Friday, August 10, 2012

Cat and Terminal

Here's a spiffy little command that can be used to combine files, look at the contents of a file and do limited text editing.

The Linux cat command at first seems so simple as to be unnecessary. In actuality, it is an excellent example of the Unix philosophy: create programs which do one thing and do it well. The thing cat does well is display the contents of a file or files. Many other utilities can handle this task, but none have all the options cat does. First, let's look at the simplest case:

cat /etc/motd

This command will display the contents of the motd (messages of the day) file to your screen. Unlike more (or less), cat will not stop when the screen is full. This is a feature, not a bug. You don't want pauses when redirecting (using the > operator) cat's output to a device, e.g., a printer or modem:

cat /etc/motd > /dev/modem

cat comes from the word conCATenate, which describes one of its best uses: to concatenate or “glue together” two or more files. If you have several individual files about animals you would like to collect together into one file, cat will do the work for you. For example:

cat tiger lion cougar > bigcats

would redirect the concatenation output, containing the three feline files in the specified order, into a new file named bigcats. If you find another file, panther, that needs to be added to the bigcats file, use cat with the append (>>) operator in the following way:

cat panther >> bigcats

Using >> ensures any prior content of bigcats is preserved. The content of panther is appended to bigcats. If you were to use the > operator here, you would replace the contents of bigcats with the contents of panther. Always use >> when you wish to add to the end of an existing file.

Be careful not to use the same file name when redirecting the output of the cat command, or you could lose one of the files. For example, don't do the following:

cat myfile yourfile > yourfile

In this case yourfile gets overwritten by myfile.

Another surprisingly handy use for cat is to redirect standard input like this:

cat > newfile
some notes I want to save in newfile.

This creates a new file (named newfile). You type as much text as you wish, then type ctrl-D to save the file. You can backspace over mistakes, but you cannot go to a previous line after you press the enter key. I often teach this particular option of cat to novice Unix users, who occasionally need to create simple files, but don't want to learn vi or other simple editors. There may be Unix systems without vi or your favorite editor, but cat is always there.

The operator >> can also be used to append notes to the end of newfile:

cat >> newfile
Adding another note to newfile.

cat Switches

Like most Unix commands, the behavior of cat can be modified by command line switches. If you use the diff command to compare files, it will show you the numbers of lines that differ between the files. However, most files don't have line numbers. Use cat with the -n switch to number each line of a file:

cat -n kittens > num_kittens

The file num_kittens is kittens with a number in front of each line, including blank lines. Use the -b switch to number only lines that are not blank.

One last cat trick: using the -v switch will show you “hidden” characters, such as control characters that may not show up in your editor. Try this experiment:

cat > catestv
CTRL-v testing CTRL-O Testing esc-b

If we use cat to view the file, we see only the normal text:

cat catestv
Testing Testing

To see what's actually in the file, use the -v option:

cat -v catestv
^V Testing ^O Testing ^[b

Here, a ^ in front of a character signifies a control character. (CTRL-[ is the same as ESC).

Courtesy : Linux Journal 
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