The Difference Between ‘wait’ and ‘sleep’ in Shell Scripting

In the world of shell scripting, understanding the nuanced differences between various commands is crucial for crafting efficient and effective scripts. Two such commands that often lead to confusion are `wait` and `sleep`. While they may seem similar at first glance, each serves a unique purpose in script execution.

This article delves into the core differences between the wait and sleep commands in shell scripting, providing clear examples and practical use cases.

Difference Between wait and sleep Commands

The difference between wait and sleep in shell scripts is essential to understand for efficient script writing and process management in Unix-like operating systems.

The sleep Command

• Function: The sleep command is used to delay for a fixed amount of time during the execution of a script. It is used to pause the execution for a specified period.
• Syntax:

  
  sleep NUMBER[SUFFIX]...
  
  

  • NUMBER can be an integer or floating-point number.
  • SUFFIX can be s for seconds (default), m for minutes, h for hours, or d for days.
• Example:

  
  #!/usr/bin/env bash
  
  echo "Wait for 5 seconds"
  sleep 5
  echo "Resumed after 5 seconds"
  
  

The wait Command

• Function: The wait command is used to pause the script until a certain condition is met, typically the completion of a background process. It is used to synchronize the script execution with the completion of background jobs.
• Syntax:

  
  wait [id]
  
  

  `id` is the job ID of the background process. If no ID is given, the command waits for all current child processes to complete.

• Example:

  
  #!/usr/bin/env bash
  
  echo "Start background process"
  some_long_running_process &
  process_id=$!
  
  echo "Wait for the process to complete"
  wait $process_id
  echo "Background process completed"
  
  

Key Differences

1. Purpose:
   • sleep: Introduces a fixed delay.
   • wait: Waits for a process to complete.
2. Use Case:
   • sleep: Useful in situations like polling, or to give time for a resource to become available.
   • wait: Ideal for managing dependencies between scripts and processes.
3. Control:
   • sleep: Control is time-based.
   • wait: Control is process-based.
4. Practical Scenarios:
   • sleep Use Case: Imagine a scenario where a script needs to give a server time to start up before sending requests. Using sleep provides a simple delay.
   • wait Use Case: In a situation where multiple data processing scripts are running in parallel, and a summary script must start only after all have completed, wait can be used to synchronize the scripts.

Conclusion

Both wait and sleep are crucial in shell scripting for different reasons. sleep is straightforward for adding delays, while wait is more sophisticated, suitable for managing process dependencies. Understanding their differences and appropriate use cases is key to effective shell scripting.