---
author: "Devoalda"
authorEmoji: 🐺
title: "Bash Cheatsheet"
date: 2020-07-06T16:47:03+08:00
description: Simple Bash Cheatsheet
draft: false
hideToc: false
enableToc: true
enableTocContent: true
tocPosition: inner
tocLevels: ["h1", "h2", "h3"]
tags:
- bash
- linux
- script
- cheetsheet
series:
- Bash
categories:
- bash
- script
image: images/postImages/Bash_256x256.png
---

# Introduction
This is a simple bash script cheatsheet.

# Variables
## Define variables
```bash
abc=123
hello=321
qwerty=asdf
```
### Assign output of command to variable
``` bash
music=$(mpc)
```

### Argument
```bash
word=$1
```

### Argument and default value
``` bash
words=${1-hello} # If variable not set or null, use default.
FOO=${VARIABLE:=default}  # If variable not set or null, set it to default.
```

{{< alert theme="info" dir="ltr" >}}
Ensure no spaces between variable name and value
{{< /alert >}}

## View variable content
```bash
$ echo $abc
# outputs: 123
$ echo "$(echo "upg")"
# outputs: upg
$ echo '$(echo "upg")'
# outputs: $(echo "upg")
```
# Loops
## For loop
``` bash
for i in {1..5}
do
	echo "Hello $i"
done
```

### {START..END..INCREMENT}
```  bash
for i in {0..10..2}
do 
	echo "Hello $i"
done
```

### Files
``` bash
for file in $HOME
do
	echo $file
done
```

## While loop
``` bash
while [ condition ]
do
	echo "hello"
done
```

### Infinite while loop
``` bash
while true
do
	echo "hello"
done
```
{{< alert theme="info" dir="ltr" >}}
Hit CTRL-C to stop the loop
{{< /alert >}}

### Read File
``` bash
#!/usr/bin/env bash
FILE=${1-hello.txt}

while read line
do
 # use $line variable to process line
	echo $line
done < "$FILE"
```
# If Else Loop
``` bash
if [ condition ]; then
	echo 1
elif [ condition2 ]; then
	echo 2
else
	echo 3
fi
```
# Conditions
| Operator | Description          |
|----------|----------------------|
| -eq      | Equal                |
| -lt      | Less than            |
| -ge      | Greater than         |
| ==       | 2 Strings equal      |
| !=       | 2 Strings not equal  |
| !        | Statement is false   |
| -d       | Directory is present |
| -e       | File is present      |

## Condition Usage Examples
``` bash
if [[ "$1" == "bash" && "$2" = "shell" ]]; then
	echo "$1 $2"

elif [ "$1" == "bash" ]; then
	echo "$1"

elif [ -e "$HOME/hello.txt" ]
	echo "File Present"

elif [ ! -d "$HOME/Documents" ]
	echo "Directory NOT Present"
else
	echo Condition
fi
```
# Case
``` bash
word=$1

case "$word" in
	hello)
		echo hello
		;;

	bye)
		echo bye 
		;;

	*)
		echo Universal
		;;
esac
```
# Functions
``` bash
function fun(){ 
	echo fun
}
```
{{< expand "Credits/References" >}}
[stackoverflow1](https://stackoverflow.com/questions/6697753/difference-between-single-and-double-quotes-in-bash)
[stackoverflow2](https://stackoverflow.com/questions/2013547/assigning-default-values-to-shell-variables-with-a-single-command-in-bash)
{{< /expand >}}