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Unix Dweeb

Linux control sequence tricks

Jun 22, 20184 mins
LinuxOpen Source

There are a lot of control sequences available on Linux systems -- maybe even some you've never used.

acer aspire e15 keyboard 4
Credit: Ben Patterson / IDG

There are quite a few control sequences available on Linux systems— many I use routinely, and some I’ve only just recently discovered— and they can be surprisingly useful. In today’s post, we’re going to run through a series of them and take a look at what they do and how they might be useful.

To start, unless you’re brand spanking new to the command line, you are undoubtedly familiar with the ctrl-c sequence that is used to terminate a running command. In print, this same sequence might be expressed as ^c or control-c and sometimes the “c” will be capitalized, but the expression always means “hold the control key and press the key specified — with no shift key or hyphen involved.

The ctrl-d sequence closes the terminal window or end terminal line input.

$ cat > file
yadda yadda
yadda yadda
$ cat file
yadda yadda
yadda yadda

You may have never tried ctrl-u. This control sequence and its “partner in crime” ctrl-y work together in an interesting way. The ctrl-u sequence removes the text you’ve just typed from command line and places it in something of a clipboard while ctrl-y puts in back. So, when you’ve just typed a complicated command, but not yet hit return, and then realize that you need to run some other commands first, you can save the command you’ve typed, take care of whatever other commands you need to run and then yank the command you saved back into place.

$ gensched 07-2018 IT summary^u ^y 

The ctrl-s and ctrl-q sequences also have a working relationship. Where ctrl-s freezes your screen, ctrl-q allows the display to continue rolling again.

$ bin/loop
^s    ^q

The ctrl-z sequence suspends the current process. You can bring it back to life with the fg (foreground) command or have the suspended process run in the background by using the bg command. If you then want to stop the process, you'll then have to list your background processes with the jobs command and use kill (e.g., kill %1). Don't forget the % that specifies the process by job number rather than its PID.

$ bin/loop
[1]+  Stopped                 bin/loop
$ bg
[1]+ bin/loop &
$ sleeping
[1]+  Running                 bin/loop &
$ sleeping
$ kill %1

The ctrl-h, ctrl-w and ctrl-u sequences erase (i.e., back over) the last letter you just typed, the last word that you just typed, or the entire line.

The ctrl-a and ctrl-e  will move your cursor to the beginning or end of the text you have just typed.

The ctrl-r sequence allows you to easily rerun recently entered commands. Type ctrl-r followed by the beginning of the command that you want to rerun. The command run will be the most recent one that began with the letters you enter.

Control sequences can be handy if you can keep them straight. Here's a quick rundown of the control sequences just covered:

    interrupts the running program
    sends an EOF (end of file) to close the terminal
    suspends the running program
    freezes the screen, stopping the display
    thaws out the screen and allows the screen display to continue
    deletes the last character typed
    deletes the last word typed
    deletes the last line typed
    retrieves previously run commands so you can run them again
    removes text from the command line and places it in the clipboard
    grabs text from the clipboard and runs it
    clears the screen
    moves cursor to the beginning of the line
    moves cursor to the end of the line

You can take control of control sequences that aren't behaving as expected with a command like this one that reasserts ctrl-h as the erase sequence.

stty erase ^h

Note that the stty -a command will list your control sequences:

$ stty -a | grep eof
intr = ^C; quit = ^; erase = ^H; kill = ^U; eof = ^D; eol = ;

More on the stty command is available at Using stty to your advantage.

The only difficult part of using control sequences is remembering which does what. For some, the letter suggests the control sequence function. For others ... well, not so much. But they can all be very useful.

Unix Dweeb

Sandra Henry-Stocker has been administering Unix systems for more than 30 years. She describes herself as "USL" (Unix as a second language) but remembers enough English to write books and buy groceries. She lives in the mountains in Virginia where, when not working with or writing about Unix, she's chasing the bears away from her bird feeders.

The opinions expressed in this blog are those of Sandra Henry-Stocker and do not necessarily represent those of IDG Communications, Inc., its parent, subsidiary or affiliated companies.

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