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` |

| 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` |

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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` |

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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: print commands

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, `set -e` for safety, and quote your variables!

When to Use

This skill is applicable to execute the workfl