Does a Bash script wait for a command to finish?

Yes, by default, a Bash script waits for each command to finish executing before proceeding to the next command in the script. This behavior is fundamental to how Bash processes commands sequentially.

Sequential Execution:

- When a Bash script is executed, commands are processed line by line. Bash will not proceed to the next line until the current command has completed. This includes commands, programs, and internal Bash functions.

Foreground Processes:

- Commands executed directly in a script without any specific modifications run as foreground processes. The script will pause and wait for the foreground process to finish. Once finished, it returns control to the shell script which continues with the next command.

Example:

Consider this simple bash script example.sh:

#!/bin/bash
echo "Start"
sleep 5
echo "Finished"


If you execute ./example.sh, it will output:

Start
(Wait for 5 seconds)
Finished

- The script will output "Start," then it will pause for five seconds while sleep 5 executes, and finally, it outputs "Finished". The echo "Finished" command is only executed when sleep has finished.

Background Processes:

- If you want a command to run without waiting for it to finish, you can run it as a background process by appending an ampersand & to the command. Bash does not wait for background processes to finish and proceeds immediately to the next command.

Example of background processes:

#!/bin/bash
echo "Start"
sleep 5 &
echo "Finished"


If you execute ./example.sh, it will output:

Start
Finished
(after 5 seconds)

- The script will output "Start" and "Finished" almost immediately, and the sleep command will run in the background.

Waiting for Background Processes:

- If you need the script to wait for a background process to finish at a certain point, you can use the wait command.

#!/bin/bash
echo "Start"
sleep 5 &
PID=$!
echo "Background process started with PID: $PID"
wait $PID
echo "Background process finished"


- The $! variable gets the process ID of the last background process. The wait $PID command makes the script pause until the process with that PID finishes.

Conclusion:

In summary, a Bash script will, by default, wait for each command to finish before moving to the next one. You can run commands in the background using & and wait for background processes to finish using the wait command. This control over process execution is very important when writing scripts for automation.

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