---
title: Bash scripting
category: CLI
layout: 2017/sheet
tags: [Featured]
updated: 2018-12-25
keywords:
  - Variables
  - Functions
  - Interpolation
  - Brace expansions
  - Loops
  - Conditional execution
  - Command substitution
---

Getting started
---------------
{: .-three-column}

### 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
```

| `{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##*.}     # 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

| `${FOO:0:3}`  | Substring _(position, length)_ |
| `${FOO:-3:3}` | Substring from the right |

### Length

| `${#FOO}` | Length of `$FOO` |

### Default values

| `${FOO:-val}`        | `$FOO`, or `val` if not set |
| `${FOO:=val}`        | Set `$FOO` to `val` if not set |
| `${FOO:+val}`        | `val` if `$FOO` is set |
| `${FOO:?message}`    | Show error message and exit if `$FOO` is not set |

The `:` is optional (eg, `${FOO=word}` works)

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
< 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 arguments                      |
| `$@`       | All arguments, starting from first |
| `$1`       | First argument                     |

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    |

| 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
if ping -c 1 google.com; then
  echo "It appears you have a working internet connection"
fi
```` 

```bash
if grep -q 'foo' ~/.bash_history; then
  echo "You appear to have typed 'foo' in the past"
fi
```

```bash
# String
if [[ -z "$string" ]]; then
  echo "String is empty"
elif [[ -n "$string" ]]; then
  echo "String is not empty"
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[@]}           # 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)
```

### 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
set -o nullglob    # Non-matching globs are removed  ('*.foo' => '')
set -o failglob    # Non-matching globs throw errors
set -o nocaseglob  # Case insensitive globs
set -o globdots    # Wildcards match dotfiles ("*.sh" => ".foo.sh")
set -o 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

| `history` | Show history |
| `shopt -s histverify` | Don't execute expanded result immediately |

### Expansions

| `!$` | 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

| `!!` | 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

| `!!: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..200
```

### 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
```

### 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

| `$?` | Exit status of last task |
| `$!` | PID of last background task |
| `$$` | PID of shell |

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
```

## 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)_
