Ten Steps to Linux Survival

Chapter 3. Step 3: Peeking at Files

Now that you know how to move around in the filesystem, it is time to learn about how to inspect the content of files. In this chapter, I show a few commands that allow you to look inside files safely, without changing them.

Cool cat

The cat (concatenate) command dumps a file to the console, as shown in Figure 3-1.

Figure 3-1. cat command

We will be using cat a lot in the rest of this report. Because most Linux configuration and log files are text, this command is handy for examining files, knowing that we can’t change them by accident. The CMD.EXE equivalent is the TYPE command.

less Is More

The less command paginates files or output, with each “page” based on the size of the console window.

In Bash, as in Windows Command Prompt, the output from one command can be redirected, or piped, to another command by using the | character. In Linux, where each command “does one thing, well,” it is common practice to combine multiple commands, piping the output from one command to the next to accomplish a series of tasks in sequence. For example, later in this report you will see how to use the ps command to produce a list of running processes and then pipe that output to the grep command to search for a specific process by name. To demonstrate, although less can be passed a filename directly, here’s how to pipe command output from cat to less:

~ $ cat /etc/passwd | less

The output from less clears the screen, and then shows the first page, as you can see in Figure 3-2.

Figure 3-2. less output

The colon at the bottom of the screen indicates that less is waiting for a command. After less displays its output, you have various navigation options:

• Space, Page Down, or the down arrow scrolls down.

• Page Up or the up arrow scrolls up.

• / finds text searching forward (down) from the current cursor position, until the end of the file is reached; for example, /error.

• ? finds text searching backward (up) from the current cursor position, until the beginning of the file is reached; for example, ?error.

• n finds next instance of the text you’re searching for (note that the meaning of this is reversed when using ?).

• p finds previous instance of the text you’re searching for (note that the meaning of this is reversed when using ?).

• q quits the less command and returns you to the prior view of the console.

tail Wind

The tail command shows the last lines in a file. It is useful when you’re looking at large log files and want to see just the last lines—for example, right after an error has occurred. By default, tail will show the last 10 lines, but you can adjust the number of lines displayed with the -n parameter. For example, Figure 3-3 shows how to display just the last five lines.

Figure 3-3. tail command

The tail command can also “follow” a file, remaining running and showing new lines on the console as they are written to the file. This is useful when you’re watching a log file for a new instance of an error message, perhaps as you are testing to see if you can trigger the condition by visiting a web page on the site that is throwing an error. Figure 3-4 shows an example using the -f parameter to follow a log file.

Figure 3-4. tail -f command