在 bash while 循环中解析和回显字符串

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:

1. The lines are printed in a different order than the original file?
2. no operation is done on picture1_123.txt and picture1 is not printed?

Thank you!

Answer 1

What Was Wrong

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.

---

How To Do It Right

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