Job Control in Bash¶

Job control is a feature of Bash that allows users to selectively stop (suspend) the execution of processes and continue (resume) their execution at a later point.

This mechanism is particularly useful for managing multiple processes from a single bash session, i.e., a single shell.

Key Concepts¶

Job: A job is associated with each pipeline and can be a single command or a group of commands connected by pipes.
- Bash maintains a table of current jobs which can be displayed using the jobs command.
Process Group: A job might consist of multiple processes, especially if pipelines are used.
- These processes share the same process group ID.
Foreground and Background Jobs: Jobs can either run in the foreground, blocking the terminal until completion, or in the background, allowing the terminal to be used for other tasks.
Standard File Descriptors (FD) are the primary means of interacting with processes:
- Standard input (stdin - FD 0)
- Standard output (stdout - FD 1)
- Standard error (stderr - FD 2)

Job Control Commands¶

jobs: Lists all current jobs with their statuses.
fg: Brings a job to the foreground.
- Usage: fg %job_number or fg %job_name.
bg: Continues a job in the background.
- Usage: bg %job_number or bg %job_name.
kill: Sends a signal to a job.
- Usage: kill %job_number.
ctrl+z: Suspends the foreground job.
ctrl+y: Delays suspending a job until it attempts to read input.

Referring to Jobs¶

%n: Refers to job number n.
%string: Refers to a job starting with string.
%?string: Refers to a job containing string.
%% or %+: Refers to the current job.
%-: Refers to the previous job.

Job States and Notifications¶

Bash reports a job's state change (e.g., from running to stopped) when it's about to display the prompt.
- Use the -b option with the set command for immediate notifications.
SIGCHLD: Bash executes any trap on SIGCHLD for each child that exits.

Examples¶

Running a Job in the Background¶

sleep 30 &

This starts the sleep command in the background, allowing the terminal to be used for other commands.

Suspending a Foreground Job¶

While a job is running in the foreground, pressing <C-z> will suspend it, returning control to the shell.

Moving a Job to the Background¶

bg %1

This resumes job 1 in the background.

Bringing a Job to the Foreground¶

fg %1

This moves job 1 into the foreground.

Listing Jobs¶

jobs

This displays the current jobs and their statuses.

Exiting Bash with Active Jobs¶

Bash warns on exit if jobs are stopped or, if checkjobs is enabled, running.
- A second exit attempt terminates stopped jobs.

Waiting for a Job¶

The wait builtin waits for a job to change state.
- With job control enabled, wait returns if the job terminates.

Important Notes¶

Naming a job can be used to bring it into the foreground:

%1 is a synonym for fg %1, bringing job 1 from the background into the foreground.
%1 & resumes job 1 in the background, equivalent to bg %1.
Background Processes and Terminal I/O: Background processes attempting to read from or write to the terminal can be suspended by the terminal driver (SIGTTIN or SIGTTOU signals).
Manipulating File Descriptors: The exec command can be used to redirect file descriptors for the current shell session.

Understanding job control enhances multitasking capabilities in Bash, providing flexibility in how processes are managed during a shell session.

This revision provides a comprehensive overview of job control in Bash, including key concepts, commands, how to refer to jobs, handling job states, and practical examples.

It clarifies ambiguous points and adds detailed explanations suitable for learners.