Bash

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 /etc/passwd file. The login shell can be changed using the chsh utility, which is part of sys-apps/coreutils package.

Files

There are many settings to consider when determining how to modify the shell's behavior. Configuration changes can be defined via variables, functions, or shell built-ins. These settings are defined in several different configuration files, where the settings in the last file parsed will overwrite entries in previously defined files.

• /etc/profile - Initial settings for all users.
• ~/.bash_profile - Settings for this user.
• ~/.bash_login - Settings for this user, if ~/.bash_profile does not exist.
• ~/.profile - Settings for this user, if ~/.bash_profile and/or ~/.bash_login do not exist.

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

• /etc/bashrc - Initial settings for all users.
• ~/.bashrc - Settings on a per-user basis.

In Gentoo, and many other Linux distributions, the /etc/bashrc file is parsed in /etc/profile to ensure that /etc/bashrc and ~/.bashrc are always checked when someone logs into the system. If it exists, the final settings are always defined by the user in each ~/.bashrc file.

<syntaxhighlight lang="bash"># Modify the PS1 variable to adjust command prompt
# /u the username of the current user
# /h the hostname up to the first `.'
# /w the  current  working  directory, with $HOME abbreviated with a tilde (uses the value of the PROMPT_DIRTRIM variable)
# /$ if the effective UID is 0, a #, otherwise a $
# For more PS1 options see the PROMPTING section of `man 1 bash`
PS1='\u@\h \w \$ '

# No double entries in the shell history.
export HISTCONTROL="$HISTCONTROL erasedups:ignoreboth"

# Do not overwrite files when redirecting output by default.
set -o noclobber

# Wrap the following commands for interactive use to avoid accidental file overwrites.
rm() { command rm -i "${@}"; }
cp() { command cp -i "${@}"; }
mv() { command mv -i "${@}"; }</syntaxhighlight>

Tab completion

The app-shells/bash-completion 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^[1]. Available completions using

and specific completions disabled using

To disable bash completion on a per user basis, create an .inputrc file with the following content in the user's home directory, then open a new bash shell:

<syntaxhighlight lang="bash"># Disable bash completion
set disable-completion on</syntaxhighlight>

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 execve() will be the union of the assignment list with the environment of the calling shell process:

To check the value of a variable:

MyUserName@MyPC: ~ $

<syntaxhighlight lang="bash">
PS1="\u@\h \w $ "
</syntaxhighlight>

<syntaxhighlight lang="bash">
PS1="\$(cut -d\  -f1 /proc/loadavg) $ "
</syntaxhighlight>

0.10 $

<syntaxhighlight lang="bash">
PS1="\e[0;32m\]\u@\h \w >\e[0m\] "
</syntaxhighlight>

Built-ins

Keyboard shortcuts

bash includes two different keyboard shortcuts modes to make editing input on the command-line easier: emacs mode and vi mode. bash defaults to emacs mode.

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:

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 ls -l into the program /usr/bin/less:

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 Ctrl+Z, 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: