Navigating directories is popularly done by using the cd command, which is probably one of the simplest commands to use. All you need to do is supply the directory you wish to change to and cd will do the rest. It also has very useful shorthands to speed up the most common tasks users perform when navigating their filesystems.

The following is what the command usage specification looks like:

cd [ -L | -P ] [directory]

In the syntax specification, [directory] is the directory you wish to change your current working directory to and [-L|-P] may be any one of the following:

The following is a typical usage example of the cd command:

cd /  

The preceding command will change your current directory to the root directory, which is named /; everything hosted on your filesystem is usually reachable from this directory.

The following are some more examples:

In the preceding command, one can have cd navigate an arbitrary number of directories above the current one, for instance, by supplying it a command as follows:

cd ../../../../../

The following are some other commands that can be used to navigate to different directories:

To see whether you have indeed changed your current working directory to the one you've specified, you can invoke the pwd command that will print your working directory. The syntax for the pwd command is as follows:

pwd [-L|-P] [--help] [--version]
pwd [--logical | --physical ] 

The –L or --logical and –P or --physical invocation options serve the same purpose as in the cd command.

It's not enough to just move between directories. You will eventually want to find out what's inside these directories. You can do this by using the ls command.

The following is the usage specification for the ls command—adapted from its man page:

ls [-aAlbBCdDfFghHiIklLmNopqQrRsStTuvwxXZ1] [FILE/DIRECTORY]

The previous command specification is another popular Linux/Unix convention. It's a shorthand to specify that any of the letters appearing in the brackets can be specified as part of the command invocation. Also, any number of them may be specified at the same time. For instance, consider the following commands:

ls –Ham
ls –and
ls –Rotti

According to the command specification, they are all acceptable ways to use the ls command. Whether or not any of these will actually do something useful depends on how each switch affects the ls command's behavior. You should keep in mind that some options may have opposing effects or certain combinations may have no effect, like a general note when reading usage specifications such as the one for ls.

The [FILE] or [DIRECTORY] argument would be any path or file at which you wish to fire ls. Without any arguments, ls will list the current working directory's entries.

Here's what some of the switches do—we will only discuss some of the most important switches here for the sake of brevity. Keep in mind that the ls command lists directory contents, so all its options will be focused on organizing and presenting a given directory's contents in a specified way.

The following are some of the ls command's invocation options:

The following are some examples of these options in action. For instance, if you'd like to say sort a bunch of files by their size, while displaying human-readable file sizes and all the access rights and creation times—which seems like a lot of work—you would run the following command:

ls –alSh

You're output could look something like the following screenshot:

Another very useful example would be checking the volume of logins to the system. This can be done by looking at the output of the following command:

ls –alSh /var/log/auth* 

Generally, keeping track of the contents of the /var/log/ directory will always be a good way to grab a good synopsis of the activity on a system.

Another important skill is being able to find resources on your filesystem in a compact yet powerful way. One of the ways you can do this is by using the aptly named find command. The following command is how find works:

find [-H] [-L] [-P] [-D debugopts] [-0level] [path…] [expression]

You can find out more about the find command by checking out the man file on it. This can be done by executing the following command:

man 1 find. 

This was discussed in the Getting help from the man pages section earlier in this chapter.

Moving on, the first three switches, namely, -H, -L, and –P, all control the way symbolic links are treated. The following list tells what they do:

The –D switch is used to allow find to print debug information if you need to know a little about what find is up to while it's searching for the files you want. -0level controls how find optimizes tests and it also allows you to reorder some tests. The level part can be specified as any number between 0 and 3 (inclusive).

The [path...] part of the argument is used to tell find where to look for files. You can also use the . and .. shorthands to specify the current and directory one level up respectively, as with the cd command.

The next argument, or rather group of arguments, is quite an important one: the [expression]. It consists of all the arguments that control the following:

The following is the structural breakdown of the find expression:

[expression] := [options][[test][OPERATOR][test][OPERATOR]...][actions]

[options] :=  [-d][-daystart][-depth][-follow][-help]...
[tests] := [-amin n][-atime file][-cmin n][-cnewer file]...
[OPERATOR] := [()][!][-not][-a][-and][-or]...
[actions] := [-delete][-exec command [;|{} +]][-execdir command]...

So now that you know where everything goes, let's look at what some of these arguments do. The find command has quite a number of very powerful options and operational modes, and one could quite literally write an entire book about find itself. So to make sure you don't get short changed—buying a book about "command line hacking" and instead learning only about find—we will only discuss some of the most common options and arguments penetration testers, system administrators, and developers use. The rest of the find command's power can be learned from the Linux manual files.

The following is a summary of some of the find command's possible arguments for options, tests, and actions.

find / -regex '^[/a-z_]*/$' #matches only the / directory
find / -regex '^[/a-z_]*/*$' #matches everything reachable from the / directory!