This section gives insight into the most important commands of your SUSE LINUX system. Along with the individual commands, parameters are listed and, where appropriate, a typical sample application is introduced. To learn more about the various commands, use the manual pages, accessed with man followed by the name of the command, for example, man ls.
In the man pages, move up and down with PgUp and PgDn. Move between the beginning and the end of a document with Home and End. End this viewing mode by pressing Q. Learn more about the man command itself with man man.
There are many more commands than listed in this chapter. For information about other commands or more detailed information, the O'Reilly publication Linux in a Nutshell is recommended. In the following overview, the individual command elements are written in different typefaces.
The actual command and its mandatory options are always printed as command option. Specifications or parameters that are not required are placed in [square brackets].
Adjust the settings to your needs. It makes no sense to write ls file(s), if no file named file(s) actually exists. You can usually combine several parameters, for example, by writing ls -la instead of ls -l -a.
If you run ls without any additional parameters, the program lists the contents of the current directory in short form.
Displays hidden files
Copies sourcefile to targetfile.
Waits for confirmation, if necessary, before an existing targetfile is overwritten
Copies recursively (includes subdirectories)
Copies sourcefile to targetfile then deletes the original sourcefile.
Creates a backup copy of the sourcefile before moving
Waits for confirmation, if necessary, before an existing targetfile is overwritten
Removes the specified files from the file system. Directories are not removed by rm unless the option -r is used.
Deletes any existing subdirectories
Waits for confirmation before deleting each file.
Creates an internal link from the sourcefile to the targetfile. Normally, such a link points directly to the sourcefile 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 the sourcefile is located, enabling linking across file systems.
Creates a symbolic link
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 user name.
Changes files and directories in all subdirectories.
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:
eXecute — executing files or changing to the directory.
Set uid bit — the application or program is started as if it were started by the owner of the file.
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 7 — the sum of all permissions.
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.
Decompresses the packed gzip files so they return to their original size and can be processed normally (like the command gunzip)
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:
Writes the output to a file and not to the screen as is usually the case
Creates a new tar archive
Adds files to an existing archive
Outputs the contents of an archive
Adds files, but only if they are newer than the files already contained in the archive
Unpacks files from an archive (extraction)
Packs the resulting archive with gzip
Compresses the resulting archive with bzip2
Lists files processed
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 21.1.8. “Archives and Data Compression”.
The locate command can find in which directory a specified file is located. If desired, use wild cards to specify file names. The program is very speedy, as 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 &).
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.
Numbers the output on the left margin
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 file name.
Only displays the names of the respective files, but not the text lines
Additionally displays the numbers of the lines in which it found a hit
Only lists the files in which searchstring does not occur
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.
Only reports whether the two files differ
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.
Specifies the file system. The most common are ext2 for Linux hard disks, msdos for MS-DOS media, vfat for the Windows file system, and iso9660 for CDs.
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.
Shows the number of occupied blocks in gigabytes, megabytes, or kilobytes — in human-readable format
Type of file system (ext2, nfs, etc.)
This command, when executed without any parameters, shows the total disk space occupied by files and subdirectories in the current directory.
Displays the size of each individual file
Output in human-readable form
Displays only the calculated total size
The command free displays information about RAM and swap space usage, showing the total and the used amount in both categories.
Output in bytes
Output in kilobytes
Output in megabytes
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 date.
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.
Displays a detailed list of all processes, independent of the owner
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:
Sends a KILL signal instead of a TERM signal. This brings the specified process to an end in almost all cases.
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.
number Determines the total number of packages to send and ends after they have been dispatched. By default, there is no limitation set.
flood ping: sends as many data packages as possible. A popular means, reserved for root, to test networks.
value Specifies the interval between two data packages in seconds. Default: one second
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.
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 user name from a running session. When using the command without specifying a user name, you will be prompted for the root password. Specify a user name and the corresponding password to use the environment of the respective user. The password is not required from root, as root is authorized to assume the identity of any user.
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.