| If the monitor option of the set command is turned on, an interactive shell associates a job with each pipeline. It keeps a table of current jobs, printed by the jobs command, and assigns them small integer numbers. When a job is started asynchronously with &, the shell prints a line which looks like: [1] 1234 indicating that the job which was started asynchronously was job number 1 and had one (top-level) process, whose process id was 1234. This paragraph and the next require features that are not in all versions of the Unix operating system and may not apply. If you are running a job and wish to do something else you may hit the key ^Z (control-Z) which sends a STOP signal to the current job. The shell will then normally indicate that the job has been 'Stopped', and print another prompt. You can then manipulate the state of this job, putting it in the background with the bg command, or run some other commands and then eventually bring the job back into the foreground with the foreground command fg. A ^Z takes effect immediately and is like an interrupt in that pending output and unread input are discarded when it is typed. A job being run in the background will stop if it tries to read from the terminal. Background jobs are normally allowed to produce output, but this can be disabled by giving the command "stty tostop". If you set this tty option, then background jobs will stop when they try to produce output like they do when they try to read input. There are several ways to refer to jobs in the shell. A job can be referred to by the process id of any process of the job or by one of the following: | %number | The job with the given number. | | %string | Any job whose command line begins with string. | | %?string | Any job whose command line contains string. | | %% | Current job. | | %+ | Equivalent to %%. | | %?/span> | Previous job. |
The shell learns immediately whenever a process changes state. It normally informs you whenever a job becomes blocked so that no further progress is possible, but only just before it prints a prompt. This is done so that it does not otherwise disturb your work. When the monitor mode is on, each background job that completes triggers any trap set for CHLD. When you try to leave the shell while jobs are running or stopped, you will be warned that "You have stopped(running) jobs.". You may use the jobs command to see what they are. If you do this or immediately try to exit again, the shell will not warn you a second time, and the stopped jobs will be terminated. | 
