Bash: command not found after piping

Question

Hi,
I'm trying something quite commonly found in an Unix environment, namely piping the output of one command into the input of a subsequent one, eg.
   bash$ ps ax | grep -i bash
But unfortunately I'm receiving--and now comes the most curious part--most of the times (not always!!) an error message
   bash: grep: command not found
When calling grep directly
   bash$ grep
the resulting output is
   Usage: grep [OPTION]... PATTERN [FILE]...
   Try `grep --help' for more information.
"which"-ing the commands
   bash$ which bash ps grep
results in
   /bin/bash
   /bin/ps
   /usr/bin/grep
Cheers,
MMHein

Answer 1

Good, you tried a different shell.  sh and bash are essentially the same executable on Mac OS X.
Did you try a new user account?  What were those results?
Have you tried installing your own bash?  Either build from sources, or use something like MacPorts.org to install one already configured for Mac OS X.
You also have ksh and zsh if they happen to rock your boat (see /etc/shells for a list of installed shells).
I think both Linc an dI agree that we have never seen this situation before either, which is why we suspect something has become corrupt.  The "Nuke and Pave" approach tends to eliminate that possibility.  But since you have lots of Unix experience (even though you missed a few Unix flavors ), and want to find the root cause, then you need to start doing tests that eliminate components.
The shells on my Mac OS X 10.7.2 Lion system have the following chksum's
cksum /bin/*sh
188949626 1371648 /bin/bash
1672788767 772992 /bin/csh
4164734636 2180736 /bin/ksh
1711289041 1371712 /bin/sh
1672788767 772992 /bin/tcsh
2366092939 1103984 /bin/zsh
I would expect yours to be identical.  But that does not take into account any libraries used.  And if you have the same values, then you need to look at something else (again, a new account can go a long way to eliminating things in your environment, or pointing to a system component).