Bash Cheatsheet#

Resources#

Devhints Cheatsheet
Learn Bash in Y Minutes: short quickstart crash course
Bash Scripting 101
Bash Beginner Series

Shell Customization#

Dotfiles#

more details

Startup Scripts#

simplified flow (reference)

Text Only

                    +-----------------+   +------FIRST------+   +-----------------+
                    |                 |   | ~/.bash_profile |   |                 |
login shell -------->|  /etc/profile   |-->| ~/.bash_login ----->|  ~/.bashrc      |
                    |                 |   | ~/.profile      |   |                 |
                    +-----------------+   +-----------------+   +-----------------+
                    +-----------------+   +-----------------+
                    |                 |   |                 |
interactive shell -->|  /etc/bashrc ------>| ~/.bashrc       |
                    |                 |   |                 |
                    +-----------------+   +-----------------+
                    +-----------------+
                    |                 |
logout shell ------->|  ~/.bash_logout |
                    |                 |
                    +-----------------+

[]-->[]: automatically sourced by workflow
[--->[]: manually sourced by convention
FIRST: only first available executed
echo "${BASH_SOURCE[0]}": use to inspect script's sourcing origins
detailed flow (reference)

	Interactive Login	Non-Interactive Login	Interactive Non-Login	Non-Interactive Non-Login	Script
/etc/profile	1	1
/etc/bash.bashrc			1
~/.bashrc			2
~/.bash_profile	2.A	2.A
~/.bash_login	2.B	2.B
~/.profile	2.C	2.C
BASH_ENV				1	1
~/.bash_logout	3	3

Variables#

generally quote your variables unless they contain wildcards to expand or command fragments

Bash

name="John"
echo $name  # see below
echo "$name"
echo "${name}!"
wildcard="*.txt"
option="iv"
cp -$options $wildcard /tmp

Special Variables#

(Reference)

Expression	Description
`$?`	Exit status of last task
`$!`	PID of last background task
`$$`	PID of shell
`$0`	Filename of the shell script
`$_`	Last argument of the previous command
`${PIPESTATUS[n]}`	return value of piped commands (array)

String Quotes#

Bash

name="John"
echo "Hi $name"  #=> Hi John
echo 'Hi $name'  #=> Hi $name

Arrays#

Array Declaration#

Bash

Fruits=('Apple' 'Banana' 'Orange')

Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"

Array Access#

Bash

echo "${Fruits[0]}"              # Element #0
echo "${Fruits[-1]}"             # Last element
echo "${Fruits[@]}"              # All elements, space-separated
echo "${Fruits[*]}"              # All elements, space-separated
echo "${#Fruits[@]}"             # Number of elements
echo "${#Fruits}"                # String length of the 1st element
echo "${#Fruits[3]}"             # String length of the Nth element
echo "${Fruits[@]:3:2}"          # Range (from position 3, length 2)
echo "${!Fruits[@]}"             # Keys of all elements, space-separated
for item in "${Fruits[@]}"; do   # Array iteration
  echo "$item"
done

Array Operations#

Bash

Fruits=("${Fruits[@]}" "Watermelon")    # Push
Fruits+=('Watermelon')                  # Also Push
Fruits=( "${Fruits[@]/Ap*/}" )          # Remove by regex match
unset Fruits[2]                         # Remove one item
Fruits=("${Fruits[@]}")                 # Duplicate
Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
lines=(`cat "logfile"`)                 # Read from file

Conditionals#

`[` vs `[[`#

[: shell command shorthand for test command
[[: shell keyword with quality of life nicities

integer comparison operators: ==,!=,<,<=,>,>=

Bash

[[ 1 < 2 ]] && echo "true"
[ 1 \< 2  ] && echo "true"
[ 1 -lt 2 ] && echo "true"

logic/grouping operators: &&,||,(,)
pattern matching: [ $name = *c* ]
regular expressions: [[ $name =~ ^Ali ]]

Conditions#

Any program that obeys the same logic (like all base utils, such as grep(1) or ping(1)) can be used as condition, see examples.

Condition	Description
`[[ -z STRING ]]`	Empty string
`[[ -n STRING ]]`	Not empty string
`[[ STRING == STRING ]]`	Equal
`[[ STRING != STRING ]]`	Not Equal

`[[ NUM -eq NUM ]]`	Equal
`[[ NUM -ne NUM ]]`	Not equal
`[[ NUM -lt NUM ]]`	Less than
`[[ NUM -le NUM ]]`	Less than or equal
`[[ NUM -gt NUM ]]`	Greater than
`[[ NUM -ge NUM ]]`	Greater than or equal

`[[ STRING =~ STRING ]]`	Regexp

`(( NUM < NUM ))`	Numeric conditions

`[[ -o noclobber ]]`	If OPTIONNAME is enabled

`[[ ! EXPR ]]`	Not
`[[ X && Y ]]`	And
`[[ X

File Conditions#

Condition	Description
`[ -e FILE ](../../%20-e%20FILE%20 " -e FILE ")`	Exists
`[ -r FILE ](../../%20-r%20FILE%20 " -r FILE ")`	Readable
`[ -h FILE ](../../%20-h%20FILE%20 " -h FILE ")`	Symlink
`[ -d FILE ](../../%20-d%20FILE%20 " -d FILE ")`	Directory
`[ -w FILE ](../../%20-w%20FILE%20 " -w FILE ")`	Writable
`[ -s FILE ](../../%20-s%20FILE%20 " -s FILE ")`	Size is > 0 bytes
`[ -f FILE ](../../%20-f%20FILE%20 " -f FILE ")`	File
`[ -x FILE ](../../%20-x%20FILE%20 " -x FILE ")`	Executable

`[ FILE1 -nt FILE2 ](../../%20FILE1%20-nt%20FILE2%20 " FILE1 -nt FILE2 ")`	1 is more recent than 2
`[ FILE1 -ot FILE2 ](../../%20FILE1%20-ot%20FILE2%20 " FILE1 -ot FILE2 ")`	2 is more recent than 1
`[ FILE1 -ef FILE2 ](../../%20FILE1%20-ef%20FILE2%20 " FILE1 -ef FILE2 ")`	Same files

Examples#

Bash

# String
if [ -z "$string" ](../../%20-z%20%22%24string%22%20 " -z "$string" "); then
  echo "String is empty"
elif [ -n "$string" ](../../%20-n%20%22%24string%22%20 " -n "$string" "); then
  echo "String is not empty"
else
  echo "This never happens"
fi

Bash

# Combinations
if [[ X && Y ]]; then
  ...
fi

Bash

# Equal
if [[ "$A" == "$B" ]]

Bash

# Regex
if [[ "A" =~ . ]]

Bash

if (( $a < $b )); then
   echo "$a is smaller than $b"
fi

Bash

if [[ -e "file.txt" ]]; then
  echo "file exists"
fi

Command Execution#

Shell Execution#

Bash

echo "I'm in $(pwd)"
echo "I'm in `pwd`"  # deprecated
# Same

Conditional Execution#

Bash

git commit && git push
git commit || echo "Commit failed"

Command substitution#

(Reference)

Strict Mode#

Reference

Bash

set -euo pipefail
IFS=$'\n\t'

Parameter Expansions#

Basic Examples#

Bash

name="John"
echo "${name}"
echo "${name/J/j}"    #=> "john" (substitution)
echo "${name:0:2}"    #=> "Jo" (slicing)
echo "${name::2}"     #=> "Jo" (slicing)
echo "${name::-1}"    #=> "Joh" (slicing)
echo "${name:(-1)}"   #=> "n" (slicing from right)
echo "${name:(-2):1}" #=> "h" (slicing from right)
echo "${food:-Cake}"  #=> $food or "Cake"

Bash

length=2
echo "${name:0:length}"  #=> "Jo"

Expanded Examples#

(Reference)

Bash

str="/path/to/foo.cpp"
echo "${str%.cpp}"    # /path/to/foo
echo "${str%.cpp}.o"  # /path/to/foo.o
echo "${str%/*}"      # /path/to

echo "${str##*.}"     # cpp (extension)
echo "${str##*/}"     # foo.cpp (basepath)

echo "${str#*/}"      # path/to/foo.cpp
echo "${str##*/}"     # foo.cpp

echo "${str/foo/bar}" # /path/to/bar.cpp

Bash

str="Hello world"
echo "${str:6:5}"   # "world"
echo "${str: -5:5}"  # "world"

Bash

src="/path/to/foo.cpp"
base=${src##*/}   #=> "foo.cpp" (basepath)
dir=${src%$base}  #=> "/path/to/" (dirpath)

Substitution#

Code	Description
`${foo%suffix}`	Remove suffix
`${foo#prefix}`	Remove prefix

`${foo%%suffix}`	Remove long suffix
`${foo/%suffix}`	Remove long suffix
`${foo##prefix}`	Remove long prefix
`${foo/#prefix}`	Remove long prefix

`${foo/from/to}`	Replace first match
`${foo//from/to}`	Replace all

`${foo/%from/to}`	Replace suffix
`${foo/#from/to}`	Replace prefix

Comments#

Bash

# Single line comment

Bash

: '
This is a
multi line
comment
'

Substrings#

Expression	Description
`${foo:0:3}`	Substring (position, length)
`${foo:(-3):3}`	Substring from the right

Length#

Expression	Description
`${#foo}`	Length of `$foo`

Manipulation#

Bash

str="HELLO WORLD!"
echo "${str,}"   #=> "hELLO WORLD!" (lowercase 1st letter)
echo "${str,,}"  #=> "hello world!" (all lowercase)

str="hello world!"
echo "${str^}"   #=> "Hello world!" (uppercase 1st letter)
echo "${str^^}"  #=> "HELLO WORLD!" (all uppercase)

Default values#

Expression	Description
`${foo:-val}`	`$foo`, or `val` if unset (or null)
`${foo:=val}`	Set `$foo` to `val` if unset (or null)
`${foo:+val}`	`val` if `$foo` is set (and not null)
`${foo:?message}`	Show error message and exit if `$foo` is unset (or null)

[!note] Omitting the : removes the (non)nullity checks
e.g. ${foo-val} expands to val if unset otherwise $foo

Loops#

Basic for loop#

Bash

for i in /etc/rc.*; do
  echo "$i"
done

C-like for loop#

Bash

for ((i = 0 ; i < 100 ; i++)); do
  echo "$i"
done

Ranges#

Bash

# Basic
for i in {1..5}; do
  echo "Welcome $i"
done

# With step size
for i in {5..50..5}; do
  echo "Welcome $i"
done

Reading lines#

Bash

while read -r line; do
  echo "$line"
done <file.txt

Forever#

Bash

while true; do
  ...
done

Functions#

Defining functions#

Bash

myfunc() {
  echo "hello $1"
}

Bash

# Same as above (alternate syntax)
function myfunc() {
  echo "hello $1"
}

Bash

myfunc "John"

Returning values#

Bash

myfunc() {
  local myresult='some value'
  echo "$myresult"
}

Bash

result=$(myfunc)

Raising errors#

Bash

myfunc() {
  return 1
}

Bash

if myfunc; then
  echo "success"
else
  echo "failure"
fi

Positional Parameters#

Expression	Description
`$#`	Number of arguments
`$*`	All positional arguments (as a single word)
`$@`	All positional arguments (as separate strings)
`$1`	First argument
`$_`	Last argument of the previous command

[!note] $@ and $* must be quoted; unquoted behavior is exactly the same i.e. args as separate strings

`$@` vs `$*` Example#

Bash

print_params() { 
  echo "\$1 = $1"
  echo "\$2 = $2"
  echo "\$3 = $3"
  echo "\$4 = $4"
}
pass_params() {
  echo -e "\n" '$* =>';   print_params $*
  echo -e "\n" '"$*" =>'; print_params "$*"
  echo -e "\n" '$@ =>';   print_params $@
  echo -e "\n" '"$@" =>'; print_params "$@"
}

pass_params "word" "words with spaces"
$* =>
  $1 = word
  $2 = words
  $3 = with
  $4 = spaces
"$*" =>
  $1 = word words with spaces
  $2 =
  $3 =
  $4 =
$@ =>
  $1 = word
  $2 = words
  $3 = with
  $4 = spaces
"$@" =>
  $1 = word
  $2 = words with spaces
  $3 =
  $4 =

Special Parameters#

(Reference)

Color Formatting#

\e: 0x1b ascii escape character
\e[38;5;(n)m: Select foreground color
\e[48;5;(n)m: Select background color
0..7: standard colors (e.g. \e[30-37m)
8..15: high intensity colors (e.g. \e[90-97m)
16..231: 6 x 6 x 6 cube (216 colors): 16 + 36 x r + 6 x g + b (0 <= r, g, b <= 5)
232..255: grayscale from black to white in 24 steps
(Reference)
more details

Bash Cheatsheet#

Resources#

Shell Customization#

Dotfiles#

Startup Scripts#

Variables#

Special Variables#

String Quotes#

Arrays#

Array Declaration#

Array Access#

Array Operations#

Conditionals#

[ vs [[#

Conditions#

File Conditions#

Examples#

Command Execution#

Shell Execution#

Conditional Execution#

Command substitution#

Strict Mode#

Parameter Expansions#

Basic Examples#

Expanded Examples#

Substitution#

Comments#

Substrings#

Length#

Manipulation#

Default values#

Loops#

Basic for loop#

C-like for loop#

Ranges#

Reading lines#

Forever#

Functions#

Defining functions#

Returning values#

Raising errors#

Positional Parameters#

$@ vs $* Example#

Special Parameters#

Color Formatting#

`[` vs `[[`#

`$@` vs `$*` Example#