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CIsForCookies
Asked: 2022-03-16 12:13:56 +0800 CST

Is it possible to alias a command prefix for redirecting stdout?

  • 772

Sorry about the non-informative heading...

What I'm looking for is something like that will allow me to shorten redirecting stdout. I know that executing a command with whitespaces before it does not write it into history, and there are a few "magic" operators, such as "!!", etc.

I'm basically looking for something like that, that would allow me to replace >/dev/null, so, I'll be able to write, for example

git pull !!! and that would be equivalent to git pull>/dev/null

Is there something like this? Am I able to override an existing operator / write a new one?

command-line

4 Answers

  1. Best Answer

    Alias expansion only looks at the first token / word of a line, so there's no way for git pull !!! to get replaced by alias expansion.

    However, redirects work even if placed before the command, so you can do it with an alias:

    alias quiet='>/dev/null '

    Leaving whitespace at the end of the alias expansion makes bash consider the next token for alias expansion. (This is why it's common to do alias sudo='sudo '). So this won't even break your aliases. Example:

    $ ll *.s xyz
ls: cannot access 'xyz': No such file or directory
-rw-r--r-- 1 peter peter 12 Mar  6 20:15 a64.s
-rw-r--r-- 1 peter peter 72 Mar  6 21:03 arm.s
-rw-r--r-- 1 peter peter 30 Mar 15 17:38 foo.s
-rw-r--r-- 1 peter peter 42 Mar  6 13:12 mips.s
$ quiet ll *.s xyz     # ll alias expanded successfully, stdout discarded
ls: cannot access 'xyz': No such file or directory
$                      # stderr is not redirected.
    • 24

  2. I'd use a function

    quietly() {
    # execute the remaining arguments as a command, but redirect output
    "$@" > /dev/null
}

quietly git pull

    Or a function that reads stdin and discards it:

    q() { cat > /dev/null; }

git pull | q

    Neither of these redirect stderr: if something goes wrong, you should know about it.

    • 16

  3. Aliasing is possible as indicated by the answer of Peter Cordes. Another approach is setting up an environment variable. This allows for a more natural redirection syntax:

    export d0='/dev/null'

    which you then can use as

    git pull >$d0
    • 5

  4. It is possible, yes, but not through aliases – instead by using bash's extdebug function, which allows making arbitrary changes to a command before it is executed (by suppressing the original command and eval'ing the new one).

    shopt -s extdebug
trap "code goes here" DEBUG

    The trap handler will get the original input in $BASH_COMMAND, and returning 1 from the handler will cancel original the command. If you enter multiple ;-separated commands, the trap is invoked separately for each one.

    Specifically:

    shopt -s extdebug
trap 'if [[ $BASH_COMMAND == *!!!* ]]; then
          eval "${BASH_COMMAND//!!!/"> /dev/null"}"; false
      fi' DEBUG

    This will replace any !!! with > /dev/null anywhere inside the command. You won't be able to do echo 'Hi!!!' anymore.

    If you prefer to match only at the end of command, that's easy too.

    trap 'if [[ $BASH_COMMAND == *\ quietly ]]; then
          eval "${BASH_COMMAND% quietly} > /dev/null"; false
      fi' DEBUG

    As another example, you can use this to alias command please to sudo command:

    trap 'case $BASH_COMMAND in *\ please) eval sudo ${BASH_COMMAND% *}; false; esac' DEBUG

    There is another way to achieve a similar thing: you can redefine keys in Bash (readline) using either bind or ~/.inputrc, which you can also use to redefine key sequences.

    For example, inputrc entry "!!!": "> /dev/null" or the equivalent shell command bind '"!!!": "> /dev/null"' will cause the text > /dev/null to be inserted whenever you attempt to type !!!.

    Note that Bash cannot predict future, which means that if you only want to type a single !, there will be a slight delay before it appears – that's Bash waiting to see whether it'll match the sequence. This can get annoying, so don't bind letters and stuff.

    In fact, it would be a better idea to bind a Ctrl or Alt hotkey instead. For example, you could create a Alt+Q hotkey using "\eq": "> /dev/null".

    Previously: https://superuser.com/a/1459211

    • 2