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

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

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

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

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

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

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

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

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