Question

If i have a do loop that looks something like

for file in *txt; do {something on $file that results in another file ending with txt being created}; done

Will that cause an infinite loop? I’m rather afraid to test it.

Answer 1

The expansion of *.txt is typically done by the shell before any commands are run. So, no, it won’t result in an infinite loop.

I say “typically” since it depends, of course, on the shell you’re using. But every shell I’ve had experience with works this way, including bash as shown below:

pax$ rm -rf *.xyzzy ; touch 1.xyzzy 2.xyzzy ; ls *.xyzzy
1.xyzzy  2.xyzzy

pax$ echo ===== ; for i in *.xyzzy ; do
>    echo Processing $i
>    echo ..... before touch ; ls *.xyzzy
>    touch 3.xyzzy
>    echo ..... after touch ; ls *.xyzzy
>    echo =====
> done
=====
Processing 1.xyzzy
..... before touch
1.xyzzy  2.xyzzy
..... after touch
1.xyzzy  2.xyzzy  3.xyzzy
=====
Processing 2.xyzzy
..... before touch
1.xyzzy  2.xyzzy  3.xyzzy
..... after touch
1.xyzzy  2.xyzzy  3.xyzzy
=====

pax$ ls *.xyzzy
1.xyzzy  2.xyzzy  3.xyzzy

You’ll note that the 3 file added during the first loop iteration doesn’t affect the loop at all, because the expansion has already been done at that point.