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Honestly I had no idea what ctrl+d even did, I just knew it was a convenient way for me to close all the REPL programs I use. The fact that it is similar to pressing enter really surprised me, so I wanted to share this knowledge with you :)

  • davel [he/him]@lemmy.mlEnglish
    221·
    2 days ago

    CTRL+M is like pressing ENTER. Kernigan & Pike, 1984: UNIX Programming Enviornment

    RETURN is an example of a control character — an invisible character that controls some aspect of input and output on the terminal. On any reasonable terminal, RETURN has a key of its own, but most control characters do not. Instead, they must be typed by holding down the CONTROL key, sometimes called CTL or CNTL or CTRL, then pressing another key, usually a letter. For example, RETURN may be typed by pressing the RETURN key or, equivalently, holding down the CONTROL key and typing an ‘m’. RETURN might therefore be called a control-m, which we will write as ctl-m.

    • tuna@discuss.tchncs.deOP
      101·
      2 days ago

      On any reasonable terminal, RETURN has a key of its own

      This reminds me of a time at work when I was not on a reasonable terminal. I was explaining to a co-worker how I automated some tasks by running some scripts, but in my demo my RETURN key didn’t work, so I had to improvise and use CTRL+M which worked, hahaha. I don’t know how the terminal got in such a bad spot but it was probably something to do with msys on Windows… honestly not sure. It was perfect timing to have happen while teaching of course ;)

      I would also be doing a disservice not to share what the book you linked says about CTRL+D. Right after your quote, it says:

      Other control characters include ctl-d, which tells a program that there is no more input

      This is pretty good for an introduction, but it is not the full story. It explains CTRL+D properly later (chapter 2, page 45):

      Now try something different: type some characters and then a ctl-d rather than a RETURN:

      $ cat -u
123<ctl-d>123

      cat prints the characters out immediately. ctl-d says, “immediately send the characters I have typed to the program that is reading from my terminal.” The ctl-d itself is not sent to the program, unlike a newline. Now type a second ctl-d, with no other characters:

      $ cat -u
123<ctl-d>123<ctl-d>$

      The shell responds with a prompt, because cat read no characters, decided that meant end of file, and stopped. ctl-d sends whatever you have typed to the program that is reading from the terminal. If you haven’t typed anything, the program will therefore read no characters, and that looks like the end of the file. That is why typing ctl-d logs you out — the shell sees no more input. Of course, ctl-d is usually used to signal an end-of-file but it is interesting that it has a more general function.

      This is why the article says it’s “like pressing enter,” because it flushes the input just like enter. The difference is that enter sends a newline, but CTRL+D does not, so you can exploit that to send no data (and the program chooses to interpret that as an EOF).