When multiple cron jobs are scheduled for identical execution times, traditional cron implementations (like Vixie cron on Debian) execute them sequentially rather than in parallel. The jobs run in the order they appear in the crontab file, with each subsequent job starting only after the previous one completes.

Consider this /etc/cron.d/mycronjobs example:

# m h dom mon dow user  command
* * * * * root /usr/bin/backup.sh
* * * * * root /usr/bin/clean_cache.sh
* * * * * root /usr/bin/generate_report.sh

In this case, generate_report.sh won't begin until clean_cache.sh finishes, which itself waits for backup.sh completion. This serialized execution can cause delays if earlier jobs are long-running.

For true parallel processing, consider these approaches:

1. Using Ampersand (&) in Commands

* * * * * root /usr/bin/long_task.sh &

2. GNU Parallel with Cron

* * * * * root ls /etc/cron.d/* | parallel -j 4 {}

3. Systemd Timers as Alternative

[Unit]
Description=Parallel job runner

[Timer]
OnCalendar=*-*-* *:*:00

[Install]
WantedBy=timers.target

When implementing parallel execution:

• Monitor system load averages with uptime
• Adjust /etc/security/limits.conf if hitting process limits
• Consider using flock for resource contention management

For Debian systems:

# Install parallel processing tools
sudo apt-get install parallel moreutils

# Example using cron-apt for parallel updates
*/30 * * * * root /usr/bin/cron-apt -o Acquire::http::Dl-Limit=100

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When multiple cron jobs are scheduled for the same execution time in Debian (or any Linux system), cron processes them sequentially by default. The jobs will run one after another in the order they appear in your crontab file or the /etc/cron.d/ directory.

Here's what happens under the hood:

# Example from /etc/cron.d/mycronjobs
* * * * * root /usr/local/bin/job1.sh
* * * * * root /usr/local/bin/job2.sh
* * * * * root /usr/local/bin/job3.sh

Cron's child process will:

1. Fork a new process for job1.sh and wait for completion
2. Only when job1.sh finishes, fork a process for job2.sh
3. Repeat the same sequential pattern for subsequent jobs

If you need true parallel processing, you have several options:

Option 1: Background Processes

* * * * * root /usr/local/bin/job1.sh &
* * * * * root /usr/local/bin/job2.sh &

Option 2: Using GNU Parallel

* * * * * root parallel ::: /usr/local/bin/job1.sh /usr/local/bin/job2.sh

Option 3: Systemd Timers (Modern Alternative)

# Create a timer unit with OnCalendar=*-*-* *:*:00
# Set AccuracySec=1us to minimize drift

Resource Contention: When jobs run simultaneously, they might compete for:

• CPU time
• Memory
• File locks
• Database connections

Solution: Implement proper locking mechanisms:

#!/bin/bash
# Using flock for file locking
(
  flock -n 200 || exit 1
  # Critical section code here
) 200>/var/lock/myjobs.lock

Use these commands to check running processes:

pgrep -fa cron
ps aux | grep 'job[1-3].sh'

For detailed logging, modify your cron jobs:

* * * * * root /usr/local/bin/job1.sh >> /var/log/job1.log 2>&1

• Use unique lock files for each job
• Implement proper error handling in scripts
• Consider using job queues (Redis, RabbitMQ) for heavy workloads
• Monitor system resources during peak loads