The ability to run commands in the background is one of the key benefits of the UNIX system. You can set any command line running in the background while you do something else at the prompt.
To set a command running in the background, type the command at the prompt as usual, but type & (ampersand) after it, before you press <Enter>. This tells the UNIX system you want the command to run in the background, so it immediately returns your prompt.
For example, if you have a lot of files to join together, or if the
files are large, you can put the command in the background:
cat bigfile1 bigfile2 bigfile3 > bigfile&
When you put a command in the background, the computer responds with a number that is the process ID of the command. See ``Controlling processes'' for information about processes and process IDs.