The command “fg” is used to run jobs in the foreground in a Unix-like operating system. In a Unix shell, when a command is executed, it typically runs in the background, allowing the user to continue interacting with the shell while the command executes. However, there may be situations where you want to bring a background job to the foreground and have it run in the foreground instead.
Here are the key points to understand about the “fg” command:
- Background Jobs: In a Unix shell, you can start a command or a script in the background by appending an ampersand (&) at the end of the command. This allows the command to execute independently of the shell, and the shell prompt is returned immediately for further commands.
- Bringing Jobs to Foreground: If you have a command or a script running in the background and you want to bring it to the foreground, you can use the “fg” command. By running “fg” followed by the job ID or the job specification, the background job will be moved to the foreground, and its output will be displayed directly on the terminal.
- Job ID or Job Specification: When you start a command or a script in the background, the shell assigns it a unique job ID. The job ID can be used with the “fg” command to specify which background job to bring to the foreground. Alternatively, you can use job specifications, such as “%1” for the first job, “%2” for the second job, and so on.
- Resuming Execution: When you bring a background job to the foreground using “fg”, its execution continues from where it was paused. The job receives input and output directly from the terminal, allowing you to interact with it as if it were running in the foreground from the beginning.
- Controlling Foreground Jobs: While a job is running in the foreground, you have control over its execution. You can pause the job by pressing Ctrl+Z, which suspends its execution and returns control to the shell. You can then resume the job in the foreground using the “fg” command or move it back to the background using the “bg” command.
- Multiple Foreground Jobs: In some cases, you may have multiple jobs running in the foreground. The shell provides mechanisms to switch between these jobs and control their execution. For example, you can use the “jobs” command to display a list of currently running jobs and their statuses, and you can use the “%” notation followed by a job ID or job specification to refer to a specific job.
fg Command Examples
1. Bring most recently suspended or running background job to foreground:
# fg
2. Bring a specific job to foreground:
# fg %job_id
Summary
In summary, the “fg” command allows you to bring background jobs to the foreground in a Unix shell. By specifying the job ID or job specification, you can resume the execution of a background job in the foreground and interact with it directly on the terminal. This command provides flexibility and control over job execution, allowing you to manage multiple jobs and switch between foreground and background execution as needed.