Bash

lastmod 2026-11-03created 2014-09-27206 edits

Bash (Borune-again shell) is a Unix shell written by Brian Fox for the GNU Project as a free software replacement for the Bourne shell. Released in 1989, it has been widely distributed as the default shell of operating systems such as the GNU operating system, Linux, Mac OS X, and Darwin. It has also been ported to DOS by DJGPP and Novell NetWare, and to Microsoft Windows through the distributions of Cygwin and MinGW.

Viewing previous screens

On Linux, in a text console rather than a GUI, you can view previous screens that have already scrolled past as follows.

• Shift + Page Up: view previous screen
• Shift + Page Down: view next screen

Alternatively, if you need to check a large amount of text one screen at a time, connect it to more with a PIP.

Category

If a command cannot be found, see the command-not-found.com page.

• Bash:read - how to use the read built-in command
• Bash:Array
• Bash:Mapfile - mapfile -t variable < <(...)
• Bash:Test
• Bash:RegularExpressions
• Bash:Quoting: quoting (wrapping strings in quotation marks).
• Bash:For: how to use for loops in bash.
• Bash:ParameterSubstitution: explanation of parameter substitution.
• Bash:Filename: extracting filenames, paths, etc.
• Bash:ExitCode: exit codes with special meanings
• Bash:InternalVariables: Internal Variables
• Bash:PatternMatching: Pattern Matching
• Bash:Completion: how to set up auto-completion
• Bash:Example
• Bash:Printf
• Bash:PromptSetting
• Bash:Source
• Environment Variables
  • Dotenv (.env) - includes how to apply it in bash, etc.
• ANSI escape code (Terminal Control Escape Sequences; Bash:Formatting; Color, etc)
• Software flow control
• Shebang: the #! on the first line
• Here document - passes a string to stdin in a form like << EOF.
• Process Substitution - <(...) or >(...) - creates a temporary file descriptor such as /dev/fd/xxx so it can be passed to a process.
• Command Substitution - `...` or $(...)
• Arithmetic Expansion - let or $((...))
• Subshells - ( ... )
• Exit status (Exit Code; return code)
• getopts
• trap
• stty
• tty
• Linux:UsefulCommandLines: a collection of frequently used or improved commands in Linux.
• Prompt Statement: the variables PS1, PS2, PS3, PS4. "Not the PlayStation! Not the PlayStation!"
• atuin - stores shell history in SQLite
• Bats - Bash Automated Testing System
• Terminal emulator
  • Terminal emulator#Truecolor - how to support truecolor in terminal emulators; covers the TERM variable.
    • For tmux configuration, see tmux#Truecolor.
    • For neovim configuration, see Neovim#Truecolor.
• Command-line argument
• terminfo
  • tput - queries the terminfo database for terminal-dependent information.
• parallel - use when you want parallel execution like make -j4.
• Bunster - a tool that compiles shell scripts into static binaries

Recommends

• ShellSpec - full-featured BDD unit testing framework
• shellcheck - ShellCheck, a static analysis tool for shell scripts
• shfmt - A shell parser, formatter, and interpreter with bash support
• Explainshell - match command-line arguments to their help text
• ble.sh (Bash Line Editor)

Key environment variables

The full list of environment variables can be checked with the env command.

Key files

• /etc/profile - system-wide environment settings and startup programs
• /etc/bashrc - system-wide aliases and functions (bash shell settings)
• /etc/inputrc - system-wide key bindings and other settings
• $HOME/.bash_profile - user environment settings and startup programs (bash shell settings)
• $HOME/.bashrc - user aliases and functions
• $HOME/.inputrc - user key bindings and other settings

Input/output streams

• (Linux) Difference between Redirect('>') and Pipe('|') - TWpower's Tech Blog

Redirection

One form of IPC (Interprocess Communication)

Redirects a standard stream to a user-defined target (a file).

Pipe

One form of IPC (Interprocess Communication)

A way to pass one program's stdout to another program's stdin. In other words, it passes the output of one program as the input of another.

STDOUT and STDERR Redirection

• What is the difference between &> and >& in bash?

&>word and >&word are equivalent. However, (in the author's experience) they may not work if the environment changes, such as the platform or the terminal program in use. Therefore, the following usage is recommended.

$ echo 'string' > word 2>&1

What does "2>&1" mean

• What does "2>&1" mean? | GeekNews

Why redirection order matters.

• command >file 2>&1 - sends stdout to the file first, then duplicates stderr to stdout → both streams go to the file
• command 2>&1 >file - duplicates stderr to the current stdout (the console), then sends only stdout to the file → stderr is still printed to the console

Bash processes redirections in order, so pay attention to the order when writing commands

Terms

Wildcard character

A wildcard character is a symbol used to specify multiple files at once when issuing a command on a computer. Symbols such as the asterisk (*) and question mark (?) are used.

Shortcuts

• Bash Shortcuts Cheat Sheet - Kapeli
• See also Shell#Sehll shortcuts.

Command editing shortcuts

• Ctrl + A: move the cursor to the start of the command line
• Ctrl + E: move the cursor to the end of the command line
• Ctrl + U: delete from the cursor to the start of the command line
• Ctrl + K: delete from the cursor to the end of the command line
• Ctrl + W: delete from the cursor to the start of the word (deletes the whole word if at a word boundary)
• Option + D: delete from the cursor to the end of the word (deletes the whole word if at a word boundary)
• Ctrl + Y: paste previously deleted text
• Ctrl + XX: move between the start of the command line and the current cursor position
• Option + B: move back one word
• Option + F: move forward one word
• Option + C: convert to uppercase from the cursor position to the end of the word (converts the whole word if the cursor is at the start of the word)
• Option + U: convert to uppercase from the cursor position to the end of the word
• Option + L: convert to lowercase from the cursor position to the end of the word
• Option + T: swap the current word with the previous word
• Ctrl + F: move forward one character
• Ctrl + B: move back one character
• Ctrl + D: delete the character at the cursor
• Ctrl + H: delete the character before the cursor
• Ctrl + T: swap the character at the cursor with the previous character

Command recall shortcuts

• Ctrl + R: reverse search through previous commands
• Ctrl + J: run the currently found command
• Ctrl + G: exit command search mode
• Ctrl + P: recall the previous command
• Ctrl + N: recall the next command
• Ctrl + _: undo the last command
• Option + .: use the last word of the previous command

Command control shortcuts

• Ctrl + L: clear the screen
• Ctrl + S: stop screen output (when running long commands) - related to XON/XOFF
• Ctrl + Q: resume screen output (when stopped with Ctrl + S) - related to XON/XOFF
• Ctrl + C: interrupt the current command
• Ctrl + Z: suspend the current command

Bash Bang(!) shortcuts

• !!: run the last command
• !blah: run the most recent command starting with 'blah'
• !blah:p: print the most recent command starting with 'blah' and add it to the command history
• !$: use the last word of the previous command (same as Option + .)
• !$:p: print the word that '!$' would substitute
• !*: use everything except the first word of the previous command (e.g., after entering 'find some_file.txt /', '! *' returns 'some_file.txt /')
• !*:p: print what '!*' would substitute

Metacharacters

ETC

• \: when placed before a command, specifies that the original command be used instead of an Alias.² (ex. \ls -la)

TIP

• !!: re-run the last executed command.
• !-2: re-run the second-to-last executed command. The number can be changed.
• !foo: run the most recent command starting with foo. (e.g. !ls)
• !foo:p: Print out the command that !foo would run also add it to the command history
• !$: Run the last word of the previous command (same as Alt + .)
• !$:p: Print out the word that !$ would substitute
• !*: Run the previous command except for the last word
• !*:p: Print out the previous command except for the last word
• ^foo^bar: Run the previous command replacing foo with bar
• $!: process ID of the most recently executed background pipeline
• $$: PID of the current shell

Arithmetic

COUNTER=0
$((COUNTER++))

Running a script downloaded with curl directly in the Bash shell

See the k3s sample example:

curl -sfL https://get.k3s.io | sh -s - --write-kubeconfig-mode 644

How to pass arguments along with a stdout-piped script

• Stackoverflow - Passing parameters to bash when executing a script fetched by curl

If the -s option is present, or if no arguments remain after option processing, then commands are read from the standard input. This option allows the positional parameters to be set when invoking an interactive shell.

curl http://foo.com/script.sh | bash -s arg1 arg2

How to get both stdout and the exit code

• Returning Values from Bash Functions

function print_checksum
{
    echo -n "TEST"
    return 1
}

function main1
{
    local actual
    actual=$(print_checksum)
    echo $?  # 1
}

function main2
{
    local actual=$(print_checksum)
    echo $?  # 0
}

Use it like the main1 function. Note that if you use local together with command substitution (Command Substitution; $()), the $? value is not received correctly. See SC2155 for related details.

Alternatively, you can pass a variable name to the function and use eval:

function myfunc()
{
    local  __resultvar=$1
    local  myresult='some value'
    eval $__resultvar="'$myresult'"
}

myfunc result
echo $result

Bash options

• KLDP - Advanced Bash-Scripting Guide: A complete guide to shell scripting with Bash - Chapter 31. Options
• Advanced Bash-Scripting Guide: Chapter 33. Options

Options let you change the behavior of the shell or a script. The set command lets you turn on options inside a script. Where the option should apply, write set -o -option-name or simply set -option-abbrev. The two forms behave the same.

#!/bin/bash
# 모든 명령어는 실행전에 echo 됨.
set -o verbose

# 위와 완전히 똑같음.
set -v

To Disable an option you have enabled, use set +o option-name or set +option-abbrev.

For the list of available options, see the set part of bash(1) - Linux manual page # SHELL BUILTIN COMMANDS. (searching for allexport finds it right away)

The list of abbreviations is as follows:

set -x -e

• (Bash Shell) How to use the set -e and set -x options (how to automatically exit a script when a command fails)

set -euo pipefail

Seems to be used a lot in production code

• -e - exits the script immediately if any command fails (exit code ≠ 0)
• -u - exits with an error when an undefined variable is referenced
• -o pipefail - treats the whole pipeline as failed if any stage fails
  • Without -o pipefail, in cmd1 | cmd2, even if cmd1 fails, only the exit code of cmd2 is checked and it is treated as success. With pipefail on, the exit code of the last failing command in the pipeline becomes the overall result.

set -a ... set +a

set -a
source ".env"
set +a

The export command in bash

• KLDP: Question about the export command in bash

Why you can never make an already running script behave as if it had been run with source:

이미 실행된 스크립트에서 source 로 실행한것처럼 하는건 절대 안됩니다.
반드시 source test.sh 또는 . test.sh (. 명령)을 써야 합니다.
그 이유는 현재 쉘이 어떤 명령을 실행할때 source나. 명령이 아닌경우 (예를 들면 $ ls ) 현재 쉘이 CHILD 를 포크해서 execlp 로 ls 명령을 실행하고 원래의 쉘은 SIGCLD 가 올때까지,
즉 자식( ls) 이 죽을때 까지 pause() 하고 있기 때문에 원래 쉘로 돌아왔을때는 자식이 export 한것은 전혀 소용 없게 됩니다.
export 는 putenv() 함수를 이용하여 특정문자열("변수명=변수값") 을 환경영역에 옮기는 명령이고 이건 CHILD 들에게만 영향을 미치기 때문입니다.

How to read a .env file and Export all variables

• bash - Set environment variables from file of key/value pairs - Stack Overflow

set -o allexport
source .env
set +o allexport

Searching Command History in the Bash shell

• Searching Command History in the Bash shell

Add the following to inputrc:

"\e[A":history-search-backward
"\e[B":history-search-forward

See GNU Readline for details.

Writing tab completion

• Writing simple tab completion for Bash and Zsh | GeekNews
  • [Original] Writing Your Own Simple Tab-Completions for Bash and Zsh :: The Mill Build Tool
• Introduces how to implement tab completion in Bash and Zsh that shows descriptions even for already completed words
• Bash and Zsh use different tab completion APIs, and only Zsh shows descriptions in completions by default
• Implements a structure where _generate_foo_completions generates the candidates, returned via COMPREPLY in Bash and compadd in Zsh
• To bring Zsh's description feature to Bash, descriptions are included in the candidate strings and removed only when there is a single candidate
• Improved so that ambiguity is intentionally added even for a single candidate, so descriptions appear on <TAB>
• The final script provides the same user experience in both shells and supports partial completion, full completion, and candidate descriptions

< vs << vs <<<

As an easy example of <<<, the following two commands are equivalent:

• echo "Hello, World!" | wc -w
• wc -w <<< "Hello, world!"

[ vs [[ vs ( vs ((

• [What's the difference between [ and [ in Bash?
• [Recommended] [shell - What is the difference between the Bash operators [[ vs vs ( vs ((? - Unix & Linux Stack Exchange
• [Recommended] [bash - Why does parameter expansion with spaces without quotes work inside double brackets "[[" but not inside single brackets ""? - Unix & Linux Stack Exchange

(…)

Denotes a subshell.

With a leading dollar sign, as in $(…), it is command substitution]:

There is a command inside the parentheses, and the command's output is used as part of the command line. ³

{…}

Braces are like parentheses in that they group commands, but they only affect parsing, not grouping.

• x=2; { x=4; }; echo $x prints 4.
• x=2; (x=4); echo $x prints 2.

Also braces being keywords need to be delimited and found in command position (hence the space after { and the ; before }) whereas parentheses don't. That's just a syntax quirk.

((…))

Double parentheses enclose an arithmetic command, a calculation on integers using syntax similar to other programming languages.

This syntax is mainly used for assignments and conditionals.

It exists only in ksh/bash/zsh, not in plain sh.

The same syntax is used in arithmetic expressions, $((…)), which expand to the integer value of the expression.

[…]

• [ is a POSIX executable file.
• [ is just an executable. And ] is simply the last argument of [.
• Ubuntu 16.04 actually has an executable at /usr/bin/[ provided by coreutils, but the bash built-in Command takes precedence.

single brackets surround conditional expressions. Conditional expressions are mostly built on operators such as -n "\(variable" to test if a variable is empty and -e "\)file" to test if a file exists. Note that you need a space around each operator (e.g. [ "\(x" = "\)y" ], not [ "\(x"="\)y" ]), and a space or a character like ; both inside and outside the brackets (e.g. [ -n "\(foo" ], not [-n "\)foo"]).

…

• <code> is a [[Korn shell-inspired Bash extension

double brackets are an alternate form of conditional expressions in ksh/bash/zsh with a few additional features, for example you can write -L $file && -f \(file</span> to test if a file is a symbolic link to a regular file whereas single brackets require [ -L "\)file" ] && [ -f "$file" ]. See Why does parameter expansion with spaces without quotes works inside double brackets but not single brackets [? for more on this topic. === Addition: =~ comparison operator == * [[ ab =~ ab?</code> is a POSIX extended regular expression match. ? is not expanded as a glob.

# test.sh file
IFS='||'
echo "$*"

# test.sh file
IFS='||'  # $@ 를 사용할 경우 작동에 아무런 영향을 끼치지 않는다.
echo "$@"

# 로그 파일 비우기
:> /var/log/application.log

command ... \
  -e FIREFOX_CLI=https://.../ `#optional` \
  ...

command -v <the_command>

[ ! -t 0 ] && echo "stdin has data" || echo "stdin is empty"

read -r -u 0 INPUT_VALUE

-bash: ~: /bin/sh^M: bad interpreter: No such file or directory

#!/bin/bash

## Warning: Don't use the quoting("...").
for cursor in $INSTALL_DIR/*.sh; do
    echo $cursor
done