Bash variable in for loop

Question

I am sure this has been answered before, but I cannot find the solution.

for i in `ls | grep ^t`; do echo $i; done

This gives me the expected results (the files and directories starting with t). If I want to use a shell variable in the for loop (with the pipe), how do I do this? This was my start but it does not work.

z="ls | grep ^t"
for i in `$z` ; do echo $i; done

EDIT: I agree this example was not wisely chosen, but what I basically need ishow to use a variable (here $z) in the for loop (for i in $z) which has at least one pipe.

I try to state my question more clearly: How do I need to define my variable (if I want to loop over the output of a command with one or more pipes) and how is the call for the for loop?

@PatrickTrentin It's syntactically correct, but you shouldn't use it. — chepner
– chepner, Commented Mar 17, 2016 at 15:28
@chepner fair enough; eta: never-mind, I looked it on my own. (: — Patrick Trentin
– Patrick Trentin, Commented Mar 17, 2016 at 15:34
@chepner I answered z=`ls | grep ^t` for i in "$z"; do echo $i; done and z="$(ls | grep ^t)"; for i in "$z"; do echo $i; done and I got -3 do you know why? Please give me some advice. — fronthem
– fronthem, Commented Mar 17, 2016 at 15:46

Tom Fenech · Accepted Answer · 2016-03-17 15:34:28Z

4

To loop through all files starting with t, you should use glob expansion instead of parsing ls output:

$ ls t*
tf  tfile
$ ls
abcd  tf  tfile
$ for i in t*; do echo "$i"; done
tf
tfile
$

Your approach will break in a number of cases, the simplest being when file names contain spaces. There is no need to use any external tool here; t* expands to all files starting with t.

As for your question, you use ls | grep ^t

And another good practice is to use subshell instead of backticks which are more readable and can be nested, use this: $(ls | grep ^t)

edited Mar 17, 2016 at 15:34

Tom Fenech

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answered Mar 17, 2016 at 15:19

riteshtch

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1 Comment

Tom Fenech Over a year ago

I only read the first part of your answer, which was why I posted my own. Your advice to use t* is the best idea - you should move that to the top of your answer then I'll delete mine.

Etan Reisner · Accepted Answer · 2016-03-17 15:54:43Z

0

You don't use a variable for this. You don't put commands in strings. See Bash FAQ 050 for discussion about why.

You shouldn't be using a for loop to read data line-by-line anyway. See Bash FAR 001 for better ways to do that.

Specifically (though really read that page):

while IFS= read -r line; do
  printf '%s\n' "$line"
done < <(some command)

answered Mar 17, 2016 at 15:54

Etan Reisner

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