Getting Loopy with Bash: using for loops
July 6th, 2009 by Shawn Powers in
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__________________________
Shawn Powers is an Associate Editor for Linux Journal. You might find him chatting on the IRC channel, or Twitter
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December 2009, #188
If last month's Infrastrucuture issue was too "big" for you then try on this month's Embedded issue. Find out how to use Player for programming mobile robots, build a humidity controller for your root cellar, find out how to reduce the boot time of your embedded system, and if you're new to embedded systems find out the basics that go into one. You can also read about the Beagle Board, the Mesh Potato and a spate of other interestingly named items. And along with our regular columns don't miss our new monthly column: Economy Size Geek.
<grumble>
On July 7th, 2009 Guy Stalnaker (not verified) says:
And no sooner to I post here than I find the answer on another site, ShellGeek (imagine that). So, thanks anyway.
Guy
Using for at the prompt
On July 7th, 2009 Guy Stalnaker (not verified) says:
I was trying to come up with a way, using for, to run an executable on the output of the ls command in a directory. That is, to have the output of ls be the input to a command:
#> for m in `ls`; do echo $m; done
This works ONLY if the file names in the directory have no spaces, e.g.,
~>ls
autosave Documents Music
bin emerald-themes
Desktop logs.05182008.sp.zip
doc man
But if the filename is a multi-word name with spaces, the for construct assigns to m the individual words and the echo output breaks the filename across separate lines. I suppose this has more to do with how ls outputs, but I cannot find an ls option (not -Q nor -1 nor -x) that seems to get the for construct to 'think' that the filenames output by ls are legitimate if they have spaces (if that makes any sense).
I have found a way to use find's -exec or -print0 with xargs to do what I wanted to do, but I was curious if any of you gurus at linux journal can come up with a way of using for on the command line:
for x in '{command with output}'; do {command on output} $x; done
for m in `ls`;do command
On July 8th, 2009 Anonymous (not verified) says:
for m in `ls`;do command "$m";done
putting $m inside " marks causes $m to be a single argument. I do this all the time when I used Cygwin on Windows (at work).
--
John
re: it should be noted though
On July 7th, 2009 pschmitt52 (not verified) says:
Isn't this the LINUX Journal after all? :)
it should be noted though
On July 7th, 2009 slacker (not verified) says:
it should be noted though that calling seq makes it less portable as you wont likely find seq on many flavors of unix... granted you can argue that you wont find bash either but hey, just sayin.
target :)
On July 7th, 2009 thogarty says:
That looks like a Target ball in the intro :)
One of these days you should do some tech tips on how you make these videos (If you have already, please post a link so I can check it out). What hardware and programs you use to record and edit? I would like to make similar instructional videos for some of my personal and work documentation.
Good to see these shell tips. Are there cases when the list would be too long for the shell to handle directly requiring something like xargs? (maximum line length) If so, that may be another good tech tip to post. These tech tips are quickly becoming my favorite part of Linux Journal :). They're concise and informative.
for + seq
On July 7th, 2009 pschmitt52 (not verified) says:
If you want use the for loop as a counting loop
then you could do this:
for x in `seq 1 1000` do echo "line: $x" doneThis will run 1000 iterations.
#!/bin/sh # doing it in C
On July 8th, 2009 Anonymous (not verified) says:
#!/bin/sh
# doing it in C way
for ((I=0;I<=1000;I++))
do
printf "line: %03d\n" $I
done
you can also use the
On July 7th, 2009 exsnafu (not verified) says:
you can also use the following in newer(3+ i think?) versions of bash:
for x in {1..1000}; do
echo "line: $x"
done
Though Not Quite Interchangeable
On July 7th, 2009 Mitch Frazier says:
Note however that this works:
j=1000 for i in `seq 1 $j` do echo $i doneWhere as this does not:
j=1000 for i in {1..$j} do echo $i doneTo "make" it work do:
j=1000 for i in `eval echo {1..$j}` do echo $i done__________________________Mitch Frazier is an Associate Editor for Linux Journal and the Web Editor for linuxjournal.com.
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