Ambiguously named executables

Question

It appears that in tcsh if I have ambiguously named executables in two separate folders in the $PATH it directs to the one in the most recently created folder. Am I correct? And is this always true?

Answer 1

In general it will be the order it is found in $PATH since that is the intended usage for $PATH.

There could be an exception to this rule but that would require that the command is hard coded with a directory to the command in the software. I would consider that bad practice though.

Answer 2

You can't have two files with the same name in the same directory. If there are several versions of prog, you have one e.g. in /usr/bin and another one in /usr/local/bin, and the PATH environment variable decides which one is picked. This is done by one of the exec(3) family which looks at the PATH (most probably execvp(3)), the shell has no saying here.

Answer 3

In Solaris, there are typically two directories (actually more than two) that contain different versions of the same name program. An example is ps which is found in /usr/ucb and /usr/bin. If both /usr/ucb and /usr/bin appear in your PATH, the version of ps that gets executed is whichever one appears first in your PATH.

Answer 4

All shells traverse the directories in the $PATH in order, and execute the first command they find. For example, if your path is /usr/local/bin:/usr/bin:/bin, then the command from /usr/local/bin is executed if there is one; if there isn't, the command from /usr/bin is executed, and so on. It doesn't matter in which order the commands were installed.

There's one exception: if you install a program while the shell is already running, the shell may have kept the location of the program in a cache, in which case it will keep executing the old program. For example, if you start tcsh, then you run foo which is at that point /usr/bin/foo, then you install another version of the program in /usr/local/bin/foo and call foo from that shell again, it will still run /usr/bin/foo. A newly started shell will pick up /usr/local/bin/foo. Run the command rehash to empty tcsh's cache of command locations. In bash and zsh, the equivalent command is hash -r (zsh also understands rehash).