CMPSC 311, Introduction to Systems Programming

Introduction to Unix

cat /etc/passwd        (the public part of the password file)
echo $HOME             (your home directory)
echo $SHELL            (which shell is started at login?)
hostname               (which system is this?)
w                      (who is logged in?)
who                    (who is logged in?)
ls                     (what files and directories do I have?)
pwd                    (which directory am I in?)

It is stated in APUE (p. 4),

"The only two characters that cannot appear in a filename are the slash character (/) and the null character.  The slash separates the filenames that form a pathname and the null character terminates a pathname.  Nevertheless, it's good practice to restrict the characters in a filename to a subset of the normal printing characters."

% ls -l x.c bits.html
-rw-rw----   1 dheller  fcse        3736 Aug 19  2002 bits.html
-rw-------   1 dheller  fcse         533 Dec  3  2004 x.c

We created a file using MS Word (Mac 2008), and saved it in four different formats.

% ls -l
-rw-------@ 1 dheller  fcse  22016 Aug 10 12:08 foo.doc
-rw-------@ 1 dheller  fcse  23398 Aug 10 12:07 foo.docx
-rw-------@ 1 dheller  fcse  26257 Aug 10 12:08 foo.rtf
-rw-------@ 1 dheller  fcse      5 Aug 10 12:13 foo.txt

% file *
foo.doc:              Microsoft Office Document
foo.docx:             Zip archive data, at least v2.0 to extract
foo.rtf:              Rich Text Format data, version 1,
foo.txt:              ASCII text, with CRLF line terminators

% cat foo.txt
foo

If you rename foo.doc to foo.bar, or foo.html, then it is still recognized as a Word file by Mac OS X.

a.out . | sort > /tmp/x
/bin/ls -a . > /tmp/y
diff /tmp/x /tmp/y
rm /tmp/x /tmp/y

The Standard I/O Library <stdio.h> defines stdin, stdout and stderr, and a lot of other stuff such as printf().  See also CP:AMA Ch. 22, or C:ARM Sec. 15.4.

stdin, stdout and stderr in C correspond to cin, cout and cerr in C++, but they are accessed via functions, and not through methods and operators.

In  command1 | command2 , the operating system maintains an intermediate buffer between command1 and command2.  The first command writes into the buffer, while the second reads from it.  If the buffer fills, the first command is temporarily suspended; if the buffer empties, the second command is temporarily suspended.  The OS also coordinates the processes to manage the buffer correctly.

A file descriptor has type int, and a file pointer has type FILE * .  FILE is a struct type defined in stdio.h.  The OS uses a file descriptor as an index into a small array of structures, while a FILE is a structure that contains a file descriptor and some additional information.  The details vary from one implementation to another, but the principles don't.

You can open a stream for reading, or for writing, or both, subject to permissions that are checked by the operating system.

When you have read all the bytes of a stream, you have reached end-of-file, and the next read operation returns a value that indicates this condition.  When entering data at the keyboard, and the keyboard is treated as stdin, you can type control-D to indicate end-of-file.  Be careful that you don't type control-D as a command, since that terminates input to your command shell.

% sleep 60             [60 seconds later we get a new prompt]
% sleep 60             [after some time, type control-C]
^C                     [the process was terminated]
% sleep 60             [after some time, type control-Z]
^Z
Suspended
% jobs
[1]  + Suspended                     sleep 60
% fg                   [allow the process to continue in the foreground]
sleep 60
% sleep 60             [that one finished, start another]
^Z
Suspended
% bg                   [allow the process to continue in the background]
[1]    sleep 60 &
% jobs
[1]    Running                       sleep 60
% jobs
[1]    Running                       sleep 60
%
[1]    Done                          sleep 60
%