Linux control sequence tricks

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

Linux control sequence tricks
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
^d <== ctrl-d typed $ 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 <== typed line will disappear
$ update cal 07-2018            <== run some other commands
$ schedCheck
$ ^y <== "gensched 07-2018 IT summary" reappears

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    <== movement stops and waits

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 ctrl-z suspends the running program ctrl-s freezes the screen, stopping the display ctrl-q thaws out the screen and allows the screen display to continue ctrl-h deletes the last character typed ctrl-w deletes the last word typed ctrl-u deletes the last line typed ctrl-r retrieves previously run commands so you can run them again ctrl-u removes text from the command line and places it in the clipboard ctrl-y grabs text from the clipboard and runs it ctrl-l 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 = <undef>;

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.

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