bash-linux

SKILL.md

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!