[solved] useless use of cat question

oliver · 2011-12-20 19:40:37

I'm trying to clean up some ksh scripts I inherited and found this:

for f in `cat file | grep -v ^#`
do
  ...
done

How would I do this without cat? I've been playing around with the following to no avail:

while read f
do
  ...
done<"grep -v ^# file"

Everything works in the scripts... it's just for my own amusement

Last edited by oliver (2011-12-20 19:56:05)

aiBo · 2011-12-20 19:45:15

this should do:

for f in $(grep -v ^# file)
do
... 
done

knopwob · 2011-12-20 19:45:40

i don't know if this will work in ksh, but in bash you could do this:

grep -v '^#' file |
while read f; do
     .....
done

edit:

actually, my solution will behave differently..
In my solution $f will contain a whole line on each iteration while in your old version with cat $f will contain only one word each iteration. This wouldn't matter, of course, if each line only contains one word.

Last edited by knopwob (2011-12-20 19:52:29)

oliver · 2011-12-20 19:55:26

thank you both... regarding the whole line versus one word I can play around with IFS to get around that.

I'll mark this badboy as solved

karol · 2011-12-20 20:02:59

Hey, wait for me!

#!/bin/bash
while read f
do
  echo $f
done < <(grep -v ^# file)

oliver · 2011-12-20 20:29:09

karol wrote:

Hey, wait for me!

#!/bin/bash
while read f
do
  echo $f
done < <(grep -v ^# file)

can you explain why the second < is needed?

alphaniner · 2011-12-20 20:48:21

The <(...) is process substitution, the lone < is the more familiar redirection. Essentially, it's like doing:

#!/bin/bash
grep -v "^#" file > file2
while read f
do
  echo $f
done < file2

In other words, the output of the process in parenthesis is treated as a file from which read pulls its STDIN.

Last edited by alphaniner (2011-12-20 20:51:33)

oliver · 2011-12-20 20:49:08

very nice... thanks again to all

Arch Linux

#1 2011-12-20 19:40:37

[solved] useless use of cat question

#2 2011-12-20 19:45:15

Re: [solved] useless use of cat question

#3 2011-12-20 19:45:40

Re: [solved] useless use of cat question

#4 2011-12-20 19:55:26

Re: [solved] useless use of cat question

#5 2011-12-20 20:02:59

Re: [solved] useless use of cat question

#6 2011-12-20 20:29:09

Re: [solved] useless use of cat question

#7 2011-12-20 20:48:21

Re: [solved] useless use of cat question

#8 2011-12-20 20:49:08

Re: [solved] useless use of cat question

Board footer