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43.12. What Can You Do with an Empty File?

It isn't a file, actually, though you can use it like one. /dev/null is a Unix device.[132] It's not a physical device. /dev/null is a special device that "eats" any text written to it and returns "end-of-file" (a file of length 0) when you read from it. So what the heck can you use it for?

[132]Well, okay. It's a device file.

  • Empty another file. Just copy /dev/null "on top of" the other file (Section 15.2).

  • Make another program "quiet" by redirecting its output there. For instance, if you're putting a program into the background and you don't want it to bother you, type:

    % progname > /dev/null &

    That redirects (Section 43.1) standard output but leaves standard error hooked to your terminal, in case there is an error.

  • Answer a program that asks a lot of questions -- you know you'll just press RETURN at each prompt. In a lot of cases, you can redirect the program's standard input from /dev/null:

    % progname < /dev/null
    Want the default setup? If yes, press RETURN:
    Enter filename or press RETURN for default:

    You should test that with each program, though, before you assume this trick will work. (If it doesn't work, try yes (Section 14.5).)

  • Where a program needs an extra filename but you don't want it to read or write an actual file. For instance, the grep (Section 13.1) programs won't give the name of the file where they find a match unless there are at least two filenames on the command line. When you use a wildcard in a directory where maybe only one file will match, use /dev/null to be sure that grep will always see more than one (Section 9.21):

    % grep "outputfile" * /dev/null

    You're guaranteed that grep won't match its regular expression in /dev/null.

  • Section 15.3 shows even more uses for /dev/null.

Another interesting device (mostly for programmers) is /dev/zero. When you read it, you'll get ASCII zeros (NUL characters) forever. There are no newlines either. For both of those reasons, many Unix commands have trouble reading it. If you want to play, the command below will give you a start (and head (Section 12.12) will give you a stop!):[133]

[133]On some Unix versions, the head program may not terminate after it has printed the first 10 lines. In that case, use sed 10q instead of head.

od Section 12.4

% fold -20 /dev/zero | od -c | head

-- JP

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