Bash Linux Patterns

Essential patterns for Bash on Linux/macOS.

1. Operator Syntax

Chaining Commands

Operator Meaning Example

;

Run sequentially cmd1; cmd2

&&

Run if previous succeeded npm install && npm run dev

||

Run if previous failed npm test || echo "Tests failed"

|

Pipe output ls | grep ".js"

2. File Operations

Essential Commands

Task Command

List all ls -la

Find files find . -name "*.js" -type f

File content cat file.txt

First N lines head -n 20 file.txt

Last N lines tail -n 20 file.txt

Follow log tail -f log.txt

Search in files grep -r "pattern" --include="*.js"

File size du -sh *

Disk usage df -h

3. Process Management

Task Command

List processes ps aux

Find by name ps aux | grep node

Kill by PID kill -9 <PID>

Find port user lsof -i :3000

Kill port kill -9 $(lsof -t -i :3000)

Background npm run dev &

Jobs jobs -l

Bring to front fg %1

4. Text Processing

Core Tools

Tool Purpose Example

grep

Search grep -rn "TODO" src/

sed

Replace sed -i 's/old/new/g' file.txt

awk

Extract columns awk '{print $1}' file.txt

cut

Cut fields cut -d',' -f1 data.csv

sort

Sort lines sort -u file.txt

uniq

Unique lines sort file.txt | uniq -c

wc

Count wc -l file.txt

5. Environment Variables

Task Command

View all env or printenv

View one echo $PATH

Set temporary export VAR="value"

Set in script VAR="value" command

Add to PATH export PATH="$PATH:/new/path"

6. Network

Task Command

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

API request curl -X GET https://api.example.com

POST JSON curl -X POST -H "Content-Type: application/json" -d '{"key":"value"}' URL

Check port nc -zv localhost 3000

Network info ifconfig or ip addr

7. Script Template

#!/bin/bash set -euo pipefail # Exit on error, undefined var, pipe fail

Colors (optional)

RED='\033[0;31m' GREEN='\033[0;32m' NC='\033[0m'

Script directory

SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"

Functions

log_info() { echo -e "${GREEN}[INFO]${NC} $1"; } log_error() { echo -e "${RED}[ERROR]${NC} $1" >&2; }

Main

main() { log_info "Starting..." # Your logic here log_info "Done!" }

main "$@"

8. Common Patterns

Check if command exists

if command -v node &> /dev/null; then echo "Node is installed" fi

Default variable value

NAME=${1:-"default_value"}

Read file line by line

while IFS= read -r line; do echo "$line" done < file.txt

Loop over files

for file in *.js; do echo "Processing $file" done

9. Differences from PowerShell

Task PowerShell Bash

List files Get-ChildItem

ls -la

Find files Get-ChildItem -Recurse

find . -type f

Environment $env:VAR

$VAR

String concat "$a$b"

"$a$b" (same)

Null check if ($x)

if [ -n "$x" ]

Pipeline Object-based Text-based

10. Error Handling

Set options

set -e # Exit on error set -u # Exit on undefined variable set -o pipefail # Exit on pipe failure set -x # Debug: print commands

Trap for cleanup

cleanup() { echo "Cleaning up..." rm -f /tmp/tempfile } trap cleanup EXIT

Remember: Bash is text-based. Use && for success chains, set -e for safety, and quote your variables!