Linux Basics

we start to learn basic linux with the commonly used commands.

Following command gives a list of files and directories in a long listing (even hidden files will be displayed), human readable sizes, KB, MB.

[root@pim /]# ls -l -h -a

Following command captures lines showing directories only page by page, to check for files use ^f. (This ^ d or ^f is NOT ctrl-d or ctrl-f)

[root@pim /]# ls -l | grep “^d” | less

To check for free space on your partitions:

[root@pim /]# df -h

To check for the used space by a particular directory:

[root@pim /]# du –c –h /home/Hashir

or

[root@pim /]# du –s –h /home/Hashir

Mounting / un-mounting floppies / cd-rom / directories :

To check for currently mounted partitions, use mount command without any arguments:

[root@pim /]# mount

You can also use the mounted files system table mtab to check for currently mounted partitions.

[root@pim /]# cat /etc/mtab

Following command mounts the floppy drive in mount point /mnt/floppy

[root@pim /]# mount /dev/fd0 /mnt/floppy

Similarly

[root@pim /]# mount /dev/cdrom /mnt/cdrom

Following command mounts your VFAT partition, C: drive, in /mnt/windir

[root@pim /]# mount -t vfat /dev/hda2 /mnt/windir

Following command mounts an NFS share /usr/SLES11 from the machine 192.168.0.20 to your local mount point /mnt/myfolder:

[root@pim /]# mount –t nfs 192.168.0.20:/usr/sles11 /mnt/myfolder

To unmount any mount point use the following command but the mount point must not be busy:

[root@pim /]# umount /mnt/myfolder

File copying / moving / renaming / deletion / compression and decompression:

Following command copies all (non-hidden) files, including any subdirectories from /home/Hashir to /mnt/myfolder

[root@pim /]# cp /home/Hashir/* /mnt/myfolder

The above command will change the ownership of the newly copied files on /mnt/myfolder to the uid and gid of the user issuing the copy (cp) command, in this case the user root. To preserve the file permissions while copying to the new location, use the -p switch as well. For example:

[root@pim /]# cp -v -r -p /home/Hashir/* /mnt/myfolder/

Following command deletes all files, including subdirectories, forcefully (without asking yes/no) from the directory /tmp.

[root@pim /]# rm -fr /tmp/*

Following command creates a tar file in /tmp as mytar.tar from the files in current directory

[root@pim /]# tar cvf /tmp/mytar.tar *

Following command extract the tar file mytar.tar from /tmp in current directory

[root@pim /]# tar xvf /tmp/mytar.tar

Following command extract the tar file mytar.tar from /tmp in /backup/restore

[root@pim /]# tar xvf /tmp/mytar.tar -C /backup/restore

Disk checking and formatting:

List of all partitions on all attached hard drives (both SCSI and IDE)

[root@pim /]# fdisk -l

To resize partitions use:

[root@pim /]# parted

Following command formats a floppy disk in ext2 format, also checks for and marks, bad sectors

[root@pim /]# mke2fs -c /dev/fd0

Following command formats a floppy disk in dos format

[root@pim /]# mkdosfs /dev/fd0

Following command checks for errors on the partition, the partition being checked should be in unmounted state during check.

[root@pim /]# e2fsck /dev/hda3

or

[root@pim /]# fsck -t ext2 /dev/hda3

Process management: 

Following command shows the list of all running processes with their process ids (PID), even owned by other users:

[root@pim /]# ps aux | less

You can also use pstree :

[root@pim /]# pstree

Following command will kill a specific process, say process id 12432:

[root@pim /]# kill 12432

or

[root@pim /]# kill –s KILL 12432

or

[root@pim /]# kill –KILL 12432

or

[root@pim /]# kill –9 12432

User Management and file permissions / ownerships: 

To add a new group called database use:

[root@pim /]# groupadd database

To add new user Hashir and make him a member of the group database, use:

[root@pim /]# useradd -s /bin/bash -g database Hashir

To change the password of user, run the following command:

[root@pim /]# passwd Hashir

To lock the password of user Hashir, run the following command as root:

[root@pim /]# passwd -l Hashir

To un-lock the password of user Hashir, run the following command as root:

[root@pim /]# passwd -u Hashir

To delete a user say Hashir, including his home directory, use:

[root@pim /]# userdel -r Hashir

To change permissions of an object (file or directory), use chmod:

[root@pim /]# chmod 770 myfile.txt

(changes the permissions of the file to rwx for both owner and group)

or

[root@pim /]# chmod ug+rwx,o-rwx myfile.txt

To change ownership of a directory /project to Hashir and set the group permissions to the group database, use chown (-R will change ownership of all files and subdirectories under the directory project:

[root@pim /]# chown –R Hashir:database /project

Package management : 

Following command lists all the currently installed packages and captures only those lines which have the word samba in them

[root@pim /]# rpm -qa | grep samba

or

[root@pim /]# rpm -qa sambal*

To check presence of the package 'bind' in the system, use the following command:

[root@pim /]# rpm -q bind

Following command lists all the information about the specific installed package.

[root@pim /]# rpm -qi samba

Following command lists all the documentation files and their location of a particular installed package.

[root@pim /]# rpm -qd samba

Following command lists file locations of all of the files of a particular installed package.

[root@pim /]# rpm -ql samba

Following command lists all information of the un-installed package:

[root@pim /]# rpm -qpi ymessenger-0.99.19-1.i386.rpm

Similarly,you can check the list of files a package contains, before actually installing it:

[root@pim /]# rpm -qpl ymessenger-0.99.19-1.i386.rpm

Install different programs from SLES-11 DVD If program not installed, by using the following command:

[root@pim /]# rpm -ivh /mnt/dvd/sles-11/RPMS/samba*.rpm

If an older version of the package is installed and you want to upgrade it to a new version, then use (U):

[root@pim /]# rpm -Uvh /mnt/dvd/sles-11/RPMS/samba*.rpm

Following command erases (removes / un-installs ) a specific package.

[root@pim /]# rpm -e samba

Miscellaneous:

To find help on some topic :

[root@pim /]# man samba

or

[root@pim /]# info samba

To find a file in the /mnt directory, use:

[root@pim /]# find /mnt -name “*.doc” -print

To find files and directories owned by the group database in the /mnt directory, use:

[root@pim /]# find /mnt -group database

Assuming a user left the organization and the files owned by his account name must be found and deleted for security reasons, use :

[root@pim /]# find / -user Hashir -exec rm '{}' ';'

To check for memory and swap space usage, you can use:

[root@pim /]# free

or

[root@pim /]# top

To check the system load status averages and the uptime:

[root@pim /]# uptime

To check when the system was booted last time:

[root@pim /]# who -b

To check the status of a particular service:

[root@pim /]# service samba status

check the kernel version of your system:

[root@pim /]# uname -r

find out the present working directory use:

[root@pim]# pwd