如何在条件语句中省略grep输出

Question

On a Mac, I want to determine if there are any sleep assertions present, using pmset. If there are, extract only that information and omit unnecessary information.

If grep returns nothing I want to print "Nothing".

if pmset -g | grep pre ; then pmset -g | grep pre | cut -d'(' -f2 | cut -d')' -f1 ; else printf "Nothing\n" ; fi

The problem is that the first grep result is printed, and so is the formatted one. For example this is what I get if a backup is in progress:

sleep 15 (sleep prevented by backupd)

sleep prevented by backupd

I don't want the first line, and want to discard it. I only want the second line to print ("sleep prevented by backupd").

If the grep result is empty I want to indicate that with the text "Nothing". The above script works OK for that.

There are probably many more elegant solutions but I've been searching days for one.

Answer 1

If i understand your question properly, you simply need to discard the output of first grep irrespective of the output it provides. If it's so, then you can use -q option provided by grep.

From the man page for 'grep':

-q, --quiet, --silent
              Quiet; do not write anything to standard output.  Exit immediately with zero status if any match  is  found,  even  if  an  error  was
              detected.  Also see the -s or --no-messages option.  (-q is specified by POSIX.)

Something like this:

if ifconfig | grep -q X; then
        ifconfig | grep Mi | cut -d'(' -f2
else
        printf "Nothing\n"
fi

Obviously in the above example, output of ifconfig will not change every time. Just used as an example. ;)

Answer 2

Redirect the output to /dev/null :

if pmset -g | grep pre >/dev/null 2>&1 ; then
    pmset -g | grep pre | cut -d'(' -f2 | cut -d')' -f1
else
    printf "Nothing\n"
fi

Answer 3

This is maybe a little more succinct. It gets grep to only output the part of the line that matches the pattern instead of the whole line, by using grep -o :

#!/bin/bash
SLEEP=$(pmset -g | grep -o "sleep prevented.*[^)]")
if [ -z "$SLEEP" ]; then
   echo Nothing
else
   echo $SLEEP
fi

The pattern is sleep prevented and any characters following until a ) is encountered.