Overview
True to its name, `wait` pauses the execution of a shell script until a background job finishes. It is typically used when multiple tasks are run in parallel in the background (using the `&` symbol), and subsequent operations in the script depend on these background tasks completing. Using `wait` also allows the script to check the exit code of the background process, which helps in determining the success or failure of the job.
Key Features
Key features of the `wait` command include:
- Waits for the termination of a specific background job.
- Can take a job number or Process ID (PID) as an argument.
- If executed without arguments, it waits for all background jobs to terminate.
- Useful for synchronizing multiple parallel tasks in shell scripts.
Main Options
The `wait` command is a simple command with few options, primarily taking job numbers or PIDs as arguments.
1) Execution Options
2) Help
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Usage Examples
Explore various usage examples of the `wait` command to understand its functionality.
Wait for All Background Jobs
sleep 5 &
sleep 10 &
jobs
wait
echo 'All jobs have completed.'Run multiple `sleep` tasks in the background, then use the `wait` command to wait for all of them to complete.
Wait for a Specific Job
sleep 10 &
sleep 5 &
jobs
wait %1
echo 'Job 1 has completed.'Wait for job number 1 to finish among two jobs. Use the `jobs` command to find the job number and then `wait %1`.
Wait by Process ID (PID)
sleep 10 &
PID=$!
echo "Job with PID $PID has started."
wait $PID
echo "Job with PID $PID has completed."In a script, save the PID of a background process into a variable using `$!`, then wait for that specific PID to terminate.
Installation
wait is a built-in command of shells (like bash, zsh), and does not require separate installation.
Tips & Cautions
Here are some points to keep in mind when using the `wait` command.
Tips
- `wait` pauses shell execution until the background process terminates. If the process does not terminate, the shell will continue to wait indefinitely.
- The exit status code of the `wait` command is the same as the exit status code of the process it waited for. This allows you to check the success or failure of the job within your script.
- To use a job number (`%1`), you must first check the current shell's background jobs using the `jobs` command. `$!` is a special variable that holds the PID of the most recently executed background process.