Info Catalog bash: Job Control bash: Job Control Builtins

bash: Job Control Basics

 
 7.1 Job Control Basics
 ======================
 
 Job control refers to the ability to selectively stop (suspend) the
 execution of processes and continue (resume) their execution at a later
 point.  A user typically employs this facility via an interactive
 interface supplied jointly by the operating system kernel's terminal
 driver and Bash.
 
    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, it prints a line that looks like:
      [1] 25647
 indicating that this job is job number 1 and that the process ID 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.
 
    To facilitate the implementation of the user interface to job
 control, the operating system maintains the notion of a current terminal
 process group ID.  Members of this process group (processes whose
 process group ID is equal to the current terminal process group ID)
 receive keyboard-generated signals such as 'SIGINT'.  These processes
 are said to be in the foreground.  Background processes are those whose
 process group ID differs from the terminal's; such processes are immune
 to keyboard-generated signals.  Only foreground processes are allowed to
 read from or, if the user so specifies with 'stty tostop', write to the
 terminal.  Background processes which attempt to read from (write to
 when 'stty tostop' is in effect) the terminal are sent a 'SIGTTIN'
 ('SIGTTOU') signal by the kernel's terminal driver, which, unless
 caught, suspends the process.
 
    If the operating system on which Bash is running supports job
 control, Bash contains facilities to use it.  Typing the SUSPEND
 character (typically '^Z', Control-Z) while a process is running causes
 that process to be stopped and returns control to Bash.  Typing the
 DELAYED SUSPEND character (typically '^Y', Control-Y) causes the process
 to be stopped when it attempts to read input from the terminal, and
 control to be returned to Bash.  The user then manipulates the state of
 this job, using the 'bg' command to continue it in the background, the
 'fg' command to continue it in the foreground, or the 'kill' command to
 kill it.  A '^Z' takes effect immediately, and has the additional side
 effect of causing pending output and typeahead to be discarded.
 
    There are a number of ways to refer to a job in the shell.  The
 character '%' introduces a job specification (JOBSPEC).
 
    Job number 'n' may be referred to as '%n'.  The symbols '%%' and '%+'
 refer to the shell's notion of the current job, which is the last job
 stopped while it was in the foreground or started in the background.  A
 single '%' (with no accompanying job specification) also refers to the
 current job.  The previous job may be referenced using '%-'.  If there
 is only a single job, '%+' and '%-' can both be used to refer to that
 job.  In output pertaining to jobs (e.g., the output of the 'jobs'
 command), the current job is always flagged with a '+', and the previous
 job with a '-'.
 
    A job may also be referred to using a prefix of the name used to
 start it, or using a substring that appears in its command line.  For
 example, '%ce' refers to a stopped 'ce' job.  Using '%?ce', on the other
 hand, refers to any job containing the string 'ce' in its command line.
 If the prefix or substring matches more than one job, Bash reports an
 error.
 
    Simply 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.  Similarly, '%1 &' resumes job 1 in the background,
 equivalent to 'bg %1'
 
    The shell learns immediately whenever a job changes state.  Normally,
 Bash waits until it is about to print a prompt before reporting changes
 in a job's status so as to not interrupt any other output.  If the '-b'
 option to the 'set' builtin is enabled, Bash reports such changes
 immediately (The Set Builtin).  Any trap on 'SIGCHLD' is
 executed for each child process that exits.
 
    If an attempt to exit Bash is made while jobs are stopped, (or
 running, if the 'checkjobs' option is enabled - see The Shopt
 Builtin), the shell prints a warning message, and if the 'checkjobs'
 option is enabled, lists the jobs and their statuses.  The 'jobs'
 command may then be used to inspect their status.  If a second attempt
 to exit is made without an intervening command, Bash does not print
 another warning, and any stopped jobs are terminated.
 

Info Catalog bash: Job Control bash: Job Control Builtins