2005/06/02 Bash looping

There are so many different ways to create bash loops. Let's just look at a few. After the first, I'll only list the initial line:

for i in 1 2 3 4 cow
do
 echo $i
done

# alternate:

for i in 1 2 3 4 cow; do echo $i;done

for i in `seq 1 10`

for ((j=1; j <= 10; j+=2))

while ((x++ < 10))

until ((x-- < -10))

for i in $(ls)

for in in `cat /tmp/somefile`

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Two other commands are useful within loops: "break" exits a loop, and "continue" goes to the next iteration:

for i in 1 2 3 4 cow 6 7 8
do
  # skips "cow"
  ((i * 1 ==  0)) && continue
 echo $i
done

for i in 1 2 3 4 cow 6 7 8
do
  # stops after "4"
  ((i * 1 ==  0)) && break
 echo $i
done

Arrays can be looped:

array=(one two three)
for x in ${array[@]}
do
   echo $x
done

array=(one two three)
for ((x=0; x < ${#array[@]} ; x++))
do
   echo ${array[$x]}
done

A problem that often comes up for new users is looping over certain files. For example, you might want to do something to every .txt file:

for i in *.txt
do
 something $i
done

That works, but if there are no ".txt" files, "something" gets passed "*.txt" just the same. If that's not a good thing, you need to do more:

if `ls *.txt > /dev/null 2>&1`
then
for i in *.txt
do
 something $i
done
fi

Or you might do something like this:

count=`ls *.txt 2>/dev/null | wc -l`
if [ "$count" -gt 0 ]
then
 echo "$count files to back up"
 for i in *.txt
 do
  something $i
 done
fi  

Yet another way to do something like that is:

set -- *.txt
[ -f "$1" ] && dosomething *.txt

If the set gets nothing, $1 will be empty. Another advantage is that you don't loop over each file if you don't need to.

As you've seen, there are a lot of different ways to do bash looping. Which you use depends mostly on on what you are trying to accomplish, and to a lesser extent on your own style and preferences.

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