What does it mean for a shell to parse its command line and to identify the PATH?

Question

After accepting the command entered by user,the shell generates a readily executable simple command line by parsing it evaluating variables(if present),performs command substitution,interprets metecharacters like * and ?(if present) and identifies the PATH.

Can anyone explain to me in simple terms the parsing part and identifying the Path part?

*text is taken from Unix and shell programming by M.G.Venkateshurthy

Answer 1

This is a question about Unix and shell programming. When you run a command like cp dir/* $destination the shell has to do several things to simplify this command before executing it:

• Replace dir/* with a list of all the paths matching dir/* e.g. dir/file1 dir/file2 dir/file3
• Replace $destination with the value of the variable destination e.g. otherdir

There are other steps not shown here, like command substitution - for example $(ls) is replaced by running the command ls and replacing it with whatever ls outputs.

Finally as a last step, generally separate from the others as it does not actually edit the command, the shell has to find the actual path of the cp command, usually at /bin/cp. It does this by looking at the variable PATH, which holds a list of directories, then looking in each directory to find an executable called cp.