Bash

Resources

GNU Bash (Bourne-again shell) is the default shell on Gentoo systems and a popular shell program found on many Linux systems.

See the terminal emulator article for some general usage pointers.

Installation

bash is part of the @system set and comes installed on every Gentoo system. It is used internally by Portage, Gentoo's default package manager, and other Gentoo system components. It is therefore highly recommended to not uninstall bash (which is usual for a package in the @system set), this would undoubtedly break the system. Do not uninstall bash just because another shell is installed, used, or set as a login shell.

USE flags

It is possible to change USE flags for the bash package:

USE flags for app-shells/bash The standard GNU Bourne again shell

`+net`	Enable /dev/tcp/host/port redirection
`+readline`	Enable support for libreadline, a GNU line-editing library that almost everyone wants
`afs`	Add OpenAFS support (distributed file system)
`bashlogger`	Log ALL commands typed into bash; should ONLY be used in restricted environments such as honeypots
`examples`	Install examples, usually source code
`mem-scramble`	Build with custom malloc/free overwriting allocated/freed memory
`nls`	Add Native Language Support (using gettext - GNU locale utilities)
`pgo`	Optimize the build using Profile Guided Optimization (PGO)
`plugins`	Add support for loading builtins at runtime via 'enable'
`static`	!!do not set this during bootstrap!! Causes binaries to be statically linked instead of dynamically
`verify-sig`	Verify upstream signatures on distfiles

Data provided by the Gentoo Package Database · Last update: 2024-10-08 21:49 More information about USE flags

Emerge

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

root #emerge --ask --changed-use --deep @world

Shell completion

Shell completion programming, sometimes called tab completion, is available to many programs and their parameters on Gentoo. To enable shell completion for bash, install the app-shells/bash-completion package. No special USE flags for packages, which support completion, are required on individual packages. Post-installation, completion functionality can be managed through eselect.

root #emerge --ask app-shells/bash-completion

The app-shells/gentoo-bashcomp package, which is installed by app-shells/bash-completion, adds Gentoo-specific completions for e.g. emerge.

Shell completion is enabled by default for all supported programs^[1].

Configuration

Login shell

Bash is the default shell on Gentoo, after installation. The default login shell for a user is defined in the /etc/passwd file.

The login shell can be changed using the chsh utility, to use another POSIX compatible shell (chsh is from sys-apps/shadow, which is part of the base profile). See the articles on login and on shell configuration for more info.

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 (global) settings for all users.
~/.bash_profile - Local bash profile settings for the current user.
~/.bash_login - Local settings for the current user, if ~/.bash_profile does not exist.
~/.profile - Settings for the current 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 (global) settings for all users.
~/.bashrc - Local settings on a per-user basis.

.bashrc

In Gentoo, and many other Linux distributions, /etc/bashrc is parsed from /etc/profile to ensure /etc/bashrc and ~/.bashrc are always checked when a user executes a new (non-login) shell. Local existing ~/.bashrc file settings always override global /etc/bashrc settings.

FILE ~/.bashrc

# This file contains suggested settings for a user's ~/.bashrc file.

# 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 "${@}"; }

Shell completion integrations

List available completions via:

user $eselect bashcomp list

Disable specific completions using

user $eselect bashcomp disable <command-name>

To disable all bash shell completion integration for a particular user, create an .inputrc file with the following content in the user's home directory, then open a new bash shell:

FILE ~/.inputrc

# Disable bash completion
set disable-completion on

Usage

Environment variables

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

user $export

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

user $export MYSTUFF=Hello

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:

user $USE=kde emerge -pv libreoffice

To check the value of a variable:

user $typeset -p MYSTUFF

PS1

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

CODE Example prompt

larry@gentoo: ~ $

Note
In the above example, larry is the user name, gentoo is the system hostname, and the ~ (tilde) symbol represents the current user's home directory (also represented by the HOME environment variable in bash).

This prompt would be the following value for the PS1 variable:

CODE PS1 definition

PS1="\u@\h \w $ "

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

Code	Effect
`\u`	Username.
`\h`	Hostname.
`\w`	Current directory.
`\d`	Current date.
`\t`	Current time.
`\$`	Indicate the root user with '#' and normal users with '$'.
`\j`	Number of currently running tasks (jobs).

Complete commands can be put into prompt using a command substitution. The following will execute the cut -d\ -f1 /proc/loadavg command to show the one-minute load average at the beginning of the prompt:

CODE PS1 definition

PS1="\$(cut -d\  -f1 /proc/loadavg) $ "

Looks like:

CODE Prompt

0.10 $

Having colors in the prompt:

CODE PS1 definition

PS1="\e[0;32m\]\u@\h \w >\e[0m\] "

The \e[0;32m\] changes the color for every next output, put \e[0m\] at the end of the variable to reset the color, otherwise the whole prompt will be in green.

Color codes:

Code	Color
`\e[0;30m\]`	Black
`\e[0;31m\]`	Red
`\e[0;32m\]`	Green
`\e[0;33m\]`	Yellow
`\e[0;34m\]`	Blue
`\e[0;35m\]`	Magenta
`\e[0;36m\]`	Cyan
`\e[0;37m\]`	White
`\e[0m\]`	Reset to standard colors

The 0; in \e[0;31m\] means foreground. If desired, other values like 1; for foreground bold and 4; for foreground underlined can be defined. Omit this number to refer to the background, e.g. \e[31m\].

This script prints out all 256 color codes available in the Bash terminal that can be used in PS1 prompts. It prints color swatches in groups of 8 and demonstrates how to set text color using escape sequences. To reset the color back to the default after using an escape sequence, end the print out with \e[0m

FILE colors.shBash Terminal Color Script

#!/bin/bash

for color in {0..255}; do
  printf "\\e[38;5;%sm%3s\\e[0m " $color "\\e[38;5;${color}m"
  if ! ((($color + 1) % 8)); then
    echo
  fi
done

The tput command in terminal provides an abstraction to control the appearance of the shell prompt. Instead of hard-coding escape sequences, tput uses terminal capabilities from the terminfo database. Here's an example that can be used for tput to change the color properties of your PS1 prompt:

CODE PS1 with tput for green text

PS1="\[$(tput setaf 2)\]\u@\h \w \$ \[$(tput sgr0)\]"

This sets the text color to green for the prompt. The tput setaf 2 command sets the foreground color using ANSI color codes, with 2 representing green. The tput sgr0 command then resets the text formatting back to the terminal's default.

The prompt can be made visually informative about the Git repository status by coloring the branch name according to the repository state. Below is a PS1 definition that color-codes the current branch as red for uncommitted changes, green for a clean directory, and yellow for stashed changes:

CODE PS1 with Colored Git Branch Status

git_branch() {
  branch=$(git branch 2>/dev/null | grep '^*' | colrm 1 2)
  if [ ! -z "$branch" ]; then
    if [ -n "$(git status --porcelain)" ]; then
      color="31"  # Red for changes
    elif [ "$(git stash list)" ]; then
      color="33"  # Yellow for stashed changes
    else
      color="32"  # Green for a clean state
    fi
    echo -e "\\e[0;${color}m${branch}\\e[0m"  
  fi
}

PS1="\u@\h \w \$(git_branch)\$ "

This prompt function checks the current Git branch and repository status, setting colors accordingly. Insert the function git_branch in your ~/.bashrc or ~/.bash_profile file, followed by the PS1 assignment for this functionality to take effect.

Built-ins

set

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

Show all current settings:

user $set -o

Disable the shell history:

user $set +o history

Enable the shell history:

user $set -o history

alias

The alias builtin can be used to define a new command or redefine an existing command:

user $alias ll='ls -l'

Now whenever ll (two lowercase Ls) is send to the shell, it will actually execute ls -l.

To remove an alias:

user $unalias ll

Note
No harm is done to the actual command being redefined.

To temporarily bypass an alias escape the first letter of the command with a backslash character:

user $\ls

Listing

Run alias without any arguments to display a list of currently defined aliases:

user $alias

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 Up and Down keys.

To show all commands in the current history:

user $history

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

user $history | grep echo

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

user $!2

To execute the last command used:

user $!!

Delete every command in the history:

user $history -c

Show the current settings for history:

user $echo $HISTCONTROL

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.

vi mode

vi mode requires an Esc key press to prefix very movement or edit, so it can be a bit awkward to learn this mode. To change the mode to vi mode, execute the following command:

user $set -o vi

Review this bash vi editing mode cheat sheet document by Peteris Krumins for more details on key bindings in vim mode.

emacs mode

To switch to emacs mode (which is the default mode):

user $set -o emacs

Navigation

The following sections are select excerpts from the bash man page. They can be retrieved locally by reading man 1 bash. They contain useful command-line navigation shortcuts and bash built-ins.

Search for Commands for Moving for the beginning of the section.

Line movement:

ctrl+a: Move cursor to the beginning of the line (home).
ctrl+e: Move cursor to the end of the line (end).
ctrl+f: Move the cursor forward one character.
ctrl+b: Move the cursor back one character.
alt+f: Move cursor forward one word.
alt+b: Move cursor backward one word.
ctrl+xx: Toggle the cursor between the current position and the beginning of the line.
ctrl+]-<char>: Move cursor to the first occurrence of the entered character to the right.
ctrl+alt-]-<char>: Move cursor to the first occurrence of the entered character to the left.

Directory movement:

cd /path: Change to /path directory.
cd -: Change to previous directory.
cd: Change to home directory.

Screen control

ctrl+s: Stop (pause) output on the screen.
ctrl+q: Resume output on the screen (after stopping it with the previous command).
ctrl+l: Clears the screen preserving the current command (very similar to the clear command).

Text manipulation

Deletion:

ctrl+u: Remove text relative to the cursor's current position to the beginning of the line.
ctrl+k: Remove text relative to the cursor's current position to the end of the line.
alt+d: Remove one word moving forward from the cursor's current position.
ctrl+w: Remove one word moving backward from the cursor's current position.
ctrl+y: Paste deleted text.

Command history

ctrl+p: Scroll backward through previous command history for the current session.
ctrl+n: Scroll forward through next entry in command history for the current session
ctrl+r: Reverse search through history (type parts of the command to initiate query).

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 by the first line of the script. This consists of two special characters #! (called the shebang), then a direct path to the shell. For example:

FILE myscript

#!/bin/bash
echo 'Hello World!'

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

Note
On many distributions /bin/sh is a symbolic link to /bin/bash. But on other distributions (like Debian) it can be a symbolic link to /bin/dash, which is a POSIX compliant variant of sh. In order to ensure good portability, be sure to test any script using the same shell as the one in its shebang.

Script execution

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

user $chmod +x myscript

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:

user $./myscript

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:

user $ls -l | less

To redirect output into a file:

user $ls -l > ls_l.txt

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:

user $./myscript && echo 'Success'

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

user $./myscript || echo 'Failure'

Jobs

Usually when script or command has been executed shell input is blocked until after execution has completed. To start a program directly in the background, append the ampersand character (&) to the end of the command:

user $./myscript &

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

When a program is already running, but the shell is needed for another task, it is possible to move programs from foreground to background and vice versa. To get a command prompt when a command is running on the shell, put it into sleep using Ctrl+z, then move it to the background:

user $bg %1

To list all jobs running in the background:

user $jobs

To move a job back to foreground:

user $fg %1

Note
Programs running as jobs usually do not terminate once they finish execution, there will be a message if a job finished and bringing it to foreground will then terminate the program.

Command substitution

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

user $emerge --ask --oneshot $(qlist -CI x11-drivers)

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

Note
This command is quite useful in Gentoo to quickly rebuild all X11 drivers.

More substitutions can be performed in one command like this:

user $emerge --ask --oneshot $(qlist -CI x11-drivers) $(qlist -CI modules)

Troubleshooting

Garbled display

The output of a shell can, in some conditions, become corrupt. See the terminal emulator article for instructions to help fix this.

External resources

Learning Modern Bash

Wikibooks Bash Scripting Guide — an easy introduction to Bash scripting.
Advanced Bash-Scripting Guide — a slightly older but still useful Bash scripting guide.
New Bash Features — a list of features added to Bash over time.
Bash Traps — a collection of common mistakes and how to avoid them.
Pure Bash Bible — a collection of pure Bash alternatives to external processes.
Awesome Bash — A curated list of Bash scripts and resources.
Exercism: Bash Track — an interactive Bash tutorial.

Debugging and Testing

Defensive Bash Programming — how not to shoot yourself in the foot with Bash.
Unofficial Strict Mode — a "strict mode" inspired by the use strict pragma in Perl and a detailed explanation of how this works.
Unit Testing in Bash — a collection of Unit testing tools.
shellcheck — a static analysis tool for shell scripts.

Code Style and Documentation

Shell Script Style Guide — Google's style guide for shell scripts.
bash-doxygen — A doxygen filter for bash scripts; great for developer documentation.
Plain Old Documentation — Perl-style POD documentation in Bash scripts; great for man page generation with pod2man.

Cheat Sheets

Bash Scripting Cheat Sheet.

Bash Reference Guides

Bash reference from the Gentoo Developer's Handbook.
Chet's Bash page.
The Bash FAQ and Bash guide on Greg Wooledge's wiki.
POSIX sh specification.
mksh, ksh93, and ksh88 manuals for cross-reference.

References

↑ News Items - bash-completion-2.1-r90, November 25th, 2014. Retrieved on May 13th, 2017.

[1] News Items - bash-completion-2.1-r90, November 25th, 2014. Retrieved on May 13th, 2017.

[1]

Bash

Installation

USE flags

USE flags for app-shells/bash The standard GNU Bourne again shell

Emerge

Shell completion

Configuration

Login shell

Files

.bashrc

Shell completion integrations

Usage

Environment variables

PS1

Built-ins

set

alias

Listing

history

Keyboard shortcuts

vi mode

emacs mode

Navigation

Screen control

Text manipulation

Command history

Scripts

Script execution

Redirection

Logical operators

Jobs

Command substitution

Troubleshooting

Garbled display

See also

External resources

Learning Modern Bash

Debugging and Testing

Code Style and Documentation

Cheat Sheets

Bash Reference Guides

References