Linux Shell Scripting Fundamentals

Once you are comfortable running commands individually (see [LinuxCommandLineEssentials](LinuxCommandLineEssentials)), the next step is combining them into **scripts** — files that run a sequence of commands automatically. If you have ever written a batch file (.bat) in Windows or used PowerShell scripts, the concept is the same. The syntax is different, and bash scripting is both more powerful and more quirky than either.

Shell scripting is where Linux becomes genuinely transformative. The ability to automate anything you can do in the terminal — backups, deployments, log analysis, system monitoring — is the practical payoff of learning the command line.

Your First Script

```bash

!/bin/bash

My first script — greet the user and show system info

echo "Hello, $(whoami)!"

echo "Today is $(date '+%A, %B %d, %Y')"

echo "You are on $(hostname)"

echo "Uptime: $(uptime -p)"

```

Save this as `info.sh`, make it executable, and run it:

```bash

chmod +x info.sh

./info.sh

```

**Key points:**

- `#!/bin/bash` (the "shebang") tells the system which interpreter to use

- `#` starts a comment

- `$(command)` runs a command and inserts its output

- `echo` prints text

Variables

```bash

Assigning variables (NO spaces around =)

name="Jake"

count=42

today=$(date '+%Y-%m-%d')

Using variables (prefix with $)

echo "Hello, $name"

echo "Count: $count"

echo "Date: $today"

Quoting matters

file="my document.txt"

cp "$file" backup/ # CORRECT: quotes preserve spaces

cp $file backup/ # WRONG: bash sees two arguments: "my" and "document.txt"

```

**Always quote your variables.** `"$variable"` prevents word splitting on spaces. This is the single most common source of bugs in shell scripts.

Conditionals

```bash

!/bin/bash

if [ -f "/etc/nginx/nginx.conf" ]; then

echo "Nginx is configured"

elif [ -f "/etc/apache2/apache2.conf" ]; then

echo "Apache is configured"

else

echo "No web server config found"

```

Common Test Operators

| Operator | Meaning | Example |

|---------|---------|--------|

| `-f file` | File exists and is a regular file | `[ -f config.txt ]` |

| `-d dir` | Directory exists | `[ -d /var/log ]` |

| `-e path` | Path exists (any type) | `[ -e /tmp/lock ]` |

| `-z "$var"` | Variable is empty | `[ -z "$name" ]` |

| `-n "$var"` | Variable is not empty | `[ -n "$name" ]` |

| `"$a" = "$b"` | Strings are equal | `[ "$user" = "root" ]` |

| `"$a" != "$b"` | Strings differ | `[ "$env" != "prod" ]` |

| `$a -eq $b` | Numbers are equal | `[ $count -eq 0 ]` |

| `$a -gt $b` | Greater than | `[ $size -gt 1000 ]` |

| `$a -lt $b` | Less than | `[ $age -lt 18 ]` |

Loops

```bash

Loop over a list

for server in web01 web02 web03; do

echo "Checking $server..."

ping -c 1 "$server" > /dev/null 2>&1 && echo " up" || echo " down"

done

Loop over files

for file in /var/log/*.log; do

echo "$file: $(wc -l < "$file") lines"

done

Loop with a counter

for i in {1..10}; do

echo "Iteration $i"

done

While loop

count=0

while [ $count -lt 5 ]; do

echo "Count: $count"

count=$((count + 1))

done

Read a file line by line

while IFS= read -r line; do

echo "Processing: $line"

done < input.txt

```

Functions

```bash

!/bin/bash

Define a function

backup_file() {

local source="$1" # First argument

local dest="$2" # Second argument

if [ ! -f "$source" ]; then

echo "ERROR: $source does not exist" >&2

return 1

cp "$source" "$dest"

echo "Backed up $source to $dest"

}

Call the function

backup_file /etc/nginx/nginx.conf /tmp/nginx.conf.bak

```

Practical Script Examples

Backup Script

```bash

!/bin/bash

Daily backup of important directories

BACKUP_DIR="/backup/$(date '+%Y-%m-%d')"

SOURCE_DIRS=("/home/jake/documents" "/home/jake/projects" "/etc")

mkdir -p "$BACKUP_DIR"

for dir in "${SOURCE_DIRS[@]}"; do

dirname=$(basename "$dir")

tar czf "$BACKUP_DIR/${dirname}.tar.gz" "$dir" 2>/dev/null

echo "Backed up $dir"

done

Remove backups older than 30 days

find /backup -maxdepth 1 -type d -mtime +30 -exec rm -rf {} +

echo "Backup complete: $BACKUP_DIR"

```

Log Monitor

```bash

!/bin/bash

Watch a log file for errors and alert

LOGFILE="/var/log/application.log"

ALERT_PATTERN="ERROR|CRITICAL|FATAL"

tail -F "$LOGFILE" | while IFS= read -r line; do

if echo "$line" | grep -qE "$ALERT_PATTERN"; then

echo "[ALERT $(date '+%H:%M:%S')] $line"

done

```

System Health Check

```bash

!/bin/bash

Quick system health report

echo "=== System Health Report ==="

echo "Date: $(date)"

echo

echo "--- Disk Usage ---"

df -h / /home | tail -n +2

echo

echo "--- Memory ---"

free -h | head -2

echo

echo "--- Load Average ---"

uptime

echo

echo "--- Top 5 CPU Processes ---"

ps aux --sort=-%cpu | head -6

echo

echo "--- Failed Services ---"

systemctl --failed --no-legend

```

Scheduling with cron

`cron` runs scripts on a schedule — the Linux equivalent of Windows Task Scheduler, but configured with a text file.

Edit your crontab:

```bash

crontab -e

```

Format: `minute hour day-of-month month day-of-week command`

```

Run backup every day at 2 AM

0 2 * * * /home/jake/scripts/backup.sh

Run health check every 5 minutes

*/5 * * * * /home/jake/scripts/health.sh >> /var/log/health.log 2>&1

Run cleanup every Sunday at midnight

0 0 * * 0 /home/jake/scripts/cleanup.sh

Run at 9 AM on the 1st of every month

0 9 1 * * /home/jake/scripts/monthly-report.sh

```

**Cron tip:** Always use full paths in cron jobs. Cron runs with a minimal environment — commands like `node` or `python3` need their full path (use `which python3` to find it).

Customizing Your Environment

Your shell configuration lives in dotfiles in your home directory:

| File | Purpose |

|------|--------|

| `~/.bashrc` | Runs every time you open a terminal (aliases, functions, prompt) |

| `~/.bash_profile` | Runs on login (environment variables, PATH) |

| `~/.profile` | Generic login configuration (used by many shells) |

Common customizations:

```bash

Add to ~/.bashrc

Useful aliases

alias ll='ls -la'

alias gs='git status'

alias update='sudo apt update && sudo apt upgrade -y'

alias ..='cd ..'

alias ...='cd ../..'

Better history

export HISTSIZE=10000

export HISTFILESIZE=20000

export HISTCONTROL=ignoredups:erasedups

Custom prompt showing git branch

parse_git_branch() {

git branch 2>/dev/null | grep '\*' | sed 's/* //'

}

export PS1='\u@\h:\w$(parse_git_branch)\$ '

```

After editing, apply changes: `source ~/.bashrc`