Linux Basic Administration Commands Guide for CentOS or Redhat

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Table of Contents
Overview.......................................................................................................................................................1
Applies To......................................................................................................................................................1
Init Levels or Run levels.................................................................................................................................1
Init Levels Table.........................................................................................................................................1
chkconfig.......................................................................................................................................................1
chmod ...........................................................................................................................................................2
chown............................................................................................................................................................2
crontab..........................................................................................................................................................3
df ...................................................................................................................................................................3
du ..................................................................................................................................................................3
free................................................................................................................................................................4
fdisk...............................................................................................................................................................4
groupadd.......................................................................................................................................................5
groupdel........................................................................................................................................................5
ifconfig ..........................................................................................................................................................5
ifup................................................................................................................................................................6
grep...............................................................................................................................................................6
find................................................................................................................................................................7
kill..................................................................................................................................................................7
passwd ..........................................................................................................................................................8
pidof..............................................................................................................................................................8
tail .................................................................................................................................................................8
tar..................................................................................................................................................................9
zip..................................................................................................................................................................9
unzip............................................................................................................................................................10
gzip..............................................................................................................................................................10
gunzip..........................................................................................................................................................10
top...............................................................................................................................................................11
touch...........................................................................................................................................................11
useradd .......................................................................................................................................................12
userdel ........................................................................................................................................................12

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mount..........................................................................................................................................................12
umount........................................................................................................................................................13

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Overview
In this guide you will be able to manage Linux system with basic administration task.
For additional and command usage information on the specific command(s); you can run “man
<command>”.
Note: Not all the parameters are explained in this document.
Applies To
Linux flavours
Init Levels or Run levels
A run level is a state of init (initialization) and the whole system that defines what system services are
operating. Run numbers identifies levels;
There is no consensus of how to use the user defined run levels (2 through 5).
Some system administrators use run levels to define which subsystems are working, e.g., whether X is
running, whether the network is operational, and so on.
Others have all subsystems always running or start and stop them individually, without changing run
levels, since run levels are too coarse for controlling their systems.
Depending on your administration needs, define the run level based on the below.
Init Levels Table
Run Level Operation (Purpose)
0 Halt System
1 Single-user mode (Recovery / special Administration)
2 By Default - Not used (user-definable)
3 By Default - Full (multi-user mode)
4 By Default - Not used (user-definable)
5 Full multi-user mode (with an X-based login screen)
6 Reboot
chkconfig
chkconfig is a basic system utility. It updates and queries run level information for system services,
manages which service(s) will start or stop automatically in a specified run levels.
chkconfig: options
--add Add an new service
--del Delete an existing service

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--level numbers, 0- 6: Update init levels
--list List Services that are added
Syntax:
chkconfig --add <service Name>
Example:
chkconfig --add httpd
chkconfig --levels 345 httpd on
chmod
chmod changes the permissions for a file; permissions should include a letter designating who gets
permissions (u for the user, g for the group, o for others, or a for all) followed by a + or - (to give or take
away the permission) followed by the kind of permission (r for read access, w for write access, x for
execute if the file is a program or script).
chmod: options
-f Silent Mode
-R Recursive
4 or “r” Read Privilege(s)
2 or “w” Write Privilege(s)
1 or “x” Execute Privilege(s)
Syntax:
chmod +x <FileName>
chmod -w <FileName>
Example:
chmod +x run.sh
chmod -x run.sh
chown
chown changes the user and/or group ownership of each given file as specified by the first non-option
argument, changes the ownership of one or more files to new owner.
chown: options
-f Silent Mode
-h Including symbolic links
-R Recursive
Syntax:
chown -fhR <UserID>:<GroupID> <FileName>

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Example:
chown -fh jboss:jboss run.sh
crontab
A short name for file /var/lib/crontab, which contains a list of Linux commands to be performed at specific
times. A system administrator can use crontab as an automatic timer to trigger the initiation of important
jobs.
To view, install, or uninstall your current crontab file.
crontab: options
-e Edit crontab entry of existing logged in user
-l List crontab entry of existing logged in user
-r Delete all the entries in crontab file
Syntax:
crontab <option>
Example:
crontab -e
crontab -l
df
df reports the amount of disk space used by the specified files, and by each directory in the hierarchies
rooted at the specified files.
df: options
-k block size will be 1K size
-h Human readable
Syntax:
df <option> <Partition>
Example:
df -kh
du
Print disk usage (as the number of 1 KB blocks used by each named directory and its
subdirectories; default is the current directory).
du: options

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-k block size will be 1K size
-h Human readable
-b Bytes
-c Calculate or sum
Syntax:
du -<Option> <FileName>
Example:
du -kh
free
Display statistics about memory usage: total free, used, physical, swap, shared, and buffers used
by the kernel.
free: options
-b Memory usage in Bytes
-k Block size will be 1K size
-m Memory usage in Megabytes
-g Memory usage in Gigabytes
Syntax:
free -<option>
Example:
free -km
fdisk
Is a menu driven program for creation and manipulation of partition tables.
fdisk: options
-l Listing of existing drive partitions.
-s Listing of existing drive partitions size.
-v fdisk version number.
Syntax:
fdisk -<option>
Example:
fdisk -l

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groupadd
Create new group of accounts for the system.
-g Assign Group ID
-f Add to group forcefully
Syntax:
groupadd -<option> <GroupName>
Example:
groupadd -g 500 oracle
groupdel
Remove group from system account files
Syntax:
groupdel <GroupName>
Example:
groupdel oracle
ifconfig
Assign an address to a network interface and/or configure network interface parameters.
ifconfig: option
-a Display all the interface details
-v Verbose mode interface details
up Activate specified interface
down Shutdown the specified interface
Syntax:
ifconfig -<option> <Network Interface>
Example:
ifconfig -a
ifconfig eth0
ifconfig eth0 up
ifconfig eth0 down

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ifup
Enable network interface
ifup: option
-a Bring up all the interfaces that are defined in /etc/network interfaces
-f Force configuration of the interface
--no-scripts Don’t run any scripts under /etc/network/if-*.d/
Syntax:
ifup <Interface Name>
Example:
ifup eth0
ifup -a
ifdown
Disable network interface
ifdown: option
-a Bring down all the interfaces that are defined in /etc/network interfaces
-f Force configuration of the interface
--no-scripts Don’t run any scripts under /etc/network/if-*.d/
Syntax:
ifdown <Interface Name>
Example:
ifdown eth0
ifdown -a
grep
grep searches the named input FILEs (or standard input if no files are named, or the file name -
is given) for lines containing a match to the given PATTERN. By default, grep prints the matching
lines.
grep: option
-c Count matching lines
-r Recursive search
-l Displays count that doesn’t match the search criteria

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Syntax:
grep <search string> <File Name>
Example:
grep vcp.muthukrishna /etc/password
grep -r "SearchString" /var/www/
grep -cw "SearchString" SearchFile.txt
grep -cvw "Search" SearchFile.txt
grep -l " SearchString" /www/*
find
Find locates files. Within each directory tree specified by the given paths, it evaluates the given expression
from left to right, according to the rules of precedence.
find: option
-print Prints the results
-name Find string
Syntax:
Find <SearchPath> -<option>
Example:
find . -name "java" -print
find . -name "Java" -H -print
kill
Send a signal to terminate one or more process IDs.
kill: option
-9 Kills process
-l Lists all possible process IDs
Syntax:
kill -<option> <Process ID>
Example:
kill –a gcc

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kill -9 13445
kill -l 1234
passwd
Change / update the password of a user or logged in user.
passwd: option
-S Status of the password set
-d Deletes set password, passwordless user
Syntax:
passwd -<option> <UserName>
Example:
passwd vcp.muthukrishna
password -S vcp.muthukrishna
password -d vcp.muthukrishna
pidof
Display the process IDs of the listed program or program.
pidof: option
-s Single shot - this instructs the program to only return one pid
Syntax:
pidof -<option> Process ID
Example:
pidof -s gdm-binary
pidof pickup
tail
Print the last 10 lines of each named file
tail: option
-f Real time file print on console
-n Number of lies to print
Syntax:

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tail -<option> <FileName>
Example:
tail -f server.log
tail –n20
tar
Copy files to or restore files from an archive medium
tar: option
-c Create an archive file
-v Verbose mode enable
-x Extract an archive file
-z Extract gunzip, gzip
-t List the files in the archive
Syntax:
tar -<option> <TarFileName>
Example:
tar -cf ArchiveFileName.tar file1 fileN
tar -cf ArchiveFileName.tar Directory/
tar -czf ArchiveFileName.tar Directory/
tar -tvf ArchiveFileName.tar
tar -xvf httpd.tar
tar -zxvf httpd.tar.gz
zip
Create a compression file.
zip: option
-R Recursive
-v Verbose mode enable
-u Update existing compression file
Syntax:
zip -R <ZipFileName> <Include FileNames>
Example:

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zip -R ZipFileName.zip *.*
zip -u ZipFileName.zip AddFileName.txt
unzip
Explodes / Extracts a compression file.
zip: option
-d Folder location where the file should be exploded
-t Test archive file
-v Verbose mode enable
Syntax:
unzip -<option> <ZipFileName>
Example:
unzip -d /usr/local/Destination_Directory ZipFileName.zip
gzip
Compress a gzip file (Compression utility like zip).
gzip: option
-v Verbose mode enable
-r Recursive
Syntax:
gzip -<option> <gzipFileName>
Example:
gzip GzipFileName.gz *.*
gzip -f GzipFileName.gz *.*
gzip TarFile.tar
gunzip
Explodes / extracts a compressed gzip file.
guzip: option
-v Verbose mode enable
-r Recursive

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Syntax:
guzip -<option> <gzipFileName>
Example:
gunzip GunzipFileName.gz
gunzip -R GunzipFileName.gz
top
Provide information (frequently refreshed) about the most CPU-intensive processes currently
running.
top: option
-u Show top info for a user
-p Show top info for a PID
-d Set delay (refresh)
Syntax:
top -<option>
Example:
top -u jboss
top -p10078
touch
Update the access time and modification time (and dates) to the current time and date.
touch: option
-c No create, if file doesn’t exists
-a Change access time only
Syntax:
top -<option> <FileName>
Example:
touch FileName.txt
touch -c FileName.ear

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useradd
Create new user accounts or update default account information.
useradd: option
-g group id of a the user
-d Location of home directory for user
-f Inactive user
-s Set Shell for the user
-k Skeleton directory
Syntax:
useradd -<option> <UserName>
Example:
useradd -g 500 jboss
useradd -s /bin/bash -g 500 jboss
useradd -s /bin/bash -g 500 jboss -d /usr/local/jboss
userdel
Delete all entries for user in system account files
userdel: option
-r Remove home directory and files of the user
-f Force Remove
Syntax:
userdel -<option> <UserName>
Example:
userdel -r jboss
userdel -f jboss
mount
Mounts a device on a location.
mount: option
-t Type of filesystem to be mounted
-a Mount all filesystem listed in /etc/fstab

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-l List mounts label
-n unmounts without writing to /etc/fstab
-v Verbose
Syntax:
mount -<option> <Device> <Mount point>
Example:
mount -t ext3 /dev/hdc1 /mnt/Linux2
mount -t iso9660 -o ro /dev/cdrom /mnt/cdrom
mount /dev/cdrom/ /mnt/cdrom
mount -o loop disk1.iso /mnt/isoDisk
Note: /mnt/cdrom and /mnt/Linux2 should be already existing directory. Never mount on
existing file system. You will lose the existing mount locations, you’ve to explicitly reboot to see
the mount points again.
umount
Unmount’s a device that is mounted
mount: option
-n unmounts without writing to /etc/fstab
-v Verbose
-f Force Unmount
Syntax:
umount -<option> <Mount point>
Example:
umount /mnt/Linux2
umount /mnt/cdrom