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
Adjust the settings to your needs. It makes no sense to write
ls file(s), if no file named
actually exists. You can usually combine several parameters, for example, by
writing ls -la instead of ls -l -a.
The following section lists the most important commands for file management. It covers anything from general file administration to manipulation of file system ACLs.
If you run ls without any additional parameters, the program lists the contents of the current directory in short form.
Displays hidden files
Waits for confirmation, if necessary, before an existing
target is overwritten
Copies recursively (includes subdirectories)
target then deletes the original
Creates a backup copy of the
Waits for confirmation, if necessary, before an existing
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
target. Normally, such a link points directly to
source on the same file system. However, if
ln is executed with the
it creates a symbolic link that only points to the directory in which
source is located, enabling linking across
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 username.
Changes files and directories in all subdirectories
Transfers the group ownership of a given
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
Changes the access permissions.
mode parameter has three parts:
group accepts the
access, grant access with
+ and deny it with
access type is controlled by the following
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 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
.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
.tar.gz. If it was compressed using
bzip2, the ending is
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
This command performs an update of the database used by
locate. To include files in all existing directories,
run the program as
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
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 filename.
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
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
Produces a “unified” diff, which makes the output more readable
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, commonly
ext2 for Linux hard disks,
msdos for MS-DOS media,
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
mount it. If the file system should also be mounted by other users, enter
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
is allowed to run the commands mount and
umount. To enable other users to run these commands,
/etc/fstab file to specify the option
user for the respective drive.
The following section lists a few of the most important commands needed for retrieving system information and process and network control.
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
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, bringing 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).
[option(s)] hostname or IP address
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
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 is authorized to assume the identity
of any user. When using the command without specifying a username, you
are prompted for the
password and change to the superuser (
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.