If you run ls without any additional parameters, the program lists the contents of the current directory in short form.

Copies source to target.

Copies source to target then deletes the original source.

Removes the specified files from the file system. Directories are not removed by rm unless the option -r is used.

Creates an internal link from source to target. Normally, such a link points directly to source on the same file system. However, if ln is executed with the -s option, it creates a symbolic link that only points to the directory in which source is located, enabling linking across file systems.

Changes the current directory. cd without any parameters changes to the user's home directory.

Creates a new directory.

Deletes the specified directory, if it is already empty.

Transfers ownership of a file to the user with the specified username.

Transfers the group ownership of a given file to the group with the specified group name. The file owner can only change group ownership if a member of both the current and the new group.

Changes the access permissions.

The mode parameter has three parts: group, access, and access type. group accepts the following characters:

For access, grant access with + and deny it with -.

The access type is controlled by the following options:

As an alternative, a numeric code can be used. The four digits of this code are composed of the sum of the values 4, 2, and 1—the decimal result of a binary mask. The first digit sets the set user ID (SUID) (4), the set group ID (2), and the sticky (1) flags. The second digit defines the permissions of the owner of the file. The third digit defines the permissions of the group members and the last digit sets the permissions for all other users. The read permission is set with 4, the write permission with 2, and the permission for executing a file is set with 1. The owner of a file would usually receive a 6 or a 7 for executable files.

This program compresses the contents of files using complex mathematical algorithms. Files compressed in this way are given the extension .gz and need to be uncompressed before they can be used. To compress several files or even entire directories, use the tar command.

tar puts one or more files into an archive. Compression is optional. tar is a quite complex command with a number of options available. The most frequently used options are:

The archive files created by tar end with .tar. If the tar archive was also compressed using gzip, the ending is .tgz or .tar.gz. If it was compressed using bzip2, the ending is .tar.bz2. Application examples can be found in Section 19.1.8, “Archives and Data Compression”.

This command is only available if you have installed the findutils-locate package. The locate command can find in which directory a specified file is located. If desired, use wild cards to specify filenames. The program is very speedy, because it uses a database specifically created for the purpose (rather than searching through the entire file system). This very fact, however, also results in a major drawback: locate is unable to find any files created after the latest update of its database. The database can be generated by root with updatedb.

This command performs an update of the database used by locate. To include files in all existing directories, run the program as root. It also makes sense to place it in the background by appending an ampersand (&), so you can immediately continue working on the same command line (updatedb &). This command usually runs as a daily cron job (see cron.daily).

With find, search for a file in a given directory. The first argument specifies the directory in which to start the search. The option -name must be followed by a search string, which may also include wild cards. Unlike locate, which uses a database, find scans the actual directory.

The cat command displays the contents of a file, printing the entire contents to the screen without interruption.

This command can be used to browse the contents of the specified file. Scroll half a screen page up or down with PgUp and PgDn or a full screen page down with Space. Jump to the beginning or end of a file using Home and End. Press Q to exit the program.

The grep command finds a specific search string in the specified file(s). If the search string is found, the command displays the line in which the searchstring was found along with the filename.

The diff command compares the contents of any two files. The output produced by the program lists all lines that do not match. This is frequently used by programmers who need only send their program alterations and not the entire source code.

This command can be used to mount any data media, such as hard disks, CD-ROM drives, and other drives, to a directory of the Linux file system.

For hard disks not defined in the file /etc/fstab, the device type must also be specified. In this case, only root can mount it. If the file system should also be mounted by other users, enter the option user in the appropriate line in the /etc/fstab file (separated by commas) and save this change. Further information is available in mount(1).

This command unmounts a mounted drive from the file system. To prevent data loss, run this command before taking a removable data medium from its drive. Normally, only root is allowed to run the commands mount and umount. To enable other users to run these commands, edit the /etc/fstab file to specify the option user for the respective drive.

The df (disk free) command, when used without any options, displays information about the total disk space, the disk space currently in use, and the free space on all the mounted drives. If a directory is specified, the information is limited to the drive on which that directory is located.

This command, when executed without any parameters, shows the total disk space occupied by files and subdirectories in the current directory.

The command free displays information about RAM and swap space usage, showing the total and the used amount in both categories.

This simple program displays the current system time. If run as root, it can also be used to change the system time. Details about the program are available in man date(1).

top provides a quick overview of the currently running processes. Press H to access a page that briefly explains the main options to customize the program.

If run without any options, this command displays a table of all your own programs or processes—those you started. The options for this command are not preceded by hyphen.

Unfortunately, sometimes a program cannot be terminated in the normal way. In most cases, you should still be able to stop such a runaway program by executing the kill command, specifying the respective process ID (see top and ps). kill sends a TERM signal that instructs the program to shut itself down. If this does not help, the following parameter can be used:

This command is similar to kill, but uses the process name (instead of the process ID) as an argument, killing all processes with that name.

The ping command is the standard tool for testing the basic functionality of TCP/IP networks. It sends a small data packet to the destination host, requesting an immediate reply. If this works, ping displays a message to that effect, which indicates that the network link is basically functioning.

The domain name system resolves domain names to IP addresses. With this tool, send queries to information servers (DNS servers).

Telnet is actually an Internet protocol that enables you to work on remote hosts across a network. telnet is also the name of a Linux program that uses this protocol to enable operations on remote computers.

	Warning
Do not use telnet over a network on which third parties can eavesdrop. Particularly on the Internet, use encrypted transfer methods, such as ssh, to avoid the risk of malicious misuse of a password (see the man page for ssh).

Users may change their own passwords at any time using this command. The administrator root can use the command to change the password of any user on the system.

The su command makes it possible to log in under a different username from a running session. Specify a username and the corresponding password to use the environment of the respective user. The password is not required from root, because root is authorized to assume the identity of any user. When using the command without specifying a username, you are prompted for the root password and change to the superuser (root).

To avoid loss of data, you should always use this program to shut down your system.

Does the same as halt except the system performs an immediate reboot.

This command cleans up the visible area of the console. It has no options.