Bash (Bourne Again SHell) is a powerful scripting language used in many Unix-like operating systems. This cheatsheet covers basic and intermediate concepts useful for writing effective Bash scripts.
1. Script Structure
1.1 Shebang
#!/bin/bash
# This line tells the system to use bash to interpret the script.1.2 Making a Script Executable
chmod +x scriptname.sh
# Adds executable permissions to the script.2. Variables
2.1 Assigning Values
name="John"
age=30
# Strings need quotes; numbers do not.2.2 Using Variables
echo $name
echo ${age} years old
# Curly braces are optional but useful for clarity.3. User Input
3.1 Reading Input
read -p "Enter your name: " name
echo "Hello, $name!"
# Prompts user for input.4. Conditional Statements
4.1 If Statement
if [ $age -gt 18 ]
then
echo "You are an adult."
elif [ $age -eq 18 ]
then
echo "Just turned 18!"
else
echo "You are not an adult."
fi4.2 Test Conditions
-eq: equal-ne: not equal-gt: greater than-lt: less than-ge: greater than or equal to-le: less than or equal to
5. Loops
5.1 For Loop
for i in {1..5}
do
echo "Looping: number $i"
done5.2 While Loop
while [ $count -le 5 ]
do
echo "Count: $count"
count=$((count + 1))
done6. Functions
6.1 Defining a Function
greet() {
echo "Hello, $1!"
}
# $1 is the first argument passed to the function.6.2 Calling a Function
greet "World"7. Arrays
7.1 Declaring an Array
colors=('red' 'green' 'blue')7.2 Accessing Array Elements
echo ${colors[0]} # red
echo ${colors[@]} # all elements8. File and Directory Operations
8.1 Checking for a File
if [ -f "$file" ]; then
echo "$file exists."
fi8.2 Reading from a File
while read line; do
echo $line
done < filename.txt9. Debugging
9.1 Debug Mode
bash -x scriptname.sh
# Prints each command and its expanded arguments before executing it.9.2 Setting Debug Mode Within a Script
set -x # Turn debug mode on
# Script lines here...
set +x # Turn debug mode off10. Best Practices
- Quote your variables to handle spaces:
"$var" - Use
[[ ]]for tests to prevent script errors. - Prefer absolute paths for files to avoid wrong file execution.