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 {1..10} (as of Bash 3.0) for i in $(ls) for in in `cat /tmp/somefile`
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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More Articles by Tony Lawrence © 2011-08-08 Tony Lawrence
The history of the world teaches us that succession is dangerous and that the strong take what they want. It's not likely to be any different with Linux. (Tony Lawrence)
Mon Aug 8 14:46:51 2011: 9679 TonyLawrence
Now that Bash 3 is becoming more common (with Bash 4 lagging behind), you should also see Bash 3 brace expansion (link)
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Bash looping Copyright © June 2005 Tony Lawrence
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