If you're new here, you may want to subscribe to my RSS feed.

Grep is a very powerful command in the Unix world. It allows you to search for words in documents. This is very useful if you are trying to find bugs in code or just tracking down certain files.

The basic structure of a grep statement is below.

grep -R "string you are looking for" *

The -R in the statement means you want to search recursively. Make sure you put the “” around your string or you will get an error when you try to execute the command. The * means that the preceding item will be matched zero or more times.

Their are many more options that can be added into the grep function to make it even more useful. To find them type the following into the command line.

man grep

I mentioned previously that Unix uses lots of text files, which are easy to create and manipulate. Unix has a variety of commands to manipulate text files. The way to move text between these commands is with pipes. The pipe symbol is the vertical bar (|) above your Enter key.

For example, suppose you have a text file full of email addresses, with one email address per line. Suppose it’s named “emails.txt” and it’s on your Desktop.

To view the emails, type the following command in the Terminal:
cat ~/Desktop/emails.txt

To view the emails sorted alphabetically, type the following:
cat ~/Desktop/emails.txt | sort

The pipe causes the output of the cat command to be fed into the sort command, then outputted to the screen. You can tack on as many pipe commands as you need.

To view the emails sorted alphabetically and with a prompt between each page, type:
cat ~/Desktop/emails.txt | sort | more

The list of emails “flows” from the cat command to the sort command to the more command, as if going through water pipes.

As Nathan Sweeney mentioned in the comments of the last post, there is actually a faster way of “typing” Unix commands: the tab key. Hitting the tab key causes Unix to fill in as much of whatever you’re typing as possible. For example, if you open Terminal (you’ll begin in your Home folder) and want to change to your Documents folder, you’d normally type cd Documents. But it’s much easier with tab completion: type cd Doc and hit tab. You’ll notice that Unix fills in the rest of the word “Documents”.

You’ll also notice that tab completion only works when whatever you’re typing is unique. (Or rather, it only works to the degree that it is unique.) For example, if you type cd D from your home directory and hit tab, Unix won’t fill in anything because it doesn’t know whether you want cd Desktop or cd Documents. But if you hit tab a 2nd time, it will display Desktop and Documents to show you your options. Hitting tab once always causes Unix to fill in as much as it can. Hitting tab the 2nd time always shows you your options, if any.

You can even use tab completion to select a command. At the beginning of a command prompt, hitting tab twice will cause Unix to display ALL the commands that are available to you. There will probably be so many that Unix will ask you if you really want to see them all. Hit “y” to confirm, and then hit “q” (quit) when you’re done.

Tab completion makes it much easier to work with Unix.

Each time you open Terminal, you begin in your Home folder. To confirm this, type pwd and press Enter. The command pwd tells you where you are (i.e. your “Present Working Directory”). Any commands you type there will by default occur in this folder (unless you refer to some other directory explicitly.)

After opening Terminal and typing pwd on my computer, it returned /Users/richard.

To navigate to a new folder, use the cd (”change directory”) command. For example, to switch to the Applications folder, type cd /Applications. You can then type ls to list all of your Applications.

To move deeper into the hierarchy, use cd and the name of the folder. For example, if I’m in /Users/richard, I can type cd Movies to move into the Movies folder. I’ll then be in /Users/richard/Movies.

To move back a folder, use two dots (..). For example, if I’m in /Users/richard and type cd .. I will then be in /Users.

In Unix, folders are called directories. Files are still files. The location of a file is called its path. The very top folder (or bottom folder depending on how you think about it) is called the root directory. This root directory is the one you see when you double click your hard drive. It contains the Applications, Library, System, and Users folders.

Here is how you refer to various parts of the file system in Unix:

• The forward slash (/) refers to the root folder.
• A path name beginning with a slash (/) is an absolute path. For example, the path /Applications refers to the Applications folder, no matter where you currently are. To see a list of Applications, type ls /Applications. It doesn’t matter where you currently are because it’s an absolute path.
• A path name beginning without a slash is a relative path. For example, if you are currently in your Home folder, the path Documents refers to the Documents folder under your Home folder. While in your Home folder, type ls Documents to see a list of your documents. If you were to type ls Applications (without the slash), you’d likely see an error since you (probably) don’t have an Application folder inside your Home folder, and the path is relative to where you currently are.
• The tilde (~) refers to your Home folder. (It’s also an absolute reference, despite not having a preceding slash.) To see what’s in your Home folder, type ls ~. To see your own Documents type ls ~/Documents. It’s an absolute reference that works from anywhere. For me, the path /Users/richard/Documents is the same as ~/Documents.
• You can use the tilde with someone else’s username to refer to their Home folder. For example, to see what’s in Mary’s Home folder, type ls ~mary or to see what’s in John’s Documents folder, type ls ~john/Documents.

You should now be able to refer to any path on your hard drive. Did I leave anything out?

When discussing what a Unix command is, I mentioned that you can alter the function of a Unix command by using flags or arguments. For example, to list all the files in your current folder, you use the ls command. To reverse its order you add the “r” argument: ls -r. But how do you know which arguments to use?

Unix has a built-in manual called the man system. Each Unix command comes with a manual page which you can access by typing man and then the name of the command. For example, to get help with the ls command, open the Terminal and type man ls.

Each manual page shows the name, synopsis, description, examples, and more for that command. You can use the up and down arrows to scroll through the entry. To skip Forward a page hit “f”; to skip Back a page hit “b”. When you’re done reading the manual page, his “q” to quit.

The man command is one of the most important commands for learning and using Unix. I use it daily.

On a side note, these manual pages are also available on the Internet, including at Apple.com: Mac OS X Man Pages

A Unix command is like an application, but it usually performs a much simpler task. There are Unix commands to show the contents of a file, to sort, to filter, and even to browse the Internet. Every Apple computer comes with hundreds of Unix commands that each do something unique. Contrast that with the dozens of applications that come with every Mac, some of which perform very complicated workflows.

Mac applications have Preferences for selecting how you’d like them to run. Unix commands, on the other hand, don’t (usually*) have any preferences or persistent settings. To specify how a Unix command runs, you use what are called flags or arguments. These flags or arguments are specified each time you run the command.

Arguments are specified with one dash and a letter (e.g. -a -b -c), or two dashes and a word (--help.) Sometimes certain settings or parameters can follow an argument (-u "stevejobs" or --user "stevejobs".) Whenever you use multiple single-letter parameters, you can combine them (-abc is the same as -a -b -c.)

Here’s an example. The ls command lists the files in a folder. Open Terminal, type ls, and hit Enter. You’ll see a list of the files and folders in your Home folder. Each name is listed one after another.

If, instead, you’d like to see all the files listed in columns, type ls -l and his Enter. You’ll see all the files listed in one column. It will also show the dates the files were created, and other information. This is the “long” format, as specified by the l flag.

You’ll notice that the list of files is in alphabetical order. If you want to reverse it, you can use the r flag. Type ls -l -r. The files will be listed in reverse alphabetical order. Since you can combine single letter flags, ls -lr will produce the same result. Most of the time the order of the flags doesn’t matter either, so ls -rl also produces the same result.

You’ll find Terminal in your Applications/Utilities folder. It’s through the Terminal that you access the command line interface of your Mac. In the same way that the graphical user interface (GUI) lets you operate your computer with a mouse, the command line interface (CLI) lets you operate your computer through typed commands. The graphical user interface is certainly easier to learn, but the command line interface can be very powerful! For instance, you can easily move a file to a new folder by dragging it with the mouse. But if you want to copy all the MP3 files anywhere in your Home folder, it’s must faster to use the command line, or Terminal. (You’ll learn how to do this.)

As long as you’re going to start using Terminal regularly, you might as well make it at home. You can drag the Terminal to your Dock for easy access. Also, you might want to change the look of Terminal by going to Window Settings in the Terminal menu. Switch the drop down menu to Color, then change the color to your liking. I personally like White on Black with a little transparency, but you can do whatever you like.

You might also want to switch to the Display menu and choose a different font and font size. When you’re done, click Use Settings as Defaults.

If you really want to get fancy, you can also access the command line interface through a 3rd party application like iTerm. iTerm adds some niceties that Terminal doesn’t have, such as tabbed windows. I know people that really like iTerm, but I personally just use Terminal.

Further reading:
Wikipedia: Terminal Application
Wikipedia: Text terminal
Wikipedia: Graphical user interface
Wikipedia: Command line interface

Is anything unclear? I’ll explain!

Unix is a computer operating system that was developed in the 60’s and 70’s at AT&T’s Bell Labs. It was designed as a robust, multi-user, multi-tasking operating system and has historically been popular on large mainframe systems at universities and governments.

Since then, many variants have descended from the first Unix, including Linus Torvald’s college project Linux. Unix was originally spelled “UNICS”, an acronym for Uniplexed Information and Computing System.

Mac OS X is built on a version of Unix called FreeBSD, which means that every modern Mac inherits the strength of a powerful, time-tested operating system. FreeBSD is widely regarded as one of the most secure operating systems. Apple combines this with a beautiful interface. Mac OS X is like the engine of a classic muscle car in a shiny, modern body.

Further reading:
Apple: Solid as a Rock
Wikipedia: Unix
The Unix family tree (diagram)

Is anything unclear? I’ll explain!