parse and echo string in a bash while loop

Question

I have a file with this structure:

picture1_123.txt
picture2_456.txt
picture3_789.txt
picture4_012.txt

I wanted to get only the first segment of the file name, that is, picture1 to picture4. I first used the following code:

cat picture | while read -r line; do cut -f1 -d "_"; echo $line; done

This returns the following output:

picture2
picture3
picture4
picture1_123.txt

This error got corrected when I changed the code to the following:

cat picture | while read line; do s=$(echo $line | cut -f1 -d "_"); echo $s; done

picture1
picture2
picture3
picture4

Why in the first:

Thank you!

Your `cut` command is reading the entire rest of stdin, so nothing is left for the next `read` command. — Charles Duffy, Oct 26 '20 at 15:43
...if you wanted it to `cut` contents in `line` only, and save the result back to `line`, that might instead be `line=$(cut -f1 -d "_" <<<"$line")` -- but don't do that, starting a new copy of `cut` per line of input is horribly inefficient. — Charles Duffy, Oct 26 '20 at 15:44
Also, note that `echo $line` is itself buggy. Always, **always** quote your expansions, as in `echo "$line"`; see [I just assigned a variable, but `echo $variable` prints something else!](https://stackoverflow.com/questions/29378566/i-just-assigned-a-variable-but-echo-variable-shows-something-else) -- or try testing with a file named `*_foo.txt` — Charles Duffy, Oct 26 '20 at 15:48

Charles Duffy · Accepted Answer · 2020-10-26T15:50:13.977

Here's what your old code did:

On the first (and only) iteration of the loop, read line read the first line into line.
The cut command read the entire rest of the file, and wrote the results of extracting only the desired field to stdout. It did not inspect, read, or modify the line variable.
Finally, your echo $line wrote the first line in entirety, with nothing being cut.
Because all input had been consumed by cut, nothing remained for the next read line to consume, so the loop never ran a second time.

The simple way to do this is to let read separate out your prefix:

while IFS=_ read -r prefix suffix; do
  echo "$prefix"
done <picture

...or to just run nothing but cut, and not use any while read loop at all:

cut -f1 -d_ <picture

1 Answers1