0

I have these 2 commands being executed one after the other.

cat a.log >> b.log
rm -r a.log

What is exactly happening here? Why is cat being used here? What about the >> sign ? And what is the -r(recursive) flag being used?

roaima
  • 107,089
  • 14
  • 139
  • 261
Dawson Smith
  • 215
  • 2
  • 10
  • 3
    Related: [What are the shell's control and redirection operators?](//unix.stackexchange.com/q/159513) – Stéphane Chazelas Jan 06 '22 at 08:12
  • 1
    If I was given that code in context I might suggest that the second command _intended_ to remove `a.log` rather than `b.log`, i.e. `rm -f a.log`. But not an answer because it's only conjecture – roaima Jan 06 '22 at 09:20
  • edited the ques – Dawson Smith Jan 06 '22 at 09:21
  • You cannot `cat` a directory (only the files within it), and `rm -r` only has meaning for directories. So one (or both) of those commands was written by somebody who did not know what they were doing. Also, there is no error checking. It would be nice to be sure the cat worked (e.g. b.log could be created if needed, and permissions to write, and disk not full) before discarding a.log. – Paul_Pedant Jan 06 '22 at 10:31

1 Answers1

3

Considering your corrected code,

cat a.log >> b.log
rm -r a.log

  1. The cat copies its list of files, or stdin if either none is supplied or a dash (-) is included in the list to its stdout. See man cat

  2. The >> is a standard shell redirection operator that appends a command's output to a named file. In the example this named file is b.log. See man bash or the documentation for your preferred shell if not bash, and What are the shell's control and redirection operators?

  3. The rm command (almost) unconditionally removes the file a.log. Because it's trying to remove a file, the -r flag is irrelevant and is ignored. If being run in an interactive session the command will ask the user for confirmation if the file does not have write permission. See man rm

If I were writing this code I would probably construct it like this

cat a.log >> b.log && rm -f a.log

Here, the removal of the a.log file happens only if the cat successfully appended its contents to the target file b.log.

roaima
  • 107,089
  • 14
  • 139
  • 261