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 |

| 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!