What to do if you get stuck in a Unix login session
Last revision August 6, 2004
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First, be patient. Wait a minute or so to make sure you are really stuck, particularly if you have any reason to believe that the command may have to do considerable computing before it is done. Also, system loads and response times vary a lot.
CTRL-S and CTRL-Q are used to control the flow of output to the terminal. CTRL-S tells the system not to send any more data to the screen until a CTRL-Q is pressed. You can use these to control fast scrolling output, although they do not work as well as you would like on network connections, since much data has already been sent to your computer over the network by the time the Unix system receives the CTRL-S character.
You might press CTRL-S by mistake and your terminal will appear to be frozen. Try pressing CTRL-Q first when your terminal appears stuck to cancel any mistakenly pressed CTRL-S characters (extra CTRL-Q characters do not cause any harm).
To stop a program, send an interrupt signal by pressing the CTRL-C key. This signal allows the program to clean up before it must exit. Some programs capture the normal interrupt and do not die. Try CTRL-\ to kill them. This kill signal forces the program to exit immediately, with no chance to clean up, so it should only be used when CTRL-C does not work.
In an emergency, close the session window or turn off the terminal and wait a few minutes. The kernel should eventually notice and kill your login process.
If nothing works, and the terminal still appears to be frozen, contact the system manager, who can find and kill the offending process that is freezing the terminal. You can do this yourself when you learn how to use the ps and kill commands.