Important Linux Commands : System Commands

System Commands
System Information

df [option(s)] [directory]

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.

-H   shows the number of occupied blocks in gigabytes, megabytes, or kilobytes – in human-readable format

-t    Type of file system (ext2, nfs, etc.)

du [option(s)] [path]

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

-a  Displays the size of each individual file

-h  Output in human-readable form

-s  Displays only the calculated total size

free [option(s)]

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

-b  Output in bytes

-k  Output in kilobytes

-m  Output in megabytes

date [option(s)]

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.

Processes

top [options(s)]

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.
ps [option(s)] [process ID]

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.

aux   Displays a detailed list of all processes, independent of the owner.

kill [option(s)] process ID

Unfortunately, sometimes a program cannot be terminated in the normal way. However, 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:

-9  Sends a KILL signal instead of a TERM signal, with which the process really is annihilated by the operating system. This brings the specific processes to an end in almost all cases.

killall [option(s)] processname

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

Network

ping [option(s)] host name|IP address

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.

-c  number Determines the total number of packages to send and ends after they have been dispatched. By default, there is no limitation set.

-f  flood ping: sends as many data packages as possible. A popular means, reserved to root, to test networks.

-i  value Specifies the interval between two data packages in seconds. Default: one second

nslookup

The Domain Name System resolves domain names to IP addresses. With this tool, send queries to information servers (DNS servers).
telnet [option(s)] host name 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.

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