Question

please consider the following:

$ cd ~/.emacs.d/lisp/tabbar
$ git pull
Already up-to-date.

$ git pull | grep -q "Already" ; echo $?
0

Still with me? Now:

$ cd ~/src/emacs-tmp/trunk
$ bzr up                           
Tree is up to date at revision 108837 of branch /home/px/src/emacs-tmp/trunk

$ bzr up | grep -q "Tree" ; echo $?     
Tree is up to date at revision 108837 of branch /home/px/src/emacs-tmp/trunk
1

Questions :

• Why is the return code of grep “0” (found) in the 1st case and
  “1” (not found) in the 2nd ?
• Why is the output of the first (git pull) command
  hidden when grepping for it ?

Answer 1

Looks like bzr is outputting to the standard error device (/dev/stderr), while grep is only inspecting the standard input. You can confirm or deny this guess by trying to redirect stderr to stdin:

bzr up 2>&1 | grep -q "Tree" ; echo $?