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OS X | UNIX

Mac OS X Unix Tutorial

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

Viewing Files - Heads or Tails

In part two I covered the commands 'cat', 'more' and 'less' to view files, which remain the best method outside of a text editor.

Use 'head' to display just the first few lines of a file - sort of like the Finder's preview.

For example:

% head *.txt

to remind you what each text file is about.

% head -25 *.txt

will display the first 25 lines.

To view the last few lines of files, use 'tail'.

For example:

tail /var/log/system.log

will display the last few entries in the system log.

A useful variant of 'tail' is the '-f' option, which causes 'tail' to continually read the file and update its output whenever the file (the end of the file) changes.

tail -f /var/log/system.log

is a command line version of the Console application. (Type control-c to quit.)

Now Where Did I Put That File? - locate

To paraphrase Douglas Adams:

The file system is big, very big. You may think your home directory is big, but that's just peanuts compared to the whole file system.

Just like the Finder (or Sherlock pre-10.2), the command line provides a means to root out files given a description of some sort. First we have 'locate'.

locate "*.txt"

will find all files ending in '.txt'. And it'll do it quickly. 'locate' uses a pre-built database which contains the names of all files in the file system.

If 'locate' does not work, it means the database has not yet been built. It is built weekly at 4:30 am on Sunday - if you have your machine up and running at that time.

To (re-)build the database type:

% sudo /usr/libexec/locate.updatedb

This will take a minute or so, and you will have to enter your admin password. When the prompt returns, try the locate command again. You should now be able to locate files anywhere on the file system.

There are a few important points to note in order to use 'locate' successfully.

'locate' matches against the whole pathname, hence:

% locate "bio.*"

will not locate the file bio.cwk (or rather the file '/Users/me/Documents/bio.cwk'), but

%locate "*bio.*"

will.

Remember to enclose the filename in quotes if it contains wildcards, otherwise the shell will interpret the '*' passing the filename expansion to 'locate'.

However, if one passes a filename with out any '*', it is treated as if it starts and ends with a '*'

% locate bio.

is equivalent to

% locate "*bio.*"

A Teaser

If your current working directory has a file called 'bio.cwk', then the command:

% locate bio.*

will locate bio.cwk, and perhaps other files like mybio.cwk. This may appear to contradict what I said above, but it doesn't. Can you think why?

Tell Me More...

Wildcards

You can make use of the wildcards covered in Part 3 - * ? [] - as part of the filename. It is essential to enclose the filename in quotes if it does contain wildcards to prevent the shell from expanding them before executing 'locate'.

The Locate Database

The locate database is updated by the Unix housekeeping scripts run periodically by 'cron'.

'cron' is a system program that runs every minute and reads a special file - '/etc/crontab' - that controls what is done and when.

Mac OS X has three scripts that run daily, weekly, and monthly.

Script 'weekly' in '/etc' updates the locate database.

If you want the Unix housekeeping daily, weekly, and monthly scripts to run (which is generally a good idea) you have several options:

don't let your Mac sleep
run the scripts manually
change the time at which the scripts run

or...

Mac Janitor

Install MacJanitor from Brian R Hill. This runs the three maintenance scripts for you.

Now Where Did I Put That File? - find

'find' is similar to locate, but it does a live search of the file system. The search will necessarily be slower, but does not rely on a potentially out of date database.

The real power behind 'find' lies in the way one specifies which files to locate, erm I mean find. One can search against just about any property of the file, not just name.

The (simplified) syntax of the find command is:

% find [options] starting-directory [search-criteria] [action]

'find' is recursive in that it searches the entire directory structure from 'starting-directory' down. It considers each file and directory selecting those that satisfy 'search-criteria', and performing 'action' on that file. The search-criteria can specify files by name, modification time, size, file attributes, and users/groups/permissions. Search criteria can be combined with 'and' and 'or' conditions. The actions can display the file (the default), list it, delete it, or execute any other command.

Note: 'find' is a very complex and powerful command. It is covered in more detail in a forthcoming advanced lesson. In this tutorial we will consider just name, time, and size criteria.

% find .

simply lists all the files and directories from the current (specified by the '.') directory down.

Note that if one searches the entire file system, it is necessary to do so with root permissions, and hence to use 'sudo'.

Find by Name

The search criterion '-name' allows you to search for files by name.

To find all files named with a '.txt' extension use:

% find . -name "*.txt"

Find by Time

The search criteria:

-mmin n is satisfied if the file was last modified n minutes ago, and
-mtime n is satisfied if the file was last modified n days ago

The elasped time is calculated from the point at which 'find' was executed, and rounded up to the nearest minute or day.

To find all files modified in ~/Sites three days ago:

% find ~/Sites -mtime 3

and 10 minutes ago:

% find ~/Sites -mmin 10

A time specified by -n means less than, while +n means more than.

To find all files in you home directory modified within the last week use:

% find ~ -mtime -7

Finally, the criterion 'newer' allows one to specify a file as the time. All files that are newer than (modified after) the given file will be found.

For example:

% find ~ -newer last-backup.log

will find all files changed (or created) since last-backup.log was modified.

Find by Size

The search criterion:

-size n

specifies a size in half-k (512 byte) blocks.

As with the time criteria, the value of 'n' can be preceded by + or - to specify larger than or smaller than.

To find all files that are larger than 2 megabytes (4000 of these 512 byte blocks):

% find ~ -size +4000

You can guess how to find empty files. However, this will not work when finding empty directories, as an empty directory does not have a size of 0. (All directories contain the '.' and '..' links). To find all empty files and directories use:

% find . -empty

The Find Action

'find' is able to perform a few actions on each file that satisfies all the search criteria. These are:

list the file details with '-ls'
delete the file with '-delete'
execute another command with '-exec' or '-ok'

To give a long list of all '.doc' files in your home directory, combine find by name with action 'ls'

% find ~ -name "*.doc" -ls

You can guess how the '-delete' option works. Obviously be careful with this one.

You can also execute any other command on each file found using '-exec'. For example, to display all '.txt' files in your home directory, we can use 'cat' on each file.

% find ~ -name "*.txt" -exec cat {} \;

The '{}' element of the command is a placeholder for the currently found file. It is replaced by the filename of the current file. The exec action must finish with a ';' character. The reason for the '\;' combination is to stop the shell seeing the ';' first.

Another example - add execute permission to all shell script files (those ending in '.sh' in this example).

% find . -name "*.sh" -ok chmod a+x {} \;

Here I have used the '-ok' action. This is the same as '-exec' and additionally asks for confirmation before the command is executed on each file.

The Find Depth Modifier

The command:

% find . -maxdepth 2

will find all files in the current directory, but will only search nested directories up to the specified maximum depth.

Remember to use:

%man find

to learn more about 'find'.

Tell Me More...

Wildcards

You can make use of the wildcards covered in Part 3 - * ? [] - as part of the filename given to '-name'. It is essential to enclose the filename in quotes if it does contain wildcards to prevent the shell from expanding them before executing 'find'.

Access, Change, Modify

'find' uses one of three timestamps when finding by time.

Access time:
This records when the file was last accessed in any way.

use '-amin' or '-atime'

Change time:
This records when a file was last changed in any way, for example written to or permissions changed.

use '-cmin' or '-ctime'

Modification time:
This records when a file's contents were last modified.

use '-mmin' or '-mtime'

More Shells - the c-shell

'csh' is the c-shell ('c' is a programming language). It is a good interactive shell, and has a scripting language that looks like 'c' language programs. However, it is not upward compatible with 'sh' and (according to many people) is flawed as a scripting shell.

The t-shell

'tcsh' is an improvement on 'csh'. It is a very good interactive shell and improves greatly on the scripting of 'csh'.

And More...

Others are the z-shell ('zsh') which is very powerful interactively and can run 'sh' scripts, and the korn shell, of which I know little.

All but the korn shell are included with Mac OS X as of 10.2

I would have preferred Mac OS X to use 'bash' as the default shell for two reasons:

It is standard on Linux and so is popular
It is upward compatible with 'sh' for scripting.

The problem with 'tcsh' is that one has to lean two scripting languages - 'sh' and 'tcsh'.

Running Another Shell

Type:

% sh

to work within a Bourne Shell, and

% exit

to exit the shell.

Next Part

In Part 6 I will introduce piping - directing the output of one command to form the input of the next command thus chaining a small number of commands together - an essential technique in harnessing the power of the command line and Unix tools.

I will also introduce the 'grep' command, which searches within files, and in Part 7 the 'sed' command, which changes the contents of files.

Until then, have a play with 'find' and get used to using it.

Enjoy :-)

Discuss this article in the Learning Center forum

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


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