bash-linux

Bash Linux Patterns

Essential patterns for Bash on Linux/macOS.

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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"

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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

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3. Process Management

Task	Command
List processes	ps aux
Find by name	ps aux | grep node
Stop owned process	kill -TERM <verified-task-PID>; verify exit, escalate only if necessary
Find port user	lsof -i :3000
Inspect port owner	lsof -nP -iTCP:3000 -sTCP:LISTEN; verify ownership before stopping
Background	npm run dev &
Jobs	jobs -l
Bring to front	fg %1

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4. Text Processing

Core Tools

Tool	Purpose	Example
grep	Search	grep -rn "TODO" src/
sed	Preview replacement	sed 's/old/new/g' file.txt (in-place flags differ on macOS/GNU)
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

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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"

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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

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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 "$@"

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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

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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

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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 only with non-secret inputs; prints expanded arguments

Trap for cleanup

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

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Remember: Bash is text-based. Use && for success chains and quote variables. set -e has conditional/pipeline exceptions; explicitly check consequential operations and never infer complete safety from shell flags.

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.