Modern Bash Cheat Sheet for Software Engineers

My notes on bash, initially generated by Claude, but will be expanded upon as I find more notable things to point out.

Basic Script Structure

#!/usr/bin/env bash
# Use this shebang - more portable than #!/bin/bash

set -euo pipefail  # THE MOST IMPORTANT LINE
# -e: exit on error
# -u: exit on undefined variable
# -o pipefail: exit if any command in a pipe fails

Variables

# Define variables (no spaces around =)
name="value"
count=42

# Use variables (always quote them!)
echo "$name"
echo "${name}"  # preferred - more explicit

# Command substitution (modern way)
current_date=$(date +%Y-%m-%d)
files=$(ls *.txt)

# Don't use backticks anymore: `date` (old style)

Conditionals

# Check if file exists
if [[ -f "myfile.txt" ]]; then
    echo "File exists"
fi

# Check if directory exists
if [[ -d "mydir" ]]; then
    echo "Directory exists"
fi

# Check if variable is empty
if [[ -z "$var" ]]; then
    echo "Variable is empty"
fi

# Check if variable is NOT empty
if [[ -n "$var" ]]; then
    echo "Variable has content"
fi

# String comparison
if [[ "$name" == "production" ]]; then
    echo "Running in production"
fi

# Numeric comparison
if [[ $count -gt 10 ]]; then
    echo "Count is greater than 10"
fi
# -eq (equal), -ne (not equal), -lt (less than), -le (less or equal), -gt, -ge

# Multiple conditions
if [[ -f "file.txt" && "$env" == "prod" ]]; then
    echo "File exists and in prod"
fi

# Use [[ ]] not [ ] - it's more modern and safer

Loops

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

# Loop over lines in a file
while IFS= read -r line; do
    echo "$line"
done < input.txt

# Loop with counter
for i in {1..5}; do
    echo "Iteration $i"
done

# Loop over array
files=("app.log" "error.log" "access.log")
for file in "${files[@]}"; do
    echo "$file"
done

Functions

# Define a function
my_function() {
    local input="$1"  # first argument
    local output="$2"  # second argument
    
    echo "Processing $input"
    return 0  # return exit code (0 = success)
}

# Call a function
my_function "value1" "value2"

# Capture function output
result=$(my_function "test")

Exit Codes & Error Handling

# Check if last command succeeded
if command; then
    echo "Success"
else
    echo "Failed"
fi

# Check exit code
some_command
if [[ $? -eq 0 ]]; then
    echo "Command succeeded"
fi

# Exit script with error
echo "Error occurred" >&2  # print to stderr
exit 1

# Run command but don't exit on failure
set +e
risky_command || echo "Command failed but continuing"
set -e

Input/Output & Pipes

# Redirect output to file (overwrites)
echo "text" > output.txt

# Append to file
echo "more text" >> output.txt

# Redirect stderr to stdout
command 2>&1

# Redirect stderr to file
command 2> error.log

# Redirect both stdout and stderr
command &> all_output.log

# Pipe commands together
cat file.txt | grep "error" | wc -l

# Better: use input redirection instead of cat
grep "error" < file.txt | wc -l

String Manipulation

text="hello world"

# Length of string
echo "${#text}"

# Substring (position:length)
echo "${text:0:5}"  # "hello"

# Replace first occurrence
echo "${text/world/universe}"

# Replace all occurrences
echo "${text//o/0}"  # "hell0 w0rld"

# Remove prefix
file="backup-2024-11-03.tar.gz"
echo "${file#backup-}"  # "2024-11-03.tar.gz"

# Remove suffix
echo "${file%.tar.gz}"  # "backup-2024-11-03"

# Uppercase/lowercase
echo "${text^^}"  # "HELLO WORLD"
echo "${text,,}"  # "hello world"

Common Patterns

# Check if script is run with sudo
if [[ $EUID -eq 0 ]]; then
    echo "Running as root"
fi

# Get script directory (works even with symlinks)
script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"

# Default value for variable
name="${1:-default_name}"  # use $1 or "default_name" if empty

# Check if command exists
if command -v docker &> /dev/null; then
    echo "Docker is installed"
fi

# Create temp file safely
tmpfile=$(mktemp)
trap "rm -f $tmpfile" EXIT  # cleanup on exit

# Run commands in parallel and wait
long_task1 &
long_task2 &
wait  # wait for all background jobs

Arrays

# Create array
servers=("web1" "web2" "db1")

# Add to array
servers+=("cache1")

# Access elements
echo "${servers[0]}"  # first element
echo "${servers[@]}"  # all elements
echo "${#servers[@]}"  # array length

# Loop over array
for server in "${servers[@]}"; do
    echo "$server"
done

Useful Commands for Daily Work

# Find files modified in last 24 hours
find . -mtime -1

# Find and delete
find . -name "*.tmp" -delete

# Search in files
grep -r "error" /var/log/

# Count lines/words/chars
wc -l file.txt

# Sort and remove duplicates
sort file.txt | uniq

# Show disk usage
du -sh /path/to/dir

# Show running processes
ps aux | grep java

# Monitor logs in real-time
tail -f /var/log/app.log

# Download file
curl -O https://example.com/file.zip

# Make HTTP request
curl -X POST -H "Content-Type: application/json" \
  -d '{"key":"value"}' \
  https://api.example.com/endpoint

# Extract tar.gz
tar -xzf archive.tar.gz

# Create tar.gz
tar -czf archive.tar.gz directory/

# Check if port is listening
netstat -tuln | grep 8080
# or on newer systems:
ss -tuln | grep 8080

Best Practices

1. Always quote variables: "$var" not $var
2. Use set -euo pipefail at the top of every script
3. Use [[ ]] not [ ] for conditions
4. Use $( ) not backticks for command substitution
5. Make scripts executable: chmod +x script.sh
6. Check your scripts: Use shellcheck - it’s amazing
7. Use local for function variables
8. Exit with meaningful codes: 0 = success, non-zero = failure

Common Pitfalls to Avoid

# BAD: Unquoted variable can break with spaces
for file in $files; do  # DON'T DO THIS

# GOOD: Quoted variable
for file in "$files"; do

# BAD: Using [ ] instead of [[ ]]
if [ $var == "test" ]; then  # can break if $var is empty

# GOOD: Using [[ ]] which handles empty vars
if [[ $var == "test" ]]; then

# BAD: Not checking if command exists
docker build .

# GOOD: Check first
if ! command -v docker &> /dev/null; then
    echo "Docker not found" >&2
    exit 1
fi
docker build .

Quick Reference Card

Task	Command
Run script	bash script.sh or ./script.sh
Make executable	chmod +x script.sh
Exit on error	set -e
Debug mode	bash -x script.sh or set -x
Check syntax	bash -n script.sh
Get exit code	$?
Current directory	$(pwd)
Script location	${BASH_SOURCE[0]}
All arguments	"$@"
Argument count	$#
Background job	command &
Wait for jobs	wait

Tools

• https://explainshell.com/
• https://www.shellcheck.net/