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---
title: Bash scripting
category: CLI
layout: 2017/sheet
tags: [Featured]
updated: 2018-11-19
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 ${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
```
### 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 arguments |
| `$@` | All arguments, starting from first |
| `$1` | First argument |
See [Special parameters](http://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).
Conditionals
------------
{: .-three-column}
### Conditions
| Condition | Description |
| --- | --- |
| `[ STRING = STRING ]` | Equal |
| `[ STRING != STRING ]` | Not Equal |
| `[ -z STRING ]` | Empty string |
| `[ -n STRING ]` | Not empty string |
| --- | --- |
| `[ NUM -eq NUM ]` | Equal |
| `[ NUM -ne NUM ]` | Not equal |
| `[ NUM -lt NUM ]` | Less than |
| `[ NUM -le NUM ]` | Less than or equal |
| `[ NUM -gt NUM ]` | Greater than |
| `[ NUM -ge NUM ]` | Greater than or equal |
| --- | --- |
| `[[ STRING =~ STRING ]]` | Regexp |
| --- | --- |
| `(( NUM < NUM ))` | Numeric conditions |
| Condition | Description |
| --- | --- |
| `[ -o noclobber ]` | If OPTIONNAME is enabled |
| --- | --- |
| `[ ! EXPR ]` | Not |
| `[ X ] && [ Y ]` | And |
| `[ X ] || [ Y ]` | Or |
### File conditions
| Condition | Description |
| --- | --- |
| `[ -e FILE ]` | Exists |
| `[ -r FILE ]` | Readable |
| `[ -h FILE ]` | Symlink |
| `[ -d FILE ]` | Directory |
| `[ -w FILE ]` | Writable |
| `[ -s FILE ]` | Size is > 0 bytes |
| `[ -f FILE ]` | File |
| `[ -x FILE ]` | Executable |
| --- | --- |
| `[ FILE1 -nt FILE2 ]` | 1 is more recent than 2 |
| `[ FILE1 -ot FILE2 ]` | 2 is more recent than 1 |
| `[ FILE1 -ef FILE2 ]` | Same files |
### Example
```bash
# String
if [ -z "$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 ))
```
```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 $i
done
```
Options
-------
### Options
```bash
set -o noclobber # Avoid overlay files (echo "hi" > foo)
set -o errexit # Used to exit upon error, avoiding cascading errors
set -o pipefail # Unveils hidden failures
set -o nounset # Exposes unset variables
```
### Glob options
```bash
set -o nullglob # Non-matching globs are removed ('*.foo' => '')
set -o failglob # Non-matching globs throw errors
set -o nocaseglob # Case insensitive globs
set -o globdots # Wildcards match dotfiles ("*.sh" => ".foo.sh")
set -o globstar # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')
```
Set `GLOBIGNORE` as a colon-separated list of patterns to be removed from glob
matches.
History
-------
### Commands
| `history` | Show history |
| `shopt -s histverify` | Don't execute expanded result immediately |
### Expansions
| `!$` | 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
```
### 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)_
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