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---
title: Bash scripting
category: CLI
layout: 2017/sheet
tags: [Featured]
updated: 2020-07-05
keywords:
- Variables
- Functions
- Interpolation
- Brace expansions
- Loops
- Conditional execution
- Command substitution
---
Getting started
---------------
{: .-three-column}
### Introduction
{: .-intro}
This is a quick reference to getting started with Bash scripting.
- [Learn bash in y minutes](https://learnxinyminutes.com/docs/bash/) _(learnxinyminutes.com)_
- [Bash Guide](http://mywiki.wooledge.org/BashGuide) _(mywiki.wooledge.org)_
### Example
```bash
#!/usr/bin/env bash
NAME="John"
echo "Hello $NAME!"
```
### Variables
```bash
NAME="John"
echo $NAME
echo "$NAME"
echo "${NAME}!"
```
### String quotes
```bash
NAME="John"
echo "Hi $NAME" #=> Hi John
echo 'Hi $NAME' #=> Hi $NAME
```
### Shell execution
```bash
echo "I'm in $(pwd)"
echo "I'm in `pwd`"
# Same
```
See [Command substitution](http://wiki.bash-hackers.org/syntax/expansion/cmdsubst)
### Conditional execution
```bash
git commit && git push
git commit || echo "Commit failed"
```
### Functions
{: id='functions-example'}
```bash
get_name() {
echo "John"
}
echo "You are $(get_name)"
```
See: [Functions](#functions)
### Conditionals
{: id='conditionals-example'}
```bash
if [[ -z "$string" ]]; then
echo "String is empty"
elif [[ -n "$string" ]]; then
echo "String is not empty"
fi
```
See: [Conditionals](#conditionals)
### Strict mode
```bash
set -euo pipefail
IFS=$'\n\t'
```
See: [Unofficial bash strict mode](http://redsymbol.net/articles/unofficial-bash-strict-mode/)
### Brace expansion
```bash
echo {A,B}.js
```
| Expression | Description |
| ---------- | ------------------- |
| `{A,B}` | Same as `A B` |
| `{A,B}.js` | Same as `A.js B.js` |
| `{1..5}` | Same as `1 2 3 4 5` |
See: [Brace expansion](http://wiki.bash-hackers.org/syntax/expansion/brace)
Parameter expansions
--------------------
{: .-three-column}
### Basics
```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"
```
See: [Parameter expansion](http://wiki.bash-hackers.org/syntax/pe)
```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##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 `$FOO` is unset (or null) |
Omitting the `:` removes the (non)nullity checks, e.g. `${FOO-val}` expands to `val` if unset otherwise `$FOO`.
Loops
-----
{: .-three-column}
### 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
for i in {1..5}; do
echo "Welcome $i"
done
```
#### With step size
```bash
for i in {5..50..5}; do
echo "Welcome $i"
done
```
### Reading lines
```bash
cat file.txt | while read line; do
echo $line
done
```
### Forever
```bash
while true; do
···
done
```
Functions
---------
{: .-three-column}
### 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
```
### Arguments
| Expression | Description |
| --- | --- |
| `$#` | Number of arguments |
| `$*` | All postional arguments (as a single word) |
| `$@` | All postitional arguments (as separate strings) |
| `$1` | First argument |
| `$_` | Last argument of the previous command |
**Note**: `$@` and `$*` must be quoted in order to perform as described.
Otherwise, they do exactly the same thing (arguments as separate strings).
See [Special parameters](http://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).
Conditionals
------------
{: .-three-column}
### Conditions
Note that `[[` is actually a command/program that returns either `0` (true) or `1` (false). 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 |
#### More conditions
| Condition | Description |
| -------------------- | ------------------------ |
| `[[ -o noclobber ]]` | If OPTIONNAME is enabled |
| --- | --- |
| `[[ ! EXPR ]]` | Not |
| `[[ X && Y ]]` | And |
| `[[ X || Y ]]` | Or |
### File conditions
| Condition | Description |
| --- | --- |
| `[[ -e FILE ]]` | Exists |
| `[[ -r FILE ]]` | Readable |
| `[[ -h FILE ]]` | Symlink |
| `[[ -d FILE ]]` | Directory |
| `[[ -w FILE ]]` | Writable |
| `[[ -s FILE ]]` | Size is > 0 bytes |
| `[[ -f FILE ]]` | File |
| `[[ -x FILE ]]` | Executable |
| --- | --- |
| `[[ FILE1 -nt FILE2 ]]` | 1 is more recent than 2 |
| `[[ FILE1 -ot FILE2 ]]` | 2 is more recent than 1 |
| `[[ FILE1 -ef FILE2 ]]` | Same files |
### Example
```bash
# String
if [[ -z "$string" ]]; then
echo "String is empty"
elif [[ -n "$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
```
Arrays
------
### Defining arrays
```bash
Fruits=('Apple' 'Banana' 'Orange')
```
```bash
Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"
```
### Working with arrays
```bash
echo ${Fruits[0]} # Element #0
echo ${Fruits[-1]} # Last element
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 ${!Fruits[@]} # Keys of all elements, space-separated
```
### Operations
```bash
Fruits=("${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
```
### Iteration
```bash
for i in "${arrayName[@]}"; do
echo $i
done
```
Dictionaries
------------
{: .-three-column}
### Defining
```bash
declare -A sounds
```
```bash
sounds[dog]="bark"
sounds[cow]="moo"
sounds[bird]="tweet"
sounds[wolf]="howl"
```
Declares `sound` as a Dictionary object (aka associative array).
### Working with dictionaries
```bash
echo ${sounds[dog]} # Dog's sound
echo ${sounds[@]} # All values
echo ${!sounds[@]} # All keys
echo ${#sounds[@]} # Number of elements
unset sounds[dog] # Delete dog
```
### Iteration
#### Iterate over values
```bash
for val in "${sounds[@]}"; do
echo $val
done
```
#### Iterate over keys
```bash
for key in "${!sounds[@]}"; do
echo $key
done
```
Options
-------
### Options
```bash
set -o noclobber # Avoid overlay files (echo "hi" > foo)
set -o errexit # Used to exit upon error, avoiding cascading errors
set -o pipefail # Unveils hidden failures
set -o nounset # Exposes unset variables
```
### Glob options
```bash
shopt -s nullglob # Non-matching globs are removed ('*.foo' => '')
shopt -s failglob # Non-matching globs throw errors
shopt -s nocaseglob # Case insensitive globs
shopt -s dotglob # Wildcards match dotfiles ("*.sh" => ".foo.sh")
shopt -s globstar # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')
```
Set `GLOBIGNORE` as a colon-separated list of patterns to be removed from glob
matches.
History
-------
### Commands
| Command | Description |
| --------------------- | ----------------------------------------- |
| `history` | Show history |
| `shopt -s histverify` | Don't execute expanded result immediately |
### Expansions
| Expression | Description |
| ------------ | ---------------------------------------------------- |
| `!$` | Expand last parameter of most recent command |
| `!*` | Expand all parameters of most recent command |
| `!-n` | Expand `n`th most recent command |
| `!n` | Expand `n`th command in history |
| `!<command>` | Expand most recent invocation of command `<command>` |
### Operations
| Code | Description |
| -------------------- | --------------------------------------------------------------------- |
| `!!` | Execute last command again |
| `!!:s/<FROM>/<TO>/` | Replace first occurrence of `<FROM>` to `<TO>` in most recent command |
| `!!:gs/<FROM>/<TO>/` | Replace all occurrences of `<FROM>` to `<TO>` in most recent command |
| `!$:t` | Expand only basename from last parameter of most recent command |
| `!$:h` | Expand only directory from last parameter of most recent command |
`!!` and `!$` can be replaced with any valid expansion.
### Slices
| Code | Description |
| -------- | ---------------------------------------------------------------------------------------- |
| `!!:n` | Expand only `n`th token from most recent command (command is `0`; first argument is `1`) |
| `!^` | Expand first argument from most recent command |
| `!$` | Expand last token from most recent command |
| `!!:n-m` | Expand range of tokens from most recent command |
| `!!:n-$` | Expand `n`th token to last from most recent command |
`!!` can be replaced with any valid expansion i.e. `!cat`, `!-2`, `!42`, etc.
Miscellaneous
-------------
### Numeric calculations
```bash
$((a + 200)) # Add 200 to $a
```
```bash
$(($RANDOM%200)) # Random number 0..199
```
### Subshells
```bash
(cd somedir; echo "I'm now in $PWD")
pwd # still in first directory
```
### Redirection
```bash
python hello.py > output.txt # stdout to (file)
python hello.py >> output.txt # stdout to (file), append
python hello.py 2> error.log # stderr to (file)
python hello.py 2>&1 # stderr to stdout
python hello.py 2>/dev/null # stderr to (null)
python hello.py &>/dev/null # stdout and stderr to (null)
```
```bash
python hello.py < foo.txt # feed foo.txt to stdin for python
```
### Inspecting commands
```bash
command -V cd
#=> "cd is a function/alias/whatever"
```
### Trap errors
```bash
trap 'echo Error at about $LINENO' ERR
```
or
```bash
traperr() {
echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
}
set -o errtrace
trap traperr ERR
```
### Case/switch
```bash
case "$1" in
start | up)
vagrant up
;;
*)
echo "Usage: $0 {start|stop|ssh}"
;;
esac
```
### Source relative
```bash
source "${0%/*}/../share/foo.sh"
```
### printf
```bash
printf "Hello %s, I'm %s" Sven Olga
#=> "Hello Sven, I'm Olga
printf "1 + 1 = %d" 2
#=> "1 + 1 = 2"
printf "This is how you print a float: %f" 2
#=> "This is how you print a float: 2.000000"
```
### Directory of script
```bash
DIR="${0%/*}"
```
### Getting options
```bash
while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in
-V | --version )
echo $version
exit
;;
-s | --string )
shift; string=$1
;;
-f | --flag )
flag=1
;;
esac; shift; done
if [[ "$1" == '--' ]]; then shift; fi
```
### Heredoc
```sh
cat <<END
hello world
END
```
### Reading input
```bash
echo -n "Proceed? [y/n]: "
read ans
echo $ans
```
```bash
read -n 1 ans # Just one character
```
### Special variables
| Expression | Description |
| ---------- | ---------------------------- |
| `$?` | Exit status of last task |
| `$!` | PID of last background task |
| `$$` | PID of shell |
| `$0` | Filename of the shell script |
See [Special parameters](http://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).
### Go to previous directory
```bash
pwd # /home/user/foo
cd bar/
pwd # /home/user/foo/bar
cd -
pwd # /home/user/foo
```
### Check for command's result
```bash
if ping -c 1 google.com; then
echo "It appears you have a working internet connection"
fi
```
### Grep check
```bash
if grep -q 'foo' ~/.bash_history; then
echo "You appear to have typed 'foo' in the past"
fi
```
## Also see
{: .-one-column}
* [Bash-hackers wiki](http://wiki.bash-hackers.org/) _(bash-hackers.org)_
* [Shell vars](http://wiki.bash-hackers.org/syntax/shellvars) _(bash-hackers.org)_
* [Learn bash in y minutes](https://learnxinyminutes.com/docs/bash/) _(learnxinyminutes.com)_
* [Bash Guide](http://mywiki.wooledge.org/BashGuide) _(mywiki.wooledge.org)_
* [ShellCheck](https://www.shellcheck.net/) _(shellcheck.net)_
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