154 csh, pipes, and find
Now suppose that “ftp” was a symbolic link (bear with me just a
while longer). Suppose that it points to the directory /com/ftp/pub/
alan. Then after “cd ftp” I’m sitting in /com/ftp/pub/alan.Like all directories /com/ftp/pub/alan contains an entry named “. .”
that refers to its superior: /com/ftp/pub. Suppose I want to go there
next. I type:% cd ..Guess what? I’m back in /home/ar/alan! Somewhere in the shell
(apparently we all use something called “tcsh” here at the AI Lab)
somebody remembers that a link was chased to get me into /com/ftp/
pub/alan, and the cd command guesses that I would rather go back to
the directory that contained the link. If I really wanted to visit /com/
ftp/pub, I should have typed “cd. /. .”.Shell Programming....................................................................
Shell programmers and the dinosaur cloners of Jurassic Park have much in
common. They don’t have all the pieces they need, so they fill in the miss-
ing pieces with random genomic material. Despite tremendous self-confi-
dence and ability, they can’t always control their creations.Shell programs, goes the theory, have a big advantage over programs writ-
ten in languages like C: shell programs are portable. That is, a program
written in the shell “programming language” can run on many different fla-
vors of Unix running on top of many different computer architectures,
because the shell interprets its programs, rather than compiling them into
machine code. What’s more, sh, the standard Unix shell, has been a central
part of Unix since 1977 and, thus, we are likely to find it on any machine.Let’s put the theory to the test by writing a shell script to print the name
and type of every file in the current directory using the file program:
Date: Fri, 24 Apr 92 14:45:48 EDT
From: Stephen Gildea <[email protected]>
Subject: Simple Shell Programming
To: UNIX-HATERS