Programming
bash
Updated Wed, 28 Sep 2022 23:50:20 GMT

For-loop over glob returns single string instead of list


I'm trying to loop over images in a directory, but instead of one image per loop, my for-loop only iterates once and the loop variable contains all the images.

#!/bin/bash
IMAGEDIR="/home/user/myimages/"
for file in "$IMAGEDIR*.jpg"; do 
  echo $file
  echo "next file"
done

Outputs:

/home/user/myimages/A.jpg /home/user/myimages/B.jpg /home/user/myimages/C.jpg
next file

I would expect it to output:

/home/user/myimages/A.jpg 
next file
/home/user/myimages/B.jpg 
next file
/home/user/myimages/C.jpg
next file

I've looked at several other questions, and I don't think I should need a nested for loop.

Additionally, I figured out that it works if I use a slightly different format

#!/bin/bash
IMAGEDIR="/home/user/myimages/"
for file in "$IMAGEDIR"*.jpg; do 
  echo $file
  echo "next file"
done

Why does the first glob return a single string and the second one return a list?

Solution

Why does the first glob return a single string and the second one return a list?

Because quotation suppresses pathname expansion, but quote removal leaves your glob subject to expansion later.

Consider your first example ...

for file in "$IMAGEDIR*.jpg"; do 
  echo $file
  echo "next file"
done

Parameter expansion applies within double quotes, so it expands "$IMAGEDIR*.jpg" to "/home/user/myimages/*.jpg". The quoting suppresses the significance of * for pathname expansion, however, and no other expansions apply, so "/home/user/myimages/*.jpg" is the final expanded version. Which is then subject to quote removal, so the first and only value that file take in that loop is (literally) /home/user/myimages/*.jpg.

In the body of the loop, then, parameter expansion expands echo $file to echo /home/user/myimages/*.jpg. The * is not quoted now, and pathname expansion happens after parameter expansion, so you get one echo command with the names of all the matching files as arguments (supposing that there are any matching files).

The difference in your second example is the placement of the quotation marks. With the * outside the quotation marks in the for line, you get pathname expansion there, resulting in several different values for variable file to take on successive iterations of the loop.

Comments (2)

  • +1 – Ah, so it's a case of two wrongs make a very confusing debugging experience. — Aug 06, 2022 at 00:41  
  • +0 – Yes, @Cecilia, I can imagine that that was so. You wanted to leave the * out of the quotation marks, but if there was any point to the quotation marks in the first place (and there was) then you should have quoted $file inside the loop body. And those issues combined to disguise both. — Aug 06, 2022 at 00:44  

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