Grep and Regular Expressions

Irrespective of the role you play with Linux (system administrator, developer or user), you often need to browse through and parse text files, and/or extract data from them. These are file manipulation operations. Thus, it is essential for the Linux user to become adept at performing certain operations on files.

Most of the time, such file manipulation is done at the command line, which allows users to perform tasks more efficiently than while using a GUI. Furthermore, the command line is more suitable for automating often executed tasks.

Indeed, experienced system administrators write customized scripts to accomplish such repetitive tasks, standardized for each particular environment. We will discuss such scripting later in much detail.

In this section, we will concentrate on command line file and text manipulation-related utilities.

cat is short for concatenate and is one of the most frequently used Linux command line utilities. It is often used to read and print files, as well as for simply viewing file contents. To view a file, use the following command:

$ cat filename

For example, cat readme.txt will display the contents of readme.txt on the terminal. However, the main purpose of cat is often to combine (concatenate) multiple files together. You can perform the actions listed in the table using cat.

The tac command (cat spelled backwards) prints the lines of a file in reverse order. Each line remains the same, but the order of lines is inverted. The syntax of tac is exactly the same as for cat, as in:

$ tac file
$ tac file1 file2 > newfile

cat can be used to read from standard input (such as the terminal window) if no files are specified. You can use the > operator to create and add lines into a new file, and the >> operator to append lines (or files) to an existing file. We mentioned this when talking about how to create files without an editor.

To create a new file, at the command prompt type

cat > <filename> and press the Enter key.

This command creates a new file and waits for the user to edit/enter the text. After you finish typing the required text, press CTRL-D at the beginning of the next line to save and exit the editing.

Another way to create a file at the terminal is

cat > <filename> << EOF

. A new file is created and you can type the required input. To exit, enter EOF at the beginning of a line.

Note that EOF is case sensitive. One can also use another word, such as STOP.

head reads the first few lines of each named file (10 by default) and displays it on standard output. You can give a different number of lines in an option.

tail prints the last few lines of each named file and displays it on standard output. By default, it displays the last 10 lines. You can give a different number of lines as an option.

To continually monitor new output in a growing log file:

$ tail -f somefile.log

This command will continuously display any new lines of output in somefile.log as soon as they appear. Thus, it enables you to monitor any current activity that is being reported and recorded.