I'm trying to run strace trough ccze, and the pipe doesn't work as expected.
The command-line I'm running to test is sudo strace -p $(pgrep apache2) | grep open, and all lines are output, ignoring grep.
Is there something special about strace that causes this behavior?
strace prints its traces on standard error, not on standard output. That's because it's common to want to redirect the standard output of the program, but usually not a problem that strace's stderr and the program's stderr are mixed.
So you should redirect strace's stderr to stdout to be able to pipe it:
sudo strace -p $(pgrep apache2) 2>&1 | grep open except that what you're really looking for is
sudo strace -p $(pgrep apache2) -e open 
open calls, I just put that as an example, what I'm really trying to do is color highlighting strace: Invalid process id: '-e' with the last command. I am on version 4.8. 2010-03-30. apache2 by the name of the process you're interested in. Check that pgrep prints a single PID, otherwise pick one and run e.g. sudo strace -p 1234 -e open vim color syntax highlighting? strace $CMD 2>&1 > /dev/null | vim -c ':set syntax=strace' -.