other package in the @system set), even if another shell is used as a login-shell.

USE flags
It is possible to change USE flags:

After making USE modifications, ask Portage to update the system so the changes take effect:

Login shell
The default login shell for a user is defined in the file. The login shell can be changed using the utility, which is part of  package.

Files
Many settings on how the shell behaves, can be defined via variables. Those variables are defined in several different configuration files, where the settings in the last file parsed do overwrite previous definitions.


 * - Initial settings for all users.
 * - Settings for this user.
 * - Settings for this user, if doesn't exist
 * - Settings for this user, if and  does not exist.

If the shell is started without login (e.g. in a terminal on a desktop), the following files are used:


 * - Initial settings for all users.
 * - Settings for this user.

In Gentoo, and many other Linux distributions, the file is parsed in  to ensure that  and  are always checked when someone logs into the system. The final settings are defined by the user in their personal file.

Tab completion
The package adds completion to many programs and their parameters. To enable completion, just merge the package. No special use flags for packages, which support completion, are required.

Bash completion for all supported programs is enabled by default. You can view the available completions with and disable specific completions with

Environment variables
See all variables for the current shell process which have the export attribute set:

Of course, users can export their own variables, which are available to the current process and inherited by child processes:

Environment variables can also be localized to an individual child process by prepending an assignment list to a simple command. The resulting environment passed to  will be the union of the assignment list with the environment of the calling shell process:

To check the value of a variable:

PS1
The special shell variable PS1 defines what the terminal prompt looks like:

This prompt would be the following value in PS1 :

The following table lists the possible placeholders that can be used in the PS1 variable:

You can also put complete commands into your prompt using a command substitution. Here we want to execute to show the one-minute load average at the beginning of the prompt:

which looks like this:

Having colors in the prompt:

The  changes the color for every next output, we have to put   at the end of our variable to reset the color, or we would type everything in green.

Color codes:

The  in   means foreground. You can define other values like  for foreground bold and   for foreground underlined. Omit this number to refer to the background, e.g..

set
The set command is used to display and change settings in the bash shell.

Show all current settings:

Disable the shell history:

Enable the shell history:

alias
You can use the builtin to define a new command or redefine an existing command:

Whenever now (two lowercase Ls) is send to the shell, it will actually execute.

To remove an alias:

If you want to temporarily bypass an alias you can escape the first letter of the command with a backslash character:

history
The history of used commands in a session is written to a file in the user's home directory. The easiest way to access the commands in the history is using the and  keys.

To show all commands in the current history:

To search for commands in the history, by piping the output through and filter for words:

The commands are numbered and can be executed using their index:

To execute the last command used:

Delete every command in the history:

Show the current settings for history:

Scripts
Shell scripts are text files which contain programs written in a certain shell scripting language. Which shell is used to interpret the commands in a script is defined in the first line (which is called the shebang):

If no shell is defined the default shell for the user who executes the script is used. Often is used, which is the father of all shells and has very limited functionalities. Nearly all shells available understand commands used when running, so those scripts are highly portable.

Start scripts
To run scripts directly from the command-line, they need to be executable. To make a shell script executable:

Now the script can be executed by using the prefix, where either the shell defined by the shebang in the script or the default shell of the user is used:

In alternative you can explicitly invoke the shell and pass the script filename as an argument (no change of permissions needed):

The file extension does not matter, but it helps to distinguish scripts from normal text files.

Redirection
In Bash it is possible to redirect the output of one program into the input of another program using a pipe, indicated by the  symbol. This enables users to create command chains. Here is an example to redirect the output of into the program :

To redirect output into a file:

The  operator will erase any previous content before adding new one. If this is not desired, use the  (append) operator instead.

Logical operators
Logical operators are very useful to chain commands together. This is helpful when checking if the previous command finished successfully or not.

(AND) - The following command prints 'Success' only if our test script is successful:

(OR) - The following command prints 'Failure' only if our test script is unsuccessful:

Jobs
Usually if we start a script or command, the input is blocked until the command is finished. To start a program directly in the background, so we can continue to work in the shell:

This will execute the script as job number 1 and the prompt expects the next input.

If a program is already running and you need to do something on the shell, it is possible to move programs from foreground to background and vice versa. To get a command prompt if a command is running on the shell, put it into sleep using +, then move it to the background:

To list all jobs running in the background:

To move a job back to foreground:

Command substitution
Using a command substitution, it is possible to run programs as parameters of other commands like here:

This will first execute the command in the brackets and append the output as parameter of emerge.

More substitutions can be performed in one command like this:

External resources

 * Bash reference from the Gentoo Developer's Handbook.
 * Advanced Bash-Scripting Guide
 * Chet's Bash page
 * Official Bash documentation
 * The Bash FAQ and Bash guide on Greg Wooledge's wiki
 * bash-hackers wiki (very good reference)
 * Bash cgit
 * POSIX sh spec
 * mksh, ksh93, and ksh88 manuals for cross-reference
 * Comprehensive Beginner Linux Tutorial