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!

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.