Some Useful Linux Commands

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In this article we are going to review some of the useful and frequently used Linux or Unix commands for Linux System Administrators that are used in their daily life. This is not a complete but it’s a compact list of commands to refer when needed. Let us start one by one how we can use those commands.
Linux Commands

Uptime

This command tells you how long your system has been running for and system load averages.

# uptime
08:59:01 up 28 min,  1 user,  load average: 0.00, 0.01, 0.05

Users

This command will show the names of users currently logged in to current host.

# users
centos root dennis steve

Sudo

This commands allows a permitted user to execute a command as the superuser or another user, as specified in the sudoers file(/etc/sudoers).

# visudo
## Allow root to run any commands anywhere
root    ALL=(ALL)       ALL
dennis  ALL=(ALL)       ALL

Now you are able to use commands with privileges and you have the root rights.

cmp

This command compare two files byte by byte and will display difference.

# cmp file1.txt file2.txt
file1.txt file2.txt differ: byte 1, line 1

lsusb

This command will list the usb devices. If lsusb command is missing then install it using following command.

# yum install usbutils
# lsusb
Bus 001 Device 002: ID 0951:16a3 Sandisk

lsmod

This command is a trivial program which nicely formats the contents of the /proc/modules, showing what kernel modules are currently loaded.

# lsmod
Module                  Size  Used by
isofs                  39844  0
intel_rapl             18773  0
crc32_pclmul           13113  0
ghash_clmulni_intel    13259  0
aesni_intel            69884  0
lrw                    13286  1 aesni_intel
gf128mul               14951  1 lrw
glue_helper            13990  1 aesni_intel
ablk_helper            13597  1 aesni_intel
cryptd                 20359  3 ghash_clmulni_intel,aesni_intel,ablk_helper
cirrus                 24694  1
syscopyarea            12529  1 cirrus
sysfillrect            12701  1 cirrus
sysimgblt              12640  1 cirrus
ppdev                  17671  0
ttm                    93441  1 cirrus
snd_pcm               105835  0
drm_kms_helper        125008  1 cirrus
snd_timer              29639  1 snd_pcm
snd                    83425  2 snd_timer,snd_pcm
soundcore              15047  1 snd
pcspkr                 12718  0
drm                   349210  4 ttm,drm_kms_helper,cirrus
i2c_piix4              22106  0
i2c_core               40582  3 drm,i2c_piix4,drm_kms_helper
parport_pc             28165  0
parport                42348  2 ppdev,parport_pc
nfsd                  302418  1
auth_rpcgss            59343  1 nfsd
nfs_acl                12837  1 nfsd
lockd                  93600  1 nfsd
grace                  13295  2 nfsd,lockd
ip_tables              27240  0
xfs                   939662  1
libcrc32c              12644  1 xfs
ata_generic            12910  0
pata_acpi              13038  0
ata_piix               35038  0
crct10dif_pclmul       14289  0
crct10dif_common       12595  1 crct10dif_pclmul
xen_netfront           26720  0
xen_blkfront           26971  2
crc32c_intel           22079  1
libata                218730  3 pata_acpi,ata_generic,ata_piix
serio_raw              13462  0
floppy                 69417  0
sunrpc                300464  7 nfsd,auth_rpcgss,lockd,nfs_acl

lsblk

This command will lists the information about all available or specified block devices gather information from filesystem. The command prints all block devices (except RAM disks) in a tree-like format by default.

# lsblk
NAME    MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvda    202:0    0  30G  0 disk
└─xvda1 202:1    0  30G  0 part /

Enable an Interface

You can enable system’s interface up by defining the name of the interface with ifconfig command.
Syntax: ifconfig interface-name up

ifconfig eth0 up

Disable an Interface

You can also disable the interface card.
Syntax: ifconfig interface-name up

ifconfig eth0 down

dd

This command will copy a file, converts and formats data according to the operands. In this command its creatig 10 MB file through if= input file and of=output file.

# dd if=/dev/zero of=/root/output.txt bs=1024 count=10240
10240+0 records in
10240+0 records out
10485760 bytes (10 MB) copied, 0.0131311 s, 799 MB/s

bc

This command is basically to calculate two variables in a CLI mode its work just like a calculator.

# bc
bc 1.06.95
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
10+12
22

25*35
875

25/5
5

Last reboot

This command will show the last reboot times or system reboots history.

# last reboot
reboot   system boot  3.10.0-327.13.1. Mon May  9 08:30 - 09:15  (00:44)
reboot   system boot  3.10.0-327.10.1. Mon May  9 08:26 - 08:29  (00:03)

wtmp begins Mon May  9 08:26:03 2016

Watch

watch runs command repeatedly, displaying its output and errors (the first screenfull). This allows you to watch the program output change over time. By default, the program is run every 2 seconds.

# watch -n 1 'ls -l; echo; du'
Every 1.0s: ls -l; echo; du                                                                                                                     Mon May  9 09:17:08 2016

total 10244
-rw-r--r--. 1 root root       18 May  9 08:38 logout
-rw-r--r--. 1 root root 10485760 May  9 09:11 output.txt

4       ./.ssh
0       ./.pki/nssdb
0       ./.pki
10276   .

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