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Mac OS X Unix Tutorial

Part 5 - Working With Unix (page 1 of 2)

The Story So Far

The previous four parts covered Unix basics:

• navigating and viewing the directory structure and the files within it
• creating and removing directories
• creating and removing files
• understanding and changing file permissions.

We are now ready to start actually interacting with our files - viewing them, searching for them, searching within them, and changing them. In the next three tutorials I will introduce a set of core commands in some detail and describe how these can be combined to perform more complex tasks.

In this part I will lay the foundations by describing the shell and what it can do for you, and detailing some useful commands:

• head, tail
• locate, find.

In Part 6 I will introduce regular expressions and two commands that make use of them:

• 'grep' for searching within files
• 'sed' for changing files.

Part 7 will cover the concepts of re-direction and pipes, and how one uses pipes to combine simple commands to perform more complex tasks.

Parts 8 and 9 will cover shell scripting.

First off, the command left dangling at the end of part four - 'file'. 'file' examines any file and (attempts to) analyse it and report on what kind of file it is.

For example:

% file ~
/Users/saruman: directory


% file /bin/ls
/bin/ls: Mach-O executable ppc


% file Documents/My\ Setup/change.log
Documents/My Setup/change.log: ASCII text


% file bin/osxfaq-css
bin/osxfaq-css: Bourne shell script text

Try it on any file; you will usually get an intelligent response. How does 'file' do this? Well, with a little inside information and a little magic - /etc/magic to be precise.

Before we begin, a note for those who have upgraded to Jaguar (Mac OS X 10.2).

Up until Mac OS X 10.2 a considerable amount of shell initialization was included by default. For whatever reason, the set up in 10.2 does not include it. Consequently you may notice that the shell behaves differently. If you wish to restore the shell initialization of pre 10.2 see the README file in /usr/share/tcsh/examples.

% which cd
/bin/ls

Where is the Bin?

You will find Unix commands in one of eight places.

/bin contains a minimum set of essential user commands.

/usr/bin contains the rest of the user commands that are delivered with your machine.

/usr/local/bin is for user commands that you (or the system administrator) have added locally.

~/bin is for user commands that you personally have installed and are not intended for other users.

For each of the above there is a corresponding 'sbin' directory for system, as opposed to user, commands. These are commands that control the Unix system and its configuration.

The Right Path

The shell does not implicitly know which directories to search in order to find an executable. It must be told via the 'path' shell variable.

Type:

to see which directories are in your search path.

It should include at least the following:

echo $path
/usr/bin  /bin  /usr/sbin  /sbin

and in that order.

Ideally, it should include:

echo $path
 /Users/your-user-name/bin
 /Developer/Tools  /Developer/Applications
 /usr/local/bin  /usr/bin  /bin
 /usr/local/sbin  /usr/sbin  /sbin
 .

You can set up 'path' from a login script in the file '.login' in your home directory. Use an editor to add these lines to the file:

## System-wide tcsh login file
#


# Set paths
set path = ( \
   ~/bin \
   /Developer/Tools /Developer/Applications \ 
   /usr/local/bin /usr/bin /bin \
   /usr/local/sbin /usr/sbin /sbin \
   . \
	   )

You only need the line:

if you have installed the developer tools.

The next time you start the Terminal, or create a new Terminal window, the shell running in that window will automatically execute '.login' and your new path should be active.

The environment variable 'PATH' also defines the search path. The shell automatically keeps 'PATH' and 'path' synchronized so you need only maintain one of these. I have not dealt with shell or environment variables before, but I shall do so in Part 8.

Note: If you add the 'set path' command to your login file, ensure that there are no characters (not even a space) after the '\' character at the end of each line.

Next Page

This page has set the scene, describing the environment in which commands are executed, and the potential for combining them.

Page two will consider some commands in more detail - head, tail, locate, and find.

Part 5 - Working With Unix (page 1 of 2)