Using a variable in a bash script for loop

Question

I have a file which has the number of .pdfs in my folder. I assign this number to a variable, fileNum like so:

fileNum=$(ls -l *.pdf | wc -l)

echo $fileNum returns this number without any problem. Now I need to use fileNum in a for loop and I am having problems with it.

My for loop is:

for i in {1..$fileNum}
do
 var=$(awk 'NR=='$i 'pdfs.file')

 gs \
 -sOutputFile="exgs_"$var \
 -sDEVICE=pdfwrite \
 -sColorConversionStrategy=Gray \
 -dProcessColorModel=/DeviceGray \
 -dCompatibilityLevel=1.4 \
 -dNOPAUSE \
 -dBATCH \
$var

done

The $ at the beginning of fileNum gives me an error message which is:

awk: line 1: syntax error at or near {

Things are fine when I actually use the number itself (which in this case is 17). Obviously, awk doesn't like this because of ... something.... I don't know what. What should I do about this?

I tried other forms such as $(fileNum) and filenum with the single quotes around it. Is it something to do with strings?

first step: debugging: echo 'NR=='$i

Karoly Horvath
– Karoly Horvath

2012-08-20 16:13:55 +00:00
Commented Aug 20, 2012 at 16:13 — Karoly Horvath
– Karoly Horvath, Commented Aug 20, 2012 at 16:13

geirha · Accepted Answer · 2012-08-20 16:56:06Z

4

I'd use bash to read the file instead of running awk for every line.

while read -r file; do
    gs -sOutputFile="exgs_$file" \
       -sDEVICE=pdfwrite \
       -sColorConversionStrategy=Gray \
       -dProcessColorModel=/DeviceGray \
       -dCompatibilityLevel=1.4 \
       -dNOPAUSE \
       -dBATCH \
       "$file"
done < pdfs.file

See also http://mywiki.wooledge.org/BashFAQ/001

Otherwise, for the general case where you want to iterate from 1 to n, I'd use a C-style for-loop.

n=10
for (( i=1; i <= n; i++)); do
   ...
done

answered Aug 20, 2012 at 16:56

geirha

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Buggabill · Accepted Answer · 2012-08-20 16:10:27Z

2

This is because Bash will do the expansion on the braces before the variable. You need to use eval in this case so that Bash expands the variable first.

for i in $(eval echo {1..$fileNum})
do
 var=$(awk 'NR=='$i 'pdfs.file')

 gs \
 -sOutputFile="exgs_"$var \
 -sDEVICE=pdfwrite \
 -sColorConversionStrategy=Gray \
 -dProcessColorModel=/DeviceGray \
 -dCompatibilityLevel=1.4 \
 -dNOPAUSE \
 -dBATCH \
$var

done

answered Aug 20, 2012 at 16:10

Buggabill

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2 Comments

Buggabill Over a year ago

Take a look here. - tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_04.html

Buggabill Over a year ago

And here is a thread on linuxquestions.org that talks about it too. linuxquestions.org/questions/programming-9/…

Stephane Rouberol · Accepted Answer · 2012-08-20 16:11:00Z

0

I would rather write:

for i in $(seq $fileNum)
do
  ....
done

answered Aug 20, 2012 at 16:11

Stephane Rouberol

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