Well, when you specify a "naked" command like "ls", it means the shell has to figure out what the heck you want. If you issue a command like /usr/bin/ping the command filename is there or it is not there. Simple. No need to figure it out.

Note: ./myscript.sh means execute a file called myscript.sh in the current directory - current directory is what the "pwd" command shows you. This pertains to your assignment so remember it.

The order of lookup is:
1. builtin: builtins are commands like cd, for, while. These are literally part of the shell code itself. They are found first, if they exist.

if #1 fails:
2. alias: alias is a hashed list of commands that are custom-defined by you. Or for you.

For example, suppose you type poorly the way I do - with your elbows. Then a looong
command is cause for frustration due to typos. So:

Code:

alias cmd='some godawful long command that I cannot type well or quickly'

So, when shell gets the command "cmd" - it goes to the alias table and "recalls" the command for you. alias is also a good way on some OS that have multiple versions of a single command - like awk on Solaris - to specify the one version you want without having to type the full filename of the command all the time.

Code:

alias

shows you all of the alias-es you currently have.

if #2 fails:
the shell has to look through a list of directories to see if it can find the command.
This list of directories is stored in the PATH envrionment variable. This is central to answering many of the questions you have to answer.

Code:

PATH="/usr/bin:/usr/local/bin:."

The colons separate the directories. The shell works from left to right with (based on the example):

/usr/bin

if that fails
/usr/local/bin

if that fails it tries the "dot"
.

. is wierd. It means the directory you are currently in. So it can literally be any directory on the machine. It is bad for privileged users to have that in the PATH variable.

Code:

echo "$PATH"

will show you your current PATH. I would assume your prof wants you to use that to answer questions. You could hedge you bets by stipulating EXACTLY what you have in your PATH.

You now have sufficient basic knowledge to answer the questions. Put the pieces together.

There is one weiner in the questions. the 'shell variable'. If you create a shell variable like

Code:

foo="ls -l myfile.dat"
# simply type
$cmd

The shell first expands the variable, then executes it following all the steps above. If the shell variable is not a valid command it fails. BUT. it requires that you ask the shell to expand it with a $[variable name]