Without the double hyphen, the letters h, e, l, and p would be interpreted as separate options. There are some commands that don't follow the double hyphen convention, using a single hyphen before a word, but most commands use double hyphens for word options.
NOTE
You can use the --help option with most commands to see the options and arguments that they support. For example, try typing hostname --help.
Many commands also accept arguments after certain options are entered or at the end of the entire command line. An argument is an extra piece of information, such as a filename, directory, username, device, or other item, that tells the command what to act on. For example, cat /etc/passwd displays the contents of the /etc/passwd file on your screen. In this case, /etc/passwd is the argument. Usually, you can have as many arguments as you want on the command line, limited only by the total number of characters allowed on a command line. Sometimes, an argument is associated with an option. In that case, the argument must immediately follow the option. With single-letter options, the argument typically follows after a space. For full-word options, the argument often follows an equal sign (=). Here are some examples:
$ ls --hide=Desktop Documents Music Public Videos Downloads Pictures Templates
In the previous example, the --hide option tells the ls command not to display the file or directory named Desktop when listing the contents of the directory. Notice that the equal sign immediately follows the option (no space) and then the argument (again, no space).
Here's an example of a single-letter option that is followed by an argument:
$ tar -cvf backup.tar /home/chris
In the tar example just shown, the options say to create (c) a file (f) named backup.tar that includes all of the contents of the /home/chris directory and its subdirectories and show verbose (v) messages as the backup is created. Because backup.tar is an argument to the f option, backup.tar must immediately follow the option.
Here are a few commands that you can try out. See how they behave differently with different options:
$ ls Desktop Documents Downloads Music Pictures Public Templates Videos $ ls -a . Desktop .gnome2_private .lesshst Public .. Documents .gnote .local Templates .bash_history Downloads .gnupg .mozilla Videos .bash_logout .emacs .gstreamer-0.10 Music .xsession-errors .bash_profile .esd_auth .gtk-bookmarks Pictures .zshrc .bashrc .fsync.log .gvfs Pictures $ uname Linux $ uname -a Linux mydesktop 5.3.7-301.fc31.x86_64 #1 SMP Mon Oct 21 19:18:58 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux $ date Wed 04 Mar 2020 09:06:25 PM EST $ date +'%d/%m/%y' 04/03/20 $ date +'%A, %B %d, %Y' Wednesday, March 04, 2020
The ls command, by itself, shows all regular files and directories in the current directory. By adding the -a, you can also see the hidden files in the directory (those beginning with a dot). The uname command shows the type of system you are running (Linux). When you add -a, you also can see the hostname, kernel release, and kernel version.
The date command has some special types of options. By itself, date simply prints the current day, date, and time as shown above. But the date command supports a special + format option, which lets you display the date in different formats. Enter date --help to see different format indicators you can use.
Try the id and who commands to get a feel for your current Linux environment, as described in the following paragraphs.
When you log in to a Linux system, Linux views you as having a particular identity, which includes your username, group name, user ID, and group ID. Linux also keeps track of your login session: It knows when you logged in, how long you have been idle, and where you logged in from.
To find out information about your identity, use the id command as follows:
$ id uid=1000(chris) gid=1000(chris) groups=1005(sales), 7(lp)
In this example, the username is chris, which is represented by the numeric user ID (uid) 1000. The primary group for chris also is called chris, which has a group ID (gid) of 1000. It is normal for Fedora and Red Hat Enterprise Linux users to have the same primary group name as their username. The user chris also belongs to other groups called sales (gid 1005) and lp (gid 7). These names and numbers represent the permissions that chris has to access computer resources.
NOTE
Linux distributions that have Security Enhanced Linux (SELinux) enabled, such as Fedora and RHEL, show additional information at the end of the id output. That output might look something like the following:
context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
SELinux provides a means of tightly locking down the security of a Linux system. See Chapter 24, “Enhancing Linux Security with SELinux,” if you want to learn about SELinux.
You can see information about your current login session by using the who command. In the following example, the -u option says to add information about idle time and the process ID and -H asks that a header be printed:
$ who -uH NAME LINE TIME IDLE PID COMMENT chris tty1 Jan 13 20:57 . 2019
The output from this who command shows that the user chris is logged in on tty1 (which is the first virtual console on the monitor connected to the computer) and his login session began at 20:57 on January 13. The IDLE time shows how long the shell has been open without any command being typed (the dot indicates that it is currently active). PID shows the process ID of the user's login shell. COMMENT would show the name of the remote computer from which the user had logged in, if that user had logged in from another computer on the network, or the name of the local X display if that user were using a Terminal window (such as :0.0).
Locating commands
Now that you have typed a few commands, you may wonder where those commands are located and how the shell finds the commands you type. To find commands you type, the shell looks in what is referred to as your path. For commands that are not in your path, you can type the complete identity of the location of the command.
If you know the directory that contains the command that you want to run, one way to run it is to type the full, or absolute, path to that command. For example, you run the date command from the /bin directory by entering the following:
$ /bin/date
Of course, this can be inconvenient, especially if the command resides in a directory with a long pathname. The better way is to have commands stored in well-known directories and then add those directories to your shell's PATH environment variable. The path consists of a list of directories that are
