Bash — Cheat Sheet

Tags: bash, shell, scripting, linux, cli

Level: intermediate

TL;DR: Essential syntax, control structures, and command-line execution patterns for Bash scripting and daily terminal usage.

Key concepts

Bash scripts execute sequentially and require a valid shebang (#!/bin/bash) at the top.

Variables are assigned without spaces (VAR="value") and referenced with a dollar sign ($VAR).

Exit status ($?) indicates the success (0) or failure (>0) of the most recently executed command.

Pipes (|) chain commands by passing standard output from one as standard input to the next.

Output redirection uses > to overwrite and >> to append to a file.

Positional parameters ($1, $2) handle arguments passed to scripts or functions.

Command substitution ($(command)) captures the output of a command into a variable.

The test construct ([[ condition ]]) is preferred over [ for string and file evaluations.

Typical questions and ready answers

Q: What is the difference between single and double quotes in Bash?R: Single quotes preserve the literal value of all characters. Double quotes allow for variable expansion and command substitution.

Q: How do you redirect both standard error and standard output to the same file?R: I use command > file.txt 2>&1 or the shorter syntax command &> file.txt.

Q: How do you handle numerical comparisons versus string comparisons?R: I use operators like -eq, -lt, and -gt for integers, and =, !=, <, > for strings.

Pitfalls and common mistakes

Adding spaces around the assignment operator (e.g., VAR = "value" causes a command not found error).

Failing to wrap variable references in double quotes (e.g., "$VAR"), which breaks scripts if the variable contains spaces or is empty.

Using a trailing slash in rsync or copy commands incorrectly, which alters whether the directory itself or its contents are copied.

Snippets and commands

# Basic if statement
if [[ -f "/etc/hosts" ]]; then
  echo "File exists."
else
  echo "File not found."
fi

# For loop over files
for file in *.txt; do
  echo "Processing $file"
done

# Function with arguments
greet() {
  local name=$1
  echo "Hello, $name!"
}
greet "World"