How does Linux decide which executable I'm trying to run?

Question

I thought I understood this, but I'm getting unexplained behavior. I have these files:

$ which -a kvm | xargs ls -l
lrwxrwxrwx 1 root root 18 2011-11-17 17:14 /usr/bin/kvm -> qemu-system-x86_64
lrwxrwxrwx 1 root root 18 2011-11-17 17:21 /usr/local/bin/kvm -> qemu-system-x86_64
$ which -a qemu-system-x86_64 | xargs ls -l
-rwxr-xr-x 1 root root 2535168 2011-10-04 02:44 /usr/bin/qemu-system-x86_64
-rwxr-xr-x 1 root root 3497304 2011-11-17 16:26 /usr/local/bin/qemu-system-x86_64

And they are different versions:

$ /usr/bin/kvm --version
QEMU PC emulator version 0.12.3 (qemu-kvm-0.12.3), Copyright (c) 2003-2008 Fabrice Bellard
$ /usr/local/bin/kvm --version
QEMU emulator version 0.15.92 (qemu-kvm-devel), Copyright (c) 2003-2008 Fabrice Bellard

My path has /usr/local/bin first, and when I do which I see that one. So I'd expect it to win. But instead the older version wins:

$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/var/lib/gems/1.8/bin
$ which kvm
/usr/local/bin/kvm
$ kvm --version
QEMU PC emulator version 0.12.3 (qemu-kvm-0.12.3), Copyright (c) 2003-2008 Fabrice Bellard

Why does kvm --version give me 0.12 instead of 0.15???

Answer 1

Linux does not make the decision in this case; it's supplied with the exact path by your shell. When using advanced shells such as bash or zsh, the which tool is inaccurate since it only checks $PATH, but there are several more possibilities. Running type kvm should tell you where the shell is looking for it.

• "hashed": The first time you run a command, the bash shell remembers its location so that it won't have to search the entire $PATH again. The remembered locations are only kept in memory. Use hash to list them and hash -r to reset, or simply start a second shell window.

• "alias": You may have created an alias "kvm" pointing to the old path. If this is the case, use unalias kvm to remove the alias.

• "function": Similar to aliases, but more expressive. Again, it's possible that you have added and forgotten a function named kvm pointing to the wrong path. Use unset kvm to remove such a function.

Answer 2

You can override the PATH search by declaring an alias

alias kvm=/sbin/kvm

Try type -a kvm

I created an alias for date

$ which -a date
/usr/local/bin/date
/bin/date

$ type -a date
date is aliased to `/bin/date'
date is /usr/local/bin/date
date is /bin/date