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The Kill Builtin

The kill command is used to send a signal to a process, usually to end it.

Basic Usage

kill PID
The PID is the process ID of the process you want to kill.

By default, kill will send the SIGTERM signal. This will tell the process to terminate.

To see a list of signals, use kill -l: 

kill -l
This will list the signals and their corresponding numbers.

A Note on Signals

SIGTERM is always preferred over SIGINT and SIGQUIT.

  • This is because SIGTERM allows the process to perform any cleanup before exiting.
  • SIGTERM is the default signal sent to a process with it is killed with kill

When bash is interactive, without any traps:

  • SIGTERM is ignored (so that kill 0 does not kill an interactive shell).
  • SIGINT is caught and handled (so that the wait builtin is interruptible).

In all cases, a bash process ignores SIGQUIT.

If job control (man://bash, /^JOB CONTROL) is in effect, bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.

Job Control

The shell associates a job with each pipeline.
It keeps a table of currently executing jobs, which may be listed with the jobs command.

When bash starts a job asynchronously (in the background), it prints a line that looks like: 

[1] 25647
This indicates that:

  • [1]: This job is job number 1.
  • 25647: The PID of the last process in the pipeline associated with this job is 25647.

All of the processes in a single pipeline are members of the same job. Bash uses the job abstraction as the basis for job control.

The % character introcues a job specification (jobspec).

  • Job number 2 can be referred to as %2.

    • It can also be referred to by using a prefix of the name used to start it, or by using a substring that appears in its command line.
    • For example, %ce refers to a stopped job whose command name begins with ce.
      • If the prefix ce matches more than 1 job, Bash gives an error.

  • The %% and %+ jobspecs refer to the current job, which is the last job stopped while it was in the foreground or started in the background.

  • The %- jobspec refers to the previous job.

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.

The jobs builtin can be used to show jobs that are under job control.

jobs
If there's no output, it means no jobs are under job control. Try backgrounding a process (^Z) and running jobs again.