Learning the Terminal in Jaguar, Part 1

Editor's note -- After reading the chapters Chris Stone contributed to Mac OS X: The Missing Manual, and previewing his work in Mac OS X in a Nutshell, I asked him to write a few articles for the Mac DevCenter to help people become more comfortable with the Terminal application in Jaguar (Mac OS X 10.2). These tutorials give you a preview of what Chris has covered in the Missing Manual.

If you've read some of his earlier tutorials, this series will feel vaguely familiar. That's because some of the examples here are the same as before. What's different is that this series is for Jaguar and the previous articles were written for Mac OS X 10 - 10.1. Jaguar changed a lot of things, and these new tutorials will show you how to work effectively in Apple's latest operating system.

Getting Comfortable

Mac OS X's Terminal application -- there it sits in your Utilities folder, foreign and mysterious. You've heard that it's a portal to the new world of the Unix command line, a world where your flurries of mouse clicks can be replaced with a just few keystrokes.

But you've been wary of rushing into this new territory where the keyboard is king, concerned that without enough knowledge you might get lost, stuck, or worse. Or maybe you're an adventurer, just waiting to dive into uncharted waters.

This article is for you. Regardless of why you've previously avoided [yourhost:~] yourname%, I'll show you how to take your first steps with the Terminal application in Jaguar. Then I'll walk you through a tutorial that will accelerate your understanding of the Unix command line.

In Part 1 of this series, you'll learn more about what Terminal does and get an overview of the tutorial procedure. In Part 2 you'll jump into the tutorial itself to learn the fundamental Unix commands necessary to get started with just about any command-line procedure.

Then, in Part 3, you'll finish the rest of the tutorial, as well as learn a few more things you can do with the command line.

The Command-Line Interface

The command-line interface (CLI) displayed in Terminal's windows provides access to the Unix shell, which is really just another way to interact with your Mac. The other method, which you're probably more comfortable with, is the Aqua interface. Aqua enables you to tell the Mac what to do by clicking on icons and menus to launch graphical applications.

The shell, on the other hand, allows you to type text commands to accomplish much of the same work. Typically these typed commands launch tiny, single-purpose Unix applications which do some specific work and then quit. The shell itself is an application that plays the go-between for the commands that you enter and the Unix kernel at the core of Mac OS X. There are in fact several shells available. By default Mac OS X uses a shell called tcsh.

If you're curious about why you would want to use the shell in the first place, see the article Why Use a Command Line Instead of Windows? for more information about the CLI versus the Aqua interface.

The Procedure

To help you learn the Terminal application more quickly, I'm going to introduce you to a Unix utility built right into your Mac OS X system. Working with this utility will help you get more comfortable with core Unix commands.

Related Reading Mac OS X in a Nutshell A Desktop Quick Reference By Jason McIntosh, Chuck Toporek, Chris Stone

Installed with Mac OS X is a mechanism that performs important fine-tuning of your system. It's called cron. By using this Unix task-scheduling utility, your system can regularly purge itself of outdated, space-hogging log files, update system databases so utilities like locate can work effectively, and do several other maintenance tasks that keep your system running lean and mean.

The cron utility fully automates this process, meaning that once everything is configured, the housecleaning will happen unattended as scheduled. The good news is that Apple has done the configuration for you. The not-so-good news is that they've scheduled these groups of tasks, or cron jobs, to run between 4:00 and 5:00 in the morning -- a time when your Mac is likely not even on! And if your Mac is never on during these times, these important tasks will never happen. If your Mac is powered on but in deep sleep, the jobs still won't run.

In this tutorial, I'll show you first how to modify the cron schedule, which is read from a file called the system crontab, so that these tasks occur at more reasonable times. I'll then explain how to configure Mac OS X's built-in mail server so that you'll receive report by email every time the cron jobs run.

The Tutorial

For this tutorial, make sure you're running Mac OS X 10.2 or newer (for older versions of OS X, refer to earlier tutorials in this series), and that you're logged in with an administrator's, though not the root, account.

Open the Terminal program, which you'll find in the Applications --> Utilities folder. Once launched, Terminal opens a single window displaying the time of the last login, a greeting, and a third line of text that comprises the prompt. With that window active, anything you type will enter just before the rectangular cursor that follows the prompt. After you type a command, simply press Return or Enter to run it.

The prompt shows the name of your computer (or rather its host name, which can vary), and then identifies your current working directory ("directory" is just the Unix term for "folder"). The current working directory is "where you are," that is, the location in your filesystem hierarchy that your next command will act on. Your initial working directory is always your home directory, which is identified in the prompt by the home directory shortcut character "~".

To fully display the path to your working directory, use the pwd command: Type pwd (which means "print working directory," though it only displays it) and press Return:

[haru:~] chris% pwd 
/Users/chris 
[haru:~] chris%

As you can see, pwd does its job by displaying the full path, or pathname, to your home directory and providing you with a new prompt when done. This path name begins with the slash character, which represents the root or top-most directory of your filesystem. Note that directories that reside on your system disk do not include that disk's name in their pathnames.

To act on a different set of files, you simply change your working directory using the cd command. Our firs step is to modify the crontab file, which exists in the /etc directory (normally invisible to the Finder). Enter cd followed by a space and the path name of the target directory, /private/etc:

[haru:/etc] chris% cd /private/etc 
[haru:/private/etc] chris% ls

Notice the change in the prompt reflecting the new working directory. If you're curious about what your working directory contains, use the ls, or list command:

[haru:/private/etc] chris% ls
[haru:/etc] chris% ls
6to4.conf                 iftab                     rc.netboot
afpovertcp.cfg            inetd.conf                resolv.conf
appletalk.cfg             kcpassword                resolver
appletalk.nvram.en0       kern_loader.conf          rmtab
atalk                     localtime                 rpc
authorization             magic                     rtadvd.conf
bashrc                    mail                      services
crontab                   mail.rc                   shells

As you can see, there are a lot of items -- quite a bit more than what's shown here -- in /private/etc, including crontab.

The Crontab File

The cron application launches automatically at system startup and runs continuously in the background executing commands as instructed by the crontab files. These files tell cron exactly what commands to run and when to run them. In fact, each user account can have its own crontab file. The system crontab found in /private/etc belongs to the super-user, or root account, and therefore can specify commands requiring the same total system access allowed to root.

cp

Before you modify the system crontab, you should make a backup copy in case you need to revert back to its default state. You'll use the cp (copy) command to do this, which lets you copy and rename a file in one step.

Normally to rename and copy a file into the same directory, you would type cp, followed by the name of the original file, and then the name of the copy:

[haru: /private/etc] chris% cp crontab crontab.bak 
cp: crontab.bak: Permission denied

Hold on. It looks like you don't have permission to write to the etc directory. In fact, only root can write to /private/etc. Because you are not logged in as root, it seems that there's no easy way to write to this directory. But there is.

sudo

The sudo utility ("substitute-user do") allows you to gain temporary root privileges on a per-command basis. To use sudo, simply preface the command you wish to run as root with sudo and a space, and sudo will prompt you for your password (not root's). If you have administrator privileges, entering your password will run the sudo'ed command as if the root user were doing it.

Warning: Use sudo with care. You can easily make mistakes with sudo that could require a complete re-installation of the OS to get going again. If that thought makes you queasy, it would be wise for now to use sudo only as directed in this article.

To perform the previous command successfully, preface it with sudo:

[haru:/private/etc] chris% sudo cp crontab crontab.bak 
Password: 
[haru:/private/etc] chris%

Notes about sudo:

• The first time you run sudo, you'll see another reminder to use sudo with care.
• You'll only need to enter your password when you haven't already used sudo within the last 5 minutes.
• It's not necessary to activate the root account to use sudo.

Now that you have a backup, you'll want to know how to restore it should you make some kind of unrecoverable mistake when editing. The restore procedure is the reverse of the backup:

[haru:/private/etc] chris% sudo cp crontab.bak crontab

In this case, you're overwriting the non-working crontab file with a fresh copy of the original you've saved previously as crontab.bak, which remains unchanged. Notice that, by default, cp will overwrite a file without warning. If you would instead like to be prompted to approve such overwrites, include cp's -i option flag.

Option flags allow you to modify the behavior of commands. The -i option flag for cp tells cp to display a prompt, asking you to allow the overwriting. Do so by typing y for yes or cancel it by typing n for no. To use the flag, simply type the cp command, add a space, type the -i flag, a space, and then the rest of the command:

[haru:/etc] chris% sudo cp -i crontab.bak crontab
overwrite crontab? y
[haru:/etc] chris%

What you need to do next, then, is edit this system crontab file, and you'll learn how by using a command-line text editor called pico. However, if you were to first examine the privileges for /etc/crontab, you would see that it's owned by root, and only root has write privileges. Sounds like another job for sudo.

pico

Of the several CLI text editors included with Mac OS X, pico is the easiest to learn. To open a text file in pico, simply enter the file name after the pico command. Used with sudo, the command to edit the crontab file in the /etc directory looks like this:

[haru:/private/etc] chris% sudo pico crontab

And this is what you'll see when you run it:

The document's text area lies between the black title bar at the top and the two rows of command prompts at the bottom. The Terminal window's scrollbar won't let you scroll through the document. Instead, you use the down-arrow to move the cursor down line by line, or use the Page commands.

All of the commands listed at the bottom are prefaced with the caret character ("^"), representing the control key. So for example, to go to the next "page" (or screenfull) of text, press the control and "V" keys as indicated. For brief descriptions of all the commands, read the pico help file by pressing control-G.

The numbers in the circled area specify the time cron runs the scripts (there are actually three of them), and this is where you'll make your changes.

Each of the three lines (numbered 1, 2, and 3) specifies one of the three scripts cron runs by default. Each script is different, performing its own appropriate set of maintenance procedures. The daily script, specified on the line labeled 1, runs once each day. The weekly script, specified on line 2, runs once each week. And the monthly script, specified on line 3, runs -- you guessed it -- once each month.

The first five columns or fields of each line specify at exactly which interval the script will run. The fields specify, from left to right, the minute, hour (on a 24-hour clock), day of the month, month, and weekday (numerically, with Sunday as 7). Asterisks used instead of numbers in these fields mean "every".

For example, line 1 specifies a time of 3:15 a.m.:

15      3       *       *       *       root    periodic daily

Since the rest of the columns contain asterisks, the daily script will run at "3:15 a.m. on every day of the month, on every month, and every day of the week," that is "every day at 3:15 a.m.".

Line 2 specifies that the weekly script runs at 4:30 a.m. on every weekday number 6, or Saturday:

30      4       *       *       6       root    periodic weekly

And line 3 specifies that the monthly script runs at 5:30 a.m. on day 1 (the first) of each month.

30      5       1       *       *       root    periodic monthly

By just changing these numbers, then, you can have these scripts run at more reasonable times. Of course, what counts as reasonable depends on your own situation, so consider these factors when deciding:

1. Choose a time when your Mac is likely to be on (and not asleep).
2. Choose a time when a few minutes of background activity won't disturb your work too much. On faster machines the activity is hardly noticeable, but it could cause some stuttering if, for example, you happened to be watching a DVD at the time.
3. Choose a time that is unique for each script. You don't want to schedule scripts to run at the same time.

For example, these times might be good for a machine that's only on during normal work hours:

• Daily -- every day at 5:15 p.m.
• Weekly -- every Monday at 8:50 a.m.
• Monthly -- the first of every month at 9:30 a.m.

Regarding the monthly job, the first of the month sometimes falls on a weekend or holiday, but for now that's the best you can do.

To modify the crontab file to reflect these new times, use the cursor keys (the four arrow keys) to move the cursor to the proper field. Except for being unable to use the mouse, you'll find that editing text with pico is similar to doing so with any GUI text editor. Use the delete key as usual, and type in the new values.

First, change the 3 in the daily script line to 17:

15      17       *       *       *       root    periodic daily

Next, change the time in the weekly script line as shown; change the day from 6 to 2 (Saturday to Tuesday).

50      8       *       *       2       root    periodic weekly

Finally, change the time in the monthly script line as shown:

30      9       1       *       *       root    periodic monthly

Once you've made the changes, save ("write out") the document by pressing control-O. You'll then be prompted to confirm the save. Just press Return to do so.

Finally, quit pico, by pressing control-X.

Once you've saved the crontab file, the new scheduling takes effect; there's no need to restart. You won't yet receive notification of the completed cron jobs, but in Part 2, you'll learn how to make that happen, as well as learn more about the scripts themselves.

Chris Stone is a Senior Macintosh Systems Administrator for O'Reilly, coauthor of Mac OS X in a Nutshell and contributing author to Mac OS X: The Missing Manual, which provides over 40 pages about the Mac OS X Terminal.

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