Bash "while read loop" does not properly recognize variables [duplicate]

Question

I am using MacBook pro terminal to execute a shell script. It loops through a text file and create filenames based on each line in the file.

#!/bin/bash

year=2010
list=list_test.txt

mydir=thisdir

i=1  # counter

while read line
do
    echo $i $line
    file1=`echo $mydir/file_$year_$line_test.tif`
    file2=`echo $mydir/file_$year_$line_test.tif`
    echo $file1 $file2 

    i=$(($i+1))

done < $list

However, the output is peculiar:

1 17019
thisdir/file_.tif thisdir/file_.tif
2 17029
thisdir/file_.tif thisdir/file_.tif
3 17039
thisdir/file_.tif thisdir/file_.tif

Within the loop, bash does not recognize some variables, like "year" which is a global, and "line" which is read from the text file. The text file is as below:

17019
17029
17039

Another script with exactly the same manner works very well. This is mysterious to me now.

Any help or comments are extremely appreciated! Thanks very much!

You want $mydir/file_${year}_${line}_test.tif (with the curly brackets), otherwise Bash thinks you mean the variables year_ and line_test. — gniourf_gniourf
– gniourf_gniourf, Commented Sep 19, 2017 at 16:51
The process of building a minimal reproducible example might have helped to narrow this down -- if you'd tried to reproduce with only the code line=foo; year=2010; echo $mydir/file_$year_$line_test.tif, it would have been obvious that the read loop is not required to produce this problem. — Charles Duffy
– Charles Duffy, Commented Sep 19, 2017 at 16:59

chepner · Accepted Answer · 2017-09-19 16:51:41Z

2

_ is a valid character for an identifier, but you want to use it as a literal character in the file name. You need to use the full form of parameter expansion, ${x} instead of $x.

(Also, the command substitution isn't necessary.)

file1=$mydir/file_${year}_${line}_test.tif
file2=$mydir/file_${year}_${line}_test.tif

answered Sep 19, 2017 at 16:51

chepner

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1 Comment

Kang Over a year ago

Thanks so much! It works now! Also thanks for the suggestion. Really appreciate it.

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