How can I get the output of a command into a bash variable?

Question

I can't remember how to capture the result of an execution into a variable in a bash script.

Basically I have a folder full of backup files of the following format: backup--my.hostname.com--1309565.tar.gz

I want to loop over a list of all files and pull the numeric part out of the filename and do something with it, so I'm doing this so far:

HOSTNAME=`hostname`
DIR="/backups/"
SUFFIX=".tar.gz"
PREFIX="backup--$HOSTNAME--"
TESTNUMBER=9999999999


#move into the backup dir
cd $DIR

#get a list of all backup files in there
FILES=$PREFIX*$SUFFIX

#Loop over the list
for F in $FILES
do
    #rip the number from the filename
    NUMBER=$F | sed s/$PREFIX//g | sed s/$SUFFIX//g

   #compare the number with another number

   if [ $NUMBER -lg $TESTNUMBER ]
      #do something

   fi

done

I know the "$F | sed s/$PREFIX//g | sed s/$SUFFIX//g" part rips the number correctly (though I appreciate there might be a better way of doing this), but I just can't remember how to get that result into NUMBER so I can reuse it in the if statement below.

Paŭlo Ebermann · Accepted Answer · 2011-06-17 15:30:29Z

5

Use the $(...) syntax (or ``).

NUMBER=$( echo $F | sed s/$PREFIX//g | sed s/$SUFFIX//g )

or

NUMBER=` echo $F | sed s/$PREFIX//g | sed s/$SUFFIX//g `

(I prefer the first one, since it is easier to see when multiple ones nest.)

edited Jun 17, 2011 at 15:30

answered Jun 17, 2011 at 15:15

Paŭlo Ebermann

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5 Comments

Orbling Over a year ago

The only problem I ever have with the $(...) style, is that these days, I think I'm looking at jQuery. ;-)

CoderChris Over a year ago

That's what I tried initially (without the cat) but this doesn't seem to do the job. Rather than capturing the number i.e. the result of the filename passed through sed a couple of times) it seems to return the whole filename...

Paŭlo Ebermann Over a year ago

About cat, this should of course be echo. I did not fully read the question ...

Paŭlo Ebermann Over a year ago

@Orbling: The problem with the `` style is that it conflicts with the inline highlighting on Stack Overflow :-p

CoderChris Over a year ago

Nice one Paŭlo thanks! Any other day of the week and I'd have realised that's what I was missing too :)

Michael Lowman · Accepted Answer · 2011-06-17 15:16:17Z

0

Backticks if you want to be portable to older shells (sh):

NUMBER=`$F | sed s/$PREFIX//g | sed s/$SUFFIX//g`.

Otherwise, use NUMBER=$($F | sed s/$PREFIX//g | sed s/$SUFFIX//g). It's better and supports nesting more readily.

answered Jun 17, 2011 at 15:16

Michael Lowman

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