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Chapter 6.4 - Functions and Cron

Learning Objectives​

By the end of this chapter, you will be able to:

  • Create and use Bash functions
  • Understand function parameters
  • Schedule tasks with cron
  • Use crontab
  • Understand cron syntax

Bash Functions​

Definition​

#!/bin/bash
# Syntax 1
function_name() {
    commands
}

# Syntax 2
function function_name {
    commands
}

# Syntax 3 (portable)
function function_name() {
    commands
}

Simple Example​

#!/bin/bash
# Simple function
hello() {
    echo "Hello $1!"
}

# Call
hello "Linux"
hello "World"

Parameters​

#!/bin/bash
# Parameters: $1, $2, etc. (local to the function)
my_function() {
    echo "First: $1"
    echo "Second: $2"
    echo "All: $@"
    echo "Count: $#"
}

# Call
my_function "arg1" "arg2" "arg3"

Return Value​

#!/bin/bash
# Return code (0-255)
is_file() {
    if [ -f "$1" ]; then
        return 0  # Success
    else
        return 1  # Failure
    fi
}

# Usage
if is_file "/etc/passwd"; then
    echo "File exists"
fi

Local Variables​

#!/bin/bash
# Local variable (function scope)
my_function() {
    local local_var="value"
    echo "Local: $local_var"
}

# Global variable
global_var="global"

my_function
echo "Global: $global_var"

Function Examples​

Utility Function​

#!/bin/bash
# Logger with timestamp
log() {
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*"
}

# Usage
log "Script start"
log "Error: File not found"
log "Script end"

Verification Function​

#!/bin/bash
# Check if user exists
user_exists() {
    id "$1" &>/dev/null
    return $?
}

# Usage
if user_exists "user1"; then
    echo "User exists"
else
    echo "User does not exist"
fi

Function with Return Value​

#!/bin/bash
# Return a value (via echo)
calculate() {
    local result=$(( $1 + $2 ))
    echo "$result"
}

# Usage
sum=$(calculate 5 3)
echo "Sum: $sum"

Cron - Task Scheduling​

What is Cron?​

Cron is a daemon that executes tasks at scheduled times.

Cron Syntax​

Format​

┌───────────── minute (0 - 59)
│ ┌───────────── hour (0 - 23)
│ │ ┌───────────── day of month (1 - 31)
│ │ │ ┌───────────── month (1 - 12)
│ │ │ │ ┌───────────── day of week (0 - 7) (0 or 7 = Sunday)
│ │ │ │ │
* * * * * command

Examples​

ScheduleDescription
0 2 * * *Every day at 2:00 AM
*/5 * * * *Every 5 minutes
0 */2 * * *Every 2 hours
0 0 * * 0Every Sunday at midnight
0 0 1 * *The 1st of every month
0 9-17 * * 1-59 AM to 5 PM, Monday-Friday
0 0,12 * * *Midnight and noon every day
*/30 * * * *Every 30 minutes

Environment Variables​

# In crontab
PATH=/usr/bin:/bin
SHELL=/bin/bash
MAILTO=admin@example.com
HOME=/home/user

# Then the tasks
0 2 * * * /path/to/script.sh

Using crontab​

Edit Crontab​

# Edit user crontab
crontab -e

# View current crontab
crontab -l

# Delete crontab
crontab -r

# Edit a user's crontab (root)
sudo crontab -u user1 -e

Crontab Format​

# Comment
# MIN HOUR DAY MONTH DAY_OF_WEEK COMMAND

# Example
0 2 * * * /usr/local/bin/backup.sh
*/15 * * * * /usr/local/bin/check.sh
0 0 1 * * /usr/local/bin/monthly-report.sh

Redirection​

# Redirect output
0 2 * * * /script.sh > /var/log/script.log 2>&1

# Ignore output
0 2 * * * /script.sh > /dev/null 2>&1

# Send by email (if MAILTO defined)
0 2 * * * /script.sh

Cron Directories​

/etc/cron.d/​

For system tasks:

# Create file in /etc/cron.d/
sudo nano /etc/cron.d/my-task

# Format (with user)
0 2 * * * user1 /usr/local/bin/script.sh

Special Directories​

/etc/cron.hourly/   # Executed every hour
/etc/cron.daily/    # Executed every day
/etc/cron.weekly/   # Executed every week
/etc/cron.monthly/  # Executed every month

Place scripts in these directories (executable, without .sh extension)

Practical Examples​

Daily Backup​

#!/bin/bash
# /usr/local/bin/backup-daily.sh
BACKUP_DIR="/backup"
SOURCE_DIR="/data"
DATE=$(date +%Y%m%d)

tar -czf "$BACKUP_DIR/backup-$DATE.tar.gz" "$SOURCE_DIR"
find "$BACKUP_DIR" -name "backup-*.tar.gz" -mtime +7 -delete

Crontab:

0 2 * * * /usr/local/bin/backup-daily.sh

Log Cleanup​

#!/bin/bash
# /usr/local/bin/cleanup-logs.sh
find /var/log -name "*.log" -mtime +30 -delete
find /var/log -name "*.gz" -mtime +90 -delete

Crontab:

0 3 * * 0 /usr/local/bin/cleanup-logs.sh

Monitoring​

#!/bin/bash
# /usr/local/bin/check-disk.sh
THRESHOLD=80
USAGE=$(df -h / | awk 'NR==2 {print $5}' | sed 's/%//')

if [ "$USAGE" -gt "$THRESHOLD" ]; then
    echo "Alert: Disk at ${USAGE}%" | mail -s "Disk Alert" admin@example.com
fi

Crontab:

*/15 * * * * /usr/local/bin/check-disk.sh

Best Practices​

Cron Scripts​

#!/bin/bash
# 1. Shebang
# 2. Environment variables
PATH=/usr/bin:/bin
# 3. Script variables
LOG_FILE="/var/log/my-script.log"
# 4. Functions
# 5. Main code with logging

Logging​

#!/bin/bash
LOG_FILE="/var/log/my-script.log"

log() {
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" >> "$LOG_FILE"
}

log "Start"
# ... processing ...
log "End"

Error Handling​

#!/bin/bash
set -euo pipefail

# Or manual handling
if ! command; then
    echo "Error" | mail -s "Script Error" admin@example.com
    exit 1
fi

Summary​

In this chapter, you learned:

Functions: Definition, parameters, return values
Cron: Automatic task scheduling
Syntax: * * * * * (minute hour day month day_of_week)
crontab: Editing, viewing, deleting
Directories: /etc/cron.d/, cron.hourly, etc.
Practice: Scripts with logging, error handling

Next Steps​

Module 7: Networking and Configuration
Module 8: Services and Daemons (with advanced crons)

Chapter created on: December 2024