How to check if while loop was executed?

Question

cat list.txt | grep '^http://.*\.osm\..*$' | while read line; do
    fn=$(basename $line)
    do_something()
done
# TODO: check if it did something

In case the grep command returns nothing, it won't enter the loop and do_something() won't be executed.

I can't check what's in $fn outside of the while loop, see Bash variable scope.

What's the least invasive solution for checking if do_something() was executed here?

@klugerama Unless you are using `bash` 4.2 and the `lastpipe` option, there is no way for a variable set inside that loop to be visible afterwards. — chepner, May 14 '13 at 16:17

Bill · Answer 1 · 2013-05-14T16:35:12.680

4

You can use a flag whose value will be changed if the loop is executed. Below is a poc.

shopt -s lastpipe   # bash 4.2 or greater only
flag="false"
cat list.txt | grep '^http://.*\.osm\..*$' | while read line; do
    fn=$(basename $line)
    do_something()
    flag="true"
done

if [ "$flag" = "true" ]
then
    echo "loop was executed"
fi

You need to use the following if while spans a sub-shell (thanks to all who commented below)

 while read line
   do
       fn=$(basename $line)
       do_something()
       flag="true"
   done < <(grep '^http://.*\.osm\..*$' list.txt)

edited May 14 '13 at 16:35

answered May 14 '13 at 16:06

Bill

5,263
6
35
50

2

Will that really work? The body of the while loop runs in a subshell, so I would have thought changes, even to existing variables are not propagated back up. – Adrian Ratnapala May 14 '13 at 16:08
1

That doesn't work because of the different scope, see the link in my description – didi_X8 May 14 '13 at 16:12
3

That depends on the particular shell. Assuming recent versions of `bash`, which seem to be most common, it will not. Older versions of, e.g. `ksh` will work, though... – twalberg May 14 '13 at 16:13

score 3 · Accepted Answer · answered May 14 '13 at 16:16

Since the while loop runs in a sub shell, it cannot propagate values back up to the parent shell. There are still ways to do what is wanted, though. Here's one:

if grep -q '^http://.*\.osm\..*$' list.txt       # loop will definitely run
then
  while read line
  do
    # do something with line
  done < <(grep '^http://.*\.osm\..*$' list.txt)
else                                             # no matches, loop will not run
  # do something else
fi

It has the side effect of running grep twice, which could be avoided by saving the output of grep and post-processing it, as suggested in another answer, but in some ways this is a bit simpler to understand...

score 2 · Answer 3 · answered May 14 '13 at 16:16

Save the output of grep in a variable and test it explicitly.

filtered=$(cat list.txt | grep ....)

if [ -z "$filtered" ] ;
  ... handle empty output ..
 else
  ... do your while loop here... 
fi

BTW: newlines are preserved inside the "$filtered", but be sure to quote it when you use it.

score 2 · Answer 4 · answered May 14 '13 at 16:21

You can use a process substitution in place of the pipeline, which would allow a flag to be usefully set:

flag=0
while read line; do
    flag=1
    fn=$(basename $line)
    do_something()
done < <( grep '^http://.*\.osm\..*$' list.txt )

if [[ $flag == 1 ]]; then
   ...
fi

Atle · Answer 5 · 2013-05-14T17:04:27.243

1

A variation on @chepner solution:

flag=0
while read line; do
    grep -q '^http://.*\.osm\..*$' <<< $line && {
      flag=1
      fn=$(basename $line)
      do_something()
    }
done < list.text

if [[ $flag == 1 ]]; then
   ...
fi

Use then one that better fits your need.

edited May 14 '13 at 17:04

answered May 14 '13 at 16:31

Atle

5,299
1
15
10

How to check if while loop was executed?

5 Answers5