---
name: bash-linux
description: Bash/Linux terminal patterns. Critical commands, piping, error handling, scripting. Use when working on macOS or Linux systems.
allowed-tools: Read, Write, Edit, Glob, Grep, Bash
---

# 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!