How to use an expression as loop counter in bash?

Question

I'm completely new in the Linux world, i-m using a Bash shell and practicing a bit. How can i perform a loop giving as counter an expression? For example looping through the amount of word in a file.

for n in(wc -w text.txt)
 do
   echo "word number $n"
 done

Unfortunately the above script its not working. The only way i have found so far is to first print the value of wc -w text.txt, then assign it to a variable and then loop:

wc -w text.txt
a=10   //result saw by wc -w text.txt
for((b=0; b<a; b++))
do
echo "$b"
done

The problem is that i need to retrieve the value of wc -c and asssing it directly to a variable, in case i have to write a script that runs automatically, the problem is that

a= wc -w test.txt

Will not work,

Any advice?

a=wc -w test.txt should be in backticks or $() to work and no space around =, i.e. a=$(wc -w test.txt) — Thor
– Thor, Commented Jul 6, 2012 at 9:08
a=$(wc -c test.txt) or a=$`wc -c test.txt` will assign to a a text value: '6 test.txt', if you try to iterate this value, you of course then will get an error (arithmetic expression needed) — JBoy
– JBoy, Commented Jul 6, 2012 at 9:15
Use a=$(wc -w < test.txt) to remove the filename in wc output. — Thor
– Thor, Commented Dec 10, 2012 at 12:14

Rody Oldenhuis · Accepted Answer · 2012-07-06 14:29:23Z

3

A clean way to do this is

tmp=$(wc -w <text.txt)
for ((i=0; i<$tmp; i++))
do 
    echo $i; 
done

The <text.txt part is necessary, because normally, the output of the wc command is

wordcount1  INPUTFILE1
wordcount2  INPUTFILE2
...

The max value for i must equal the word count of your file, so you must take away the INPUTFILE1 part before your loop works as intended. Using an input stream < rather than a filename takes care of this issue.

Using a temp variable prevents wc from being called on each iteration.

edited Jul 6, 2012 at 14:29

answered Jul 6, 2012 at 9:14

Rody Oldenhuis

38.1k7 gold badges53 silver badges98 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Nahuel Fouilleul Over a year ago

(set -v; your code) shows that wc is called for each iteration; use a variable instead

Rody Oldenhuis Over a year ago

Thanks! added your suggestions to the answer.

JBoy Over a year ago

is the dollar sign in this case used as indicator for an expression evaluation?

Rody Oldenhuis Over a year ago

@JBoy Yes, the $(...) notation works the same as backquoting in bash

Collectives™ on Stack Overflow

How to use an expression as loop counter in bash?

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related