855 lines
20 KiB
Markdown
855 lines
20 KiB
Markdown
---
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title: Bash scripting
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category: CLI
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tags: [Featured]
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updated: 2020-07-05
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keywords:
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- Variables
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- Functions
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- Interpolation
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- Brace expansions
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- Loops
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- Conditional execution
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- Command substitution
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---
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## Getting started
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{: .-three-column}
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### Introduction
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{: .-intro}
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This is a quick reference to getting started with Bash scripting.
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- [Learn bash in y minutes](https://learnxinyminutes.com/docs/bash/) _(learnxinyminutes.com)_
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- [Bash Guide](http://mywiki.wooledge.org/BashGuide) _(mywiki.wooledge.org)_
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- [Bash Hackers Wiki](https://web.archive.org/web/20230406205817/https://wiki.bash-hackers.org/) _(wiki.bash-hackers.org)_
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### Example
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```bash
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#!/usr/bin/env bash
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name="John"
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echo "Hello $name!"
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```
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### Variables
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```bash
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name="John"
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echo $name # see below
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echo "$name"
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echo "${name}!"
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```
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Generally quote your variables unless they contain wildcards to expand or command fragments.
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```bash
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wildcard="*.txt"
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options="iv"
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cp -$options $wildcard /tmp
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```
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### String quotes
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```bash
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name="John"
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echo "Hi $name" #=> Hi John
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echo 'Hi $name' #=> Hi $name
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```
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### Shell execution
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```bash
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echo "I'm in $(pwd)"
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echo "I'm in `pwd`" # obsolescent
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# Same
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```
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See [Command substitution](https://web.archive.org/web/20230326081741/https://wiki.bash-hackers.org/syntax/expansion/cmdsubst)
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### Conditional execution
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```bash
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git commit && git push
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git commit || echo "Commit failed"
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```
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### Functions
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{: id='functions-example'}
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```bash
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get_name() {
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echo "John"
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}
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echo "You are $(get_name)"
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```
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See: [Functions](#functions)
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### Conditionals
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{: id='conditionals-example'}
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```bash
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if [[ -z "$string" ]]; then
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echo "String is empty"
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elif [[ -n "$string" ]]; then
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echo "String is not empty"
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fi
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```
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See: [Conditionals](#conditionals)
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### Strict mode
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```bash
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set -euo pipefail
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IFS=$'\n\t'
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```
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See: [Unofficial bash strict mode](http://redsymbol.net/articles/unofficial-bash-strict-mode/)
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### Brace expansion
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```bash
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echo {A,B}.js
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```
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| Expression | Description |
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| ---------------------- | --------------------- |
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| `{A,B}` | Same as `A B` |
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| `{A,B}.js` | Same as `A.js B.js` |
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| `{1..5}` | Same as `1 2 3 4 5` |
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| <code>{{1..3},{7..9}}</code> | Same as `1 2 3 7 8 9` |
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See: [Brace expansion](https://web.archive.org/web/20230207192110/https://wiki.bash-hackers.org/syntax/expansion/brace)
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## Parameter expansions
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{: .-three-column}
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### Basics
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```bash
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name="John"
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echo "${name}"
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echo "${name/J/j}" #=> "john" (substitution)
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echo "${name:0:2}" #=> "Jo" (slicing)
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echo "${name::2}" #=> "Jo" (slicing)
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echo "${name::-1}" #=> "Joh" (slicing)
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echo "${name:(-1)}" #=> "n" (slicing from right)
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echo "${name:(-2):1}" #=> "h" (slicing from right)
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echo "${food:-Cake}" #=> $food or "Cake"
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```
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```bash
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length=2
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echo "${name:0:length}" #=> "Jo"
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```
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See: [Parameter expansion](https://web.archive.org/web/20230408142504/https://wiki.bash-hackers.org/syntax/pe)
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```bash
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str="/path/to/foo.cpp"
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echo "${str%.cpp}" # /path/to/foo
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echo "${str%.cpp}.o" # /path/to/foo.o
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echo "${str%/*}" # /path/to
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echo "${str##*.}" # cpp (extension)
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echo "${str##*/}" # foo.cpp (basepath)
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echo "${str#*/}" # path/to/foo.cpp
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echo "${str##*/}" # foo.cpp
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echo "${str/foo/bar}" # /path/to/bar.cpp
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```
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```bash
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str="Hello world"
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echo "${str:6:5}" # "world"
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echo "${str: -5:5}" # "world"
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```
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```bash
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src="/path/to/foo.cpp"
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base=${src##*/} #=> "foo.cpp" (basepath)
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dir=${src%$base} #=> "/path/to/" (dirpath)
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```
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### Prefix name expansion
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```bash
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prefix_a=one
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prefix_b=two
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echo ${!prefix_*} # all variables names starting with `prefix_`
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prefix_a prefix_b
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```
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### Indirection
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```bash
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name=joe
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pointer=name
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echo ${!pointer}
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joe
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```
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### Substitution
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| Code | Description |
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| ----------------- | ------------------- |
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| `${foo%suffix}` | Remove suffix |
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| `${foo#prefix}` | Remove prefix |
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| --- | --- |
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| `${foo%%suffix}` | Remove long suffix |
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| `${foo/%suffix}` | Remove long suffix |
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| `${foo##prefix}` | Remove long prefix |
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| `${foo/#prefix}` | Remove long prefix |
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| --- | --- |
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| `${foo/from/to}` | Replace first match |
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| `${foo//from/to}` | Replace all |
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| --- | --- |
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| `${foo/%from/to}` | Replace suffix |
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| `${foo/#from/to}` | Replace prefix |
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### Comments
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```bash
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# Single line comment
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```
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```bash
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: '
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This is a
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multi line
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comment
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'
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```
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### Substrings
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| Expression | Description |
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| --------------- | ------------------------------ |
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| `${foo:0:3}` | Substring _(position, length)_ |
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| `${foo:(-3):3}` | Substring from the right |
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### Length
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| Expression | Description |
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| ---------- | ---------------- |
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| `${#foo}` | Length of `$foo` |
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### Manipulation
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```bash
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str="HELLO WORLD!"
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echo "${str,}" #=> "hELLO WORLD!" (lowercase 1st letter)
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echo "${str,,}" #=> "hello world!" (all lowercase)
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str="hello world!"
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echo "${str^}" #=> "Hello world!" (uppercase 1st letter)
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echo "${str^^}" #=> "HELLO WORLD!" (all uppercase)
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```
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### Default values
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| Expression | Description |
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| ----------------- | -------------------------------------------------------- |
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| `${foo:-val}` | `$foo`, or `val` if unset (or null) |
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| `${foo:=val}` | Set `$foo` to `val` if unset (or null) |
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| `${foo:+val}` | `val` if `$foo` is set (and not null) |
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| `${foo:?message}` | Show error message and exit if `$foo` is unset (or null) |
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Omitting the `:` removes the (non)nullity checks, e.g. `${foo-val}` expands to `val` if unset otherwise `$foo`.
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## Loops
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{: .-three-column}
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### Basic for loop
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```bash
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for i in /etc/rc.*; do
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echo "$i"
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done
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```
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### C-like for loop
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```bash
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for ((i = 0 ; i < 100 ; i++)); do
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echo "$i"
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done
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```
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### Ranges
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```bash
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for i in {1..5}; do
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echo "Welcome $i"
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done
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```
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#### With step size
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```bash
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for i in {5..50..5}; do
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echo "Welcome $i"
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done
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```
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### Reading lines
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```bash
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while read -r line; do
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echo "$line"
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done <file.txt
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```
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### Forever
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```bash
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while true; do
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···
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done
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```
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## Functions
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{: .-three-column}
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### Defining functions
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```bash
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myfunc() {
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echo "hello $1"
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}
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```
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```bash
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# Same as above (alternate syntax)
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function myfunc {
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echo "hello $1"
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}
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```
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```bash
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myfunc "John"
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```
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### Returning values
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```bash
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myfunc() {
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local myresult='some value'
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echo "$myresult"
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}
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```
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```bash
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result=$(myfunc)
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```
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### Raising errors
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```bash
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myfunc() {
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return 1
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}
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```
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```bash
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if myfunc; then
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echo "success"
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else
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echo "failure"
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fi
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```
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### Arguments
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| Expression | Description |
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| ---------- | ---------------------------------------------- |
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| `$#` | Number of arguments |
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| `$*` | All positional arguments (as a single word) |
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| `$@` | All positional arguments (as separate strings) |
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| `$1` | First argument |
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| `$_` | Last argument of the previous command |
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**Note**: `$@` and `$*` must be quoted in order to perform as described.
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Otherwise, they do exactly the same thing (arguments as separate strings).
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See [Special parameters](https://web.archive.org/web/20230318164746/https://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).
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## Conditionals
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{: .-three-column}
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### Conditions
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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.
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| Condition | Description |
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| ------------------------ | --------------------- |
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| `[[ -z STRING ]]` | Empty string |
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| `[[ -n STRING ]]` | Not empty string |
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| `[[ STRING == STRING ]]` | Equal |
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| `[[ STRING != STRING ]]` | Not Equal |
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| --- | --- |
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| `[[ NUM -eq NUM ]]` | Equal |
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| `[[ NUM -ne NUM ]]` | Not equal |
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| `[[ NUM -lt NUM ]]` | Less than |
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| `[[ NUM -le NUM ]]` | Less than or equal |
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| `[[ NUM -gt NUM ]]` | Greater than |
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| `[[ NUM -ge NUM ]]` | Greater than or equal |
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| --- | --- |
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| `[[ STRING =~ STRING ]]` | Regexp |
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| --- | --- |
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| `(( NUM < NUM ))` | Numeric conditions |
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#### More conditions
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| Condition | Description |
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| -------------------- | ------------------------ |
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| `[[ -o noclobber ]]` | If OPTIONNAME is enabled |
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| --- | --- |
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| `[[ ! EXPR ]]` | Not |
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| `[[ X && Y ]]` | And |
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| `[[ X || Y ]]` | Or |
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### File conditions
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| Condition | Description |
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| ----------------------- | ----------------------- |
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| `[[ -e FILE ]]` | Exists |
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| `[[ -r FILE ]]` | Readable |
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| `[[ -h FILE ]]` | Symlink |
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| `[[ -d FILE ]]` | Directory |
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| `[[ -w FILE ]]` | Writable |
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| `[[ -s FILE ]]` | Size is > 0 bytes |
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| `[[ -f FILE ]]` | File |
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| `[[ -x FILE ]]` | Executable |
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| --- | --- |
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| `[[ FILE1 -nt FILE2 ]]` | 1 is more recent than 2 |
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| `[[ FILE1 -ot FILE2 ]]` | 2 is more recent than 1 |
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| `[[ FILE1 -ef FILE2 ]]` | Same files |
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### Example
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```bash
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# String
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if [[ -z "$string" ]]; then
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echo "String is empty"
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elif [[ -n "$string" ]]; then
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echo "String is not empty"
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else
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echo "This never happens"
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fi
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```
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```bash
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# Combinations
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if [[ X && Y ]]; then
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...
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fi
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```
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```bash
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# Equal
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if [[ "$A" == "$B" ]]
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```
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```bash
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# Regex
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if [[ "A" =~ . ]]
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```
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```bash
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if (( $a < $b )); then
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echo "$a is smaller than $b"
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fi
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```
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```bash
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if [[ -e "file.txt" ]]; then
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echo "file exists"
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fi
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```
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## Arrays
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### Defining arrays
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```bash
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Fruits=('Apple' 'Banana' 'Orange')
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```
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```bash
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Fruits[0]="Apple"
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Fruits[1]="Banana"
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Fruits[2]="Orange"
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```
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### Working with arrays
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```bash
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echo "${Fruits[0]}" # Element #0
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echo "${Fruits[-1]}" # Last element
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echo "${Fruits[@]}" # All elements, space-separated
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echo "${#Fruits[@]}" # Number of elements
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echo "${#Fruits}" # String length of the 1st element
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echo "${#Fruits[3]}" # String length of the Nth element
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echo "${Fruits[@]:3:2}" # Range (from position 3, length 2)
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echo "${!Fruits[@]}" # Keys of all elements, space-separated
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```
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### Operations
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```bash
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Fruits=("${Fruits[@]}" "Watermelon") # Push
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Fruits+=('Watermelon') # Also Push
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Fruits=( "${Fruits[@]/Ap*/}" ) # Remove by regex match
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unset Fruits[2] # Remove one item
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Fruits=("${Fruits[@]}") # Duplicate
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Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
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lines=(`cat "logfile"`) # Read from file
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```
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### Iteration
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```bash
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for i in "${arrayName[@]}"; do
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echo "$i"
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done
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```
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## Dictionaries
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{: .-three-column}
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### Defining
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```bash
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declare -A sounds
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```
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```bash
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sounds[dog]="bark"
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sounds[cow]="moo"
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sounds[bird]="tweet"
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sounds[wolf]="howl"
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```
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Declares `sound` as a Dictionary object (aka associative array).
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### Working with dictionaries
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```bash
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echo "${sounds[dog]}" # Dog's sound
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echo "${sounds[@]}" # All values
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echo "${!sounds[@]}" # All keys
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echo "${#sounds[@]}" # Number of elements
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unset sounds[dog] # Delete dog
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```
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### Iteration
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#### Iterate over values
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```bash
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for val in "${sounds[@]}"; do
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echo "$val"
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done
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```
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#### Iterate over keys
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```bash
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for key in "${!sounds[@]}"; do
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echo "$key"
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done
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```
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## Options
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### Options
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```bash
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set -o noclobber # Avoid overlay files (echo "hi" > foo)
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set -o errexit # Used to exit upon error, avoiding cascading errors
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set -o pipefail # Unveils hidden failures
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set -o nounset # Exposes unset variables
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```
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### Glob options
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```bash
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shopt -s nullglob # Non-matching globs are removed ('*.foo' => '')
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shopt -s failglob # Non-matching globs throw errors
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shopt -s nocaseglob # Case insensitive globs
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shopt -s dotglob # Wildcards match dotfiles ("*.sh" => ".foo.sh")
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shopt -s globstar # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')
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```
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Set `GLOBIGNORE` as a colon-separated list of patterns to be removed from glob
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matches.
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## History
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### Commands
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| Command | Description |
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| --------------------- | ----------------------------------------- |
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| `history` | Show history |
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| `shopt -s histverify` | Don't execute expanded result immediately |
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### Expansions
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| Expression | Description |
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| ------------ | ---------------------------------------------------- |
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| `!$` | Expand last parameter of most recent command |
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| `!*` | Expand all parameters of most recent command |
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| `!-n` | Expand `n`th most recent command |
|
|
| `!n` | Expand `n`th command in history |
|
|
| `!<command>` | Expand most recent invocation of command `<command>` |
|
|
|
|
### Operations
|
|
|
|
| Code | Description |
|
|
| -------------------- | --------------------------------------------------------------------- |
|
|
| `!!` | Execute last command again |
|
|
| `!!:s/<FROM>/<TO>/` | Replace first occurrence of `<FROM>` to `<TO>` in most recent command |
|
|
| `!!:gs/<FROM>/<TO>/` | Replace all occurrences of `<FROM>` to `<TO>` in most recent command |
|
|
| `!$: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
|
|
```
|
|
|
|
```bash
|
|
declare -i count # Declare as type integer
|
|
count+=1 # Increment
|
|
```
|
|
|
|
### 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 >output.txt 2>&1 # stdout and stderr to (file), equivalent to &>
|
|
python hello.py &>/dev/null # stdout and stderr to (null)
|
|
echo "$0: warning: too many users" >&2 # print diagnostic message to stderr
|
|
```
|
|
|
|
```bash
|
|
python hello.py < foo.txt # feed foo.txt to stdin for python
|
|
diff <(ls -r) <(ls) # Compare two stdout without files
|
|
```
|
|
|
|
### 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"
|
|
|
|
printf '%s\n' '#!/bin/bash' 'echo hello' >file
|
|
# format string is applied to each group of arguments
|
|
printf '%i+%i=%i\n' 1 2 3 4 5 9
|
|
```
|
|
|
|
### Transform strings
|
|
|
|
| Command option | Description |
|
|
| -------------- | --------------------------------------------------- |
|
|
| `-c` | Operations apply to characters not in the given set |
|
|
| `-d` | Delete characters |
|
|
| `-s` | Replaces repeated characters with single occurrence |
|
|
| `-t` | Truncates |
|
|
| `[:upper:]` | All upper case letters |
|
|
| `[:lower:]` | All lower case letters |
|
|
| `[:digit:]` | All digits |
|
|
| `[:space:]` | All whitespace |
|
|
| `[:alpha:]` | All letters |
|
|
| `[:alnum:]` | All letters and digits |
|
|
|
|
#### Example
|
|
|
|
```bash
|
|
echo "Welcome To Devhints" | tr '[:lower:]' '[:upper:]'
|
|
WELCOME TO DEVHINTS
|
|
```
|
|
|
|
### 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 [[ "$1" == '--' ]]; then shift; fi
|
|
```
|
|
|
|
### Heredoc
|
|
|
|
```sh
|
|
cat <<END
|
|
hello world
|
|
END
|
|
```
|
|
|
|
### Reading input
|
|
|
|
```bash
|
|
echo -n "Proceed? [y/n]: "
|
|
read -r ans
|
|
echo "$ans"
|
|
```
|
|
|
|
The `-r` option disables a peculiar legacy behavior with backslashes.
|
|
|
|
```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 |
|
|
| `$0` | Filename of the shell script |
|
|
| `$_` | Last argument of the previous command |
|
|
| `${PIPESTATUS[n]}` | return value of piped commands (array) |
|
|
|
|
See [Special parameters](https://web.archive.org/web/20230318164746/https://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](https://web.archive.org/web/20230406205817/https://wiki.bash-hackers.org/) _(bash-hackers.org)_
|
|
- [Shell vars](https://web.archive.org/web/20230318164746/https://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)_
|