$ sleep 500 &  7582
Puts the sleep command in background. 7582 is the process id of the background process.
$ jobs  Running sleep 500 & (wd: ~) - Running sleep 600 & (wd: ~) + Running ./Fritzing &
First field shows the job ids. The + and - sign that follows the job id for two jobs denote the default job and next candidate default job when the current default job ends respectively. The default job is used when the
bg commands are used without any argument.
Second field gives the status of the job. Third field is the command used to start the process.
The last field (wd: ~) says that the sleep commands were started from the working directory ~ (Home).
$ fg %2 sleep 600
%2 specifies job no. 2. If fg is used without any arguments if brings the last process put in background to the foreground.
$ fg %?sle sleep 500
?sle refers to the baground process command containing "sle". If multiple background commands contain the string, it will produce an error.
Press Ctrl + Z to stop a foreground process and put it in background
$ sleep 600 ^Z + Stopped sleep 600
$ bg + sleep 600 &