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!

When to Use

This skill is applicable to execute the workflow or actions described in the overview.

Limitations

• Use this skill only when the task clearly matches the scope described above.

• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.

• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.