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

bash
#!/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

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

Default variable value

bash
NAME=${1:-"default_value"}

Read file line by line

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

Loop over files

bash
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

bash
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

bash
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!