如何保留每个Modify块首行并删除后续Modify行？sed命令求助

Got it, let's fix this problem. You want to retain the first Modify: line that follows each File: entry, and discard any subsequent consecutive Modify: lines. Your initial sed attempts didn't work because they either deleted all Modify: lines or used invalid syntax for block-specific line numbering. Here are two reliable approaches:

1. Using sed with a State Flag #

This uses sed's hold space to track whether we've already kept the first Modify: line for the current File: entry:

sed '/^ File:/{h;p;b};/^ Modify:/{x;/./{x;d};x;p;h}' sample.txt

How it works: #

• When we hit a line starting with File::
  • h: Save the line to the hold space (sets our "waiting for first Modify" flag)
  • p: Print the File: line
  • b: Skip the rest of the script for this line
• When we hit a line starting with Modify::
  • x: Swap the pattern space (current line) with the hold space
  • /./{x;d}: If the hold space isn't empty (meaning we already kept a Modify: line for this File:), swap back and delete the current line
  • If the hold space is empty (first Modify: line for this File:), swap back, print the line, and save it to the hold space (h) to mark that we've already kept one

2. Using awk (More Intuitive Syntax) #

awk makes state tracking straightforward with variables:

awk '/^ File:/ {print; keep_modify=1; next} /^ Modify:/ {if (keep_modify) {print; keep_modify=0} next} 1' sample.txt

How it works: #

• When a File: line is found: print it, set keep_modify to 1 (we want to keep the next Modify: line), then move to the next line
• When a Modify: line is found: if keep_modify is 1, print the line and set keep_modify to 0 (so subsequent Modify: lines are skipped); otherwise, skip the line
• The final 1 ensures any other lines (if present) are printed (not necessary for your input, but adds robustness)

Testing the Solution #

If you save your input to sample.txt and run either command, you'll get exactly the output you're expecting:

File: /Music/David Bowe/1.mp3 Modify: 2010-11-24 09:36:33.000000000 -0200 File: /Music/Music/David Bowe/Tom.mp3 Modify: 2010-10-22 11:12:15.000000000 -0200 File: /Music/Music/David Bowe/Hero.mp3 Modify: 2010-10-26 07:21:56.000000000 -0200 File: /Music/Music/David Bowe/AbsoluteBeginners.mp3 Modify: 2010-10-22 11:12:07.000000000 -0200 File: /Music/Music/David Bowe/ImAfraidOfAmericans.mp3 Modify: 2011-11-13 08:54:57.000000000 -0200

内容的提问来源于stack exchange，提问作者Rossano Fenner