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Basic Linux Commands (Tips and Tricks)

Linux Developing History

This post is to collect some basic linux commands I used while working on some Linux system:
  • Upgrade System
  • Basic Commands
  • Iptable firewalls
  • User and Group
  • Change Interface IP Address 
  • Fold and Disk Commands
  • Cron Job
  • Create SSH Trust Relationship between two Linux Machines
  • Find Big Files in Linux File System 
  • Find Out My Linux Distribution Name and Version
  • Check Your Public IP Address from CLI
  • PS command
  • VI Command
  • Check Hardware Info
  • Install a software on Linux
  • Use ssh key to encrypt / decrypt files
  • AWS Amazon Linux Instance Commands
  • Change Time Zone
  • Add/Remove Route
  • Remove Specific SSH Host Key 

    Upgrade System

    For Ubuntu:

    apt-get -y update & apt-get -y upgrade & apt-get -y dist-upgrade


    Basic Commands

    man ： manual

    ls ：List Directory Contents
    pwd     ：print working directory
    cd     ：change directory
    mkdir     ：Make directory
    cp     ：Copy
    mv     ：Move
    find,     locate, which and whereis : four commands to search file / folder in Linux
    kill:
    passwd ：Password
    md5sum ：Compute and Check MD5 Message Digest
    history ：History (Event) Record。
    sudo ：(super user do)
    touch ：Update the access and modification times of each FILE to the current time
    chmod ：change file mode bits
    chown ：change file owner and group
    apt ：Advanced Package Tool
    dd: Convert and Copy a file
           [email protected]:~# dd if=/home/user/Downloads/debian.iso of=/dev/sdb1 bs=512M; sync
    tar : Tape Archive
    cal : Calendar
    cat : Concatenation. Concatenate (join) two or more plain file and/or print contents of a file on standard output.
    grep : searches the given file for lines containing a match to the given strings or words
    ps : (Process)
    service : command controls the Starting, Stopping or Restarting of a ‘service‘
    df : disk usages of file system
    du : disk usages
    cmp : compare
    wget : a free utility for non-interactive (i.e., can work in background) download of files from the Web
    mount
    gcc : is the in-built compiler for ‘c‘ language in Linux Environment.
    g++ is the in-built compiler for ‘C++‘ , the first object oriented programming language.
    Java is one of the world’s highly used programming language and is considered fast, secure, and reliable. Most of the the web based service of today runs on java.

    Iptable firewalls

    2.1 Delete IPtable firewall rules

    [[email protected] ~]# /sbin/iptables -L -v -n
    Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
     pkts bytes target     prot opt in     out     source               destination         
      74M   53G RH-Firewall-1-INPUT  all  --  *      *       0.0.0.0/0            0.0.0.0/0           

     Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
     pkts bytes target     prot opt in     out     source               destination         
        0     0 RH-Firewall-1-INPUT  all  --  *      *       0.0.0.0/0            0.0.0.0/0           

     Chain OUTPUT (policy ACCEPT 18M packets, 1069M bytes)
     pkts bytes target     prot opt in     out     source               destination         

     Chain RH-Firewall-1-INPUT (2 references)
     pkts bytes target     prot opt in     out     source               destination         
     5462  734K ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0           
    46700 2228K ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0           icmp type 255 
        0     0 ACCEPT     esp  --  *      *       0.0.0.0/0            0.0.0.0/0           
        0     0 ACCEPT     ah   --  *      *       0.0.0.0/0            0.0.0.0/0           
        0     0 ACCEPT     udp  --  *      *       0.0.0.0/0            224.0.0.251         udp dpt:5353 
        0     0 ACCEPT     udp  --  *      *       0.0.0.0/0            0.0.0.0/0           udp dpt:631 
      719 34592 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp dpt:631 
      63M   52G ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED 
     3094  150K ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           state NEW tcp dpt:22 
      10M 1029M REJECT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           reject-with icmp-host-prohibited

     [[email protected] ~]# /sbin/service iptables save
    Saving firewall rules to /etc/sysconfig/iptables: [  OK  ]
    [[email protected] ~]# /sbin/service iptables stop
    Flushing firewall rules: [  OK  ]
    Setting chains to policy ACCEPT: filter [  OK  ]
    Unloading iptables modules: [  OK  ]
    [[email protected] ~]# /sbin/iptables -L -v -n
    Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
     pkts bytes target     prot opt in     out     source               destination      

     Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
     pkts bytes target     prot opt in     out     source               destination      

     Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
     pkts bytes target     prot opt in     out     source               destination      
    [[email protected] ~]# /sbin/service iptables start
    Flushing firewall rules: [  OK  ]
    Setting chains to policy ACCEPT: filter [  OK  ]
    Unloading iptables modules: [  OK  ]
    Applying iptables firewall rules: [  OK  ]
    Loading additional iptables modules: ip_conntrack_netbios_ns [  OK  ]

    Or we can use the following command or script to stop the rules:
    #!/bin/sh
    echo "Saving current firewall rules at /root/current.firewall file..."
    iptables-save > /root/current.firewall
    echo "Stopping firewall and allowing everyone..."
    iptables -F
    iptables -X
    iptables -t nat -F
    iptables -t nat -X
    iptables -t mangle -F
    iptables -t mangle -X
    iptables -P INPUT ACCEPT
    iptables -P FORWARD ACCEPT
    iptables -P OUTPUT ACCEPT

     2.2. Changing Debian IPTABLES Rules To Survive Reboot
    2.2.1. iptables scripts to enhance the rules at /usr/local/scripts/rc.iptables during a reboot
    Linux1~# cat /etc/init.d/iptables
    #!/bin/sh
    #
    IPTABLES_CONFIG=/usr/local/scripts/rc.iptables
    PATH=/usr/bin:/bin:/sbin:/usr/sbin:/usr/local/bin:/usr/local/sbin

     if [ ! -x /sbin/iptables ]; then
            exit 0
    fi

     start() {
            if [ -f $IPTABLES_CONFIG ]; then
                iptables -F
                iptables -X
                echo $"Applying iptables firewall rules: "
                $IPTABLES_CONFIG
                echo
                touch /var/lock/subsys/iptables
            fi
    }

     stop() {
            iptables -P INPUT ACCEPT
            iptables -P OUTPUT ACCEPT
            iptables -P FORWARD ACCEPT
            iptables -F
            iptables -X
            echo
            rm -f /var/lock/subsys/iptables
    }

     case "$1" in
      start)
            start
            ;;

       stop)
            stop
            ;;

       restart)
            start
            ;;
      *)
            echo $"Usage: $0 {start|stop|restart}"
            exit 1
    esac

     exit 0

     Linux1~# vi /usr/local/scripts/rc.iptables

     Linux1~# /etc/init.d/iptables restart

     Linux1~#iptables -L -v -n | more


     2.2.2. using iptables-restore and iptables-save to edit iptables rules
    iptables-save > /etc/iptables.test.rule

    editor /etc/iptables.test.rule
    iptables-restore < /etc/iptables.test.rule
    iptables-save > /etc/iptables.up.rule
    editor /etc/network/if-pre-up.d/iptables

    Add these lines to iptables file:
                      #!/bin/sh
                      /sbin/iptables-restore < /etc/iptables.up.rule

    The iptables file under     /etc/network/if-pre-up.d/ needs to be executable so change the permissions:
                        chmod +x /etc/network/if-pre-up.d/iptables

    Note: What I found is in some old Debian system, method b does not work. But method a works all the time.

    User and Group

    [[email protected] ~]# useradd test1
    [[email protected] ~]# passwd test1
    Changing password for user test1.
    New UNIX password:
    Retype new UNIX password:
    passwd: all authentication tokens updated successfully.

    [[email protected] ~]# usermod -a -G root test
    [[email protected] ~]# id test
    uid=501(test) gid=501(test) groups=501(test),0(root) context=root:system_r:unconfined_t:s0-s0:c0.c1023
    [[email protected] ~]# groups
    root bin daemon sys adm disk wheel
    [[email protected] ~]# users
    root root
    [[email protected] ~]# groupadd network

    [[email protected] ~]# groups
    root bin daemon sys adm disk wheel
    [[email protected] ~]# cat /etc/group
    root:x:0:root,test,test1
    test:x:501:
    test1:x:502:
    network:x:503:
    [[email protected] ~]# cat /etc/passwd
    root:x:0:0:root:/root:/bin/bash
    xfs:x:43:43:X Font Server:/etc/X11/fs:/sbin/nologin
    test1:x:502:502::/home/test1:/bin/bash

    Change Interface IP Address 
    • Temporary:
      • ifconfig eth1 192.168.2.50 netmask 255.255.255.0 up
    Restart the networking service, enter:
    # /etc/init.d/network restart

    Fold and Disk Commands

    [[email protected] var]# rm -r dbbackup/ -f
    [[email protected] var]# df -h
    Filesystem            Size  Used Avail Use% Mounted on
    /dev/hda3             7.6G  7.3G     0 100% /
    /dev/hda1             244M   12M  219M   6% /boot
    tmpfs                 504M     0  504M   0% /dev/shm
    /dev/hdb1             197G  197G     0 100% /data

     [[email protected] var]# du -s
    4779468 .

    Cron Job

    [[email protected] ~]$ sudo su -
    Password:
    [[email protected] ~]# crontab -l
    @daily scp -r find /var/netscreen/dbbackup/ -mtime -1 -type d -print [email protected]:/data
    @daily mv /root/CP_MGMT_*.tgz /data/backup/cp/

     [[email protected] ~]# crontab -e
    [[email protected] ~]# 

    There are 5 fields before the actual command:
    field                   allowed values
    -----                   --------------
    minute               0-59
    hour                  0-23
    day of month    1-31
    month               1-12 (or names)
    day of week      0-7 (0 or 7 is Sun, or use names)

    Run a command once/week scheduled Saturday morning at 6am:

    0 6 * * sat /path/to/command
    or
    0 6 * * 6 /path/to/command

     Note: Website crontab.guru to write a proper cron job . https://wdt.io/ can provide cron job monitor service. For example, reboot httpd service every four hour: 

    [[email protected] log]# vi /etc/crontab 

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/

# For details see man 4 crontabs

# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name command to be executed
    0 */4 * * * root sudo service httpd restart && curl -sm 30 k.wdt.io/[email protected]/reboot_httpd_4h?c=0_*/4_*_*_*
    

    

    

    Create SSH Trust Relationship between two Linux Machines
    
Become root:
    
sudo su - 
    

    

    
Change to user nsm: 
    
su nsm 
    

    

    
Go to the /home/nsm directory: 
    
cd /home/nsm 
    

    

    
Create the keys: (Path should be /home/nsm/.ssh/id_rsa. Leave the passphrase blank.)
    

      

    
ssh-keygen -t rsa
    

    
Secure copy the public key to the other server as the admin user: (use admin password)
    

      

    
scp /home/nsm/.ssh/id_rsa.pub [email protected]<ipAddressOfOtherServer>:/home/admin/authorized_keys
    

      

    • or Go to the remote server. The command below will add the key that is in temp1 file to the end of the authorized_keys file.
      • 

    
cat temp1 >> authorized_keys
    

      

    • Repeat steps 2-6 on  deviceB.   On deviceB, become root: (from user nsm, exit to root). Move the authorized_keys file that was copied to admin into nsm/.ssh:
      • 

    
mv /home/admin/authorized_keys /home/nsm/.ssh/authorized_keys
    

      

    • Change ownership of authorized_keys: 
      • 

    
chown nsm:nsm /home/nsm/.ssh/authorized_keys
    

      

    • At this point, you will be able to SSH between both servers without it asking for a password.
      • 

    
ssh [email protected]
    

    
Find Big Files in Linux File System 
    

      

    • find . -type f -size +10000 -exec ls -lh {} \; 
      • 

    • find . -type f -size +50000k -exec ls -lh {} \; | awk '{ print $9 ": " $5 }'
      • 

    • Find large files (>10M) in current folder
      • 
find . -type f -size +10000k 

    

    
a. Juniper Firewall  
    
Sample output:
    

    
[email protected]% find . -type f -size +10000 -exec ls -lh {} \; 
    
-rw-r--r--  1 930  929   134M Jan  5 17:34 ./cf/packages/junos-11.4R6.6-domestic
    
-rw-r--r--  1 root  wheel   139M Sep  8  2011 ./cf/var/log/junos-srxsme-11.2R2.4-domestic.tgz
    
-rw-r-----  1 root  wheel   4.9M Feb 11 17:12 ./cf/var/db/idpd/db/secdb_02.db
    
-rw-r-----  1 root  wheel   6.7M Feb 11 17:13 ./cf/var/db/idpd/db/secdb_03.db
    
-rw-r-----  1 root  wheel    64M Feb 11 17:13 ./cf/var/db/idpd/db/secdb_06.db
    
-rwxr-xr-x  1 admin  20    24M May 23 08:38 ./cf/var/db/idpd/nsm-download/SignatureUpdate.xml
    
.....
    
b. Checkpoint Firewall gateway:
    
[[email protected]]# find . -type f -size +50000k -exec ls -lh {} \; | awk '{ print $9 ": " $5 }'
    
./sysimg/CPwrapper/linux/CPEndpointSecurity/EndpointSecurityServer.bin: 145M
    
./sysimg/CPwrapper/linux/windows/SmartConsole.exe: 194M
    
./sysimg/CPwrapper/linux/CPrt/CPrt-R75.40-00.i386.rpm: 53M
    
./sysimg/CPwrapper/linux/CPportal/CPportal-R75.40-00.i386.rpm: 59M
    
./var/log/db: 336M
    
....
    

    
Find Out My Linux Distribution Name and Version
    

    

    

    

    
[[email protected] ~]# cat /etc/*-release
    
Red Hat Enterprise Linux Server release 5.5 Beta (Tikanga)
    

    
[[email protected] ~]# cat /proc/version
    
Linux version 2.6.18-186.el5 ([email protected]) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-46)) #1 SMP Wed Jan 27 18:14:15 EST 2010
    

    
Linux1:~# cat /proc/version
    
Linux version 2.6.26-2-amd64 (Debian 2.6.26-27) ([email protected]) (gcc version 4.1.3 20080704 (prerelease) (Debian 4.1.2-25)) #1 SMP Wed Sep 21 03:36:44 UTC 2011
    

    

    
[[email protected] ~]# lsb_release -a
    
LSB Version:    :core-3.1-ia32:core-3.1-noarch:graphics-3.1-ia32:graphics-3.1-noarch
    
Distributor ID: RedHatEnterpriseServer
    
Description:    Red Hat Enterprise Linux Server release 5.5 Beta (Tikanga)
    
Release:        5.5
    
Codename:       Tikanga
    

    
Linux1:~# lsb_release -a
    
No LSB modules are available.
    
Distributor ID: Debian
    
Description:    Debian GNU/Linux 5.0.9 (lenny)
    
Release:        5.0.9
    
Codename:       lenny
    

    
uname = (Unix Name),
    

    
[[email protected] ~]# uname -a
    
Linux Linux01p 2.6.18-186.el5 #1 SMP Wed Jan 27 18:14:15 EST 2010 i686 i686 i386 GNU/Linux
    

    
[[email protected] ~]# uname -mrs
    
Linux 2.6.18-186.el5 i686
    

    

    

    
    		

    

    

    

    

    

    
Check Your Public IP Address from CLI
    

      

    • curl -s checkip.dyndns.org|sed -e 's/.*Current IP Address: //' -e 's/<.*$//'
      • 

    • curl icanhazip.com
      • 

    • telnet www.checkmyip.com 80 | grep confidence | grep -Eo '([0-9]{1,3}\.){3}[0-9]{1,3}'
      • 

    • wget -O - -q icanhazip.com
      • 

    • wget http://ipinfo.io/ip -qO -
      • 

    • curl ifconfig.me
      • 

    

    

    
PS command
    

    

    

    
Display the top 5 processes consuming most of the cpu:
    

    

    

    
[[email protected]]# ps aux --sort=-pcpu | head -5
    
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
    
admin     3935 14.9  1.0  33032 10344 ?        Ss   09:27   5:13 /bin/confd
    
admin     3941  5.0 58.1 559724 556864 ?       Ss   09:27   1:46 /bin/monitord
    
admin     4215  1.4  3.6 251040 35412 ?        Ssl  09:28   0:28 cpd
    

    
admin     3937  0.7  0.2  26076  2808 ?        Ssl  09:27   0:15 /bin/searchd
    

    

    

    

    
VI Command
    

    
Cut and paste:
    

      

    • Position the cursor where you want to begin cutting.
      • 

    • Press v to select characters (or uppercase V to select whole lines).
      • 

    • Move the cursor to the end of what you want to cut.
      • 

    • Press d to cut (or y to copy).
      • 

    • Move to where you would like to paste.
      • 

    • Press P to paste before the cursor, or p to paste after.
      • 

    

    
Check Hardware Info
    
For CPU:
    
$ cat /proc/cpuinfo
    
$ lscpu
    

    
For Memory :$ free -m (give you result by MB)
    
$ cat /proc/meminfo
    

    
For HDD:$ df -h (give you human readable result)
    
$ sudo fdisk -l
    
$ hdparm -i /dev/device (for example sda1, hda3...)
    


    

    

    Install/Uninstall a software on Linux
    

    

    
For Red Hat/Fedora:
    
$ yum install firefox
    

    
If you are using Red Hat Enterprise Linux, it happens that the package you are looking for is in EPEL, so you can install that:
    
sudo rpm -Uvh http://download.fedora.redhat.com/pub/epel/5/i386/epel-release-5-4.noarch.rpm
    

    
and then you can:
    
yum install ncdu.
    

    
For Ubuntu ( run this as root ) :
    
# apt-get install firefox
    

    
For Debian/Ubuntu
    
# aptitude install firefox
    

    

    

    

    
Use ssh key to encrypt / decrypt files
    

    

    

    

    
Create a file:
    
echo ‘This is a sekret’ >/tmp/msg.txt
    

    
Export public key:
    
openssl rsa -in ~/private.pem -out /tmp/public.pub -outform PEM -pubout
    

    
Encrypt file with public key (anyone can have this key):
    
openssl rsautl -encrypt -inkey /tmp/public.pub -pubin -in /tmp/msg.txt -out /tmp/file.enc
    

    
Decrypt the file with private key (only you should have the private key):
    
openssl rsautl -decrypt -inkey ~/private.pem -in /tmp/file.enc -out /tmp/decrypted.txt
    

    
Check decoded message:
    
cat /tmp/decrypted.txt
    

    

    

    

    
AWS Amazon Linux Instance Commands
    

    

    
sudo yum update -y
    
sudo yum install -y httpd24 php70 mysql56-server php70-mysqlnd
    
sudo service httpd star
    

    

    
sudo chkconfig httpd on
    
chkconfig --list httpd
    
curl http://localhost
    

    

    
sudo usermod -a -G apache ec2-user
    
groups
    
sudo chown -R ec2-user:apache /var/www
    
sudo chmod 2775 /var/www
    
find /var/www -type d -exec sudo chmod 2775 {} \;
    
find /var/www -type f -exec sudo chmod 0664 {} \;
    
echo "<?php phpinfo(); ?>" > /var/www/html/phpinfo.php
    
sudo yum list installed httpd24 php70 mysql56-server php70-mysqlnd
    
sudo service mysqld start
    
sudo chkconfig mysqld on
    
sudo service httpd restart
    

    

    

    

    

    
Change Time Zone
    

    

    

    

    
Ubuntu
    

    [email protected]:/var/log/apache2$ timedatectl list-timezones | grep Toronto
America/Toronto
[email protected]:/var/log/apache2$ sudo timedatectl set-timezone America/Toronto
sudo: unable to resolve host ip-10-1-1-50
[email protected]:/var/log/apache2$ date
Fri Sep 29 22:09:11 EDT 2017


    

    

    

    

    
AWS Linux/CentOS/RHEL 6/5 
    

    [[email protected] ~]$ sudo su
[[email protected] ec2-user]# mv /etc/localtime /root/localtime.old
[[email protected] ec2-user]# ln -s /usr/share/zoneinfo/America/Toronto /etc/localtime
[[email protected] ec2-user]# date
Fri Sep 29 22:11:00 EDT 2017
[[email protected] ec2-user]#


    

    

    

    

    

    

    
Add/Remove Route
    

    
LinuxSvr:~# cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug eth0
iface eth0 inet static
        address 10.9.2.10
        netmask 255.255.255.0
        network 10.9.2.0
        broadcast 10.9.2.255
        gateway 10.94.22.1
        # dns-* options are implemented by the resolvconf package, if installed
        dns-nameservers 10.9.1.5
        dns-search accounts.intern gdc.intern intern
        #
        #up route add -net 19.18.0.0/16 gw 10.9.2.3
        #up route add -net 172.1.0.0/16 gw 10.9.2.3
        #up route add -net 10.0.0.0/24 gw 10.9.2.3
mta:~# 

    

    

    

    

    
Remove Specific SSH Host Key 
    

    

    

    

    
If you have remote server system has changed, your ssh session might failed because ssh fingerprint is not matching what you have now for that host. To fix this issue, you will just need to remove existing host's fingerprint. 
    

    

    

    
users-Mac-mini:~ user$ ssh -lroot 111.67.205.98
    

    
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
    

    
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
    

    
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
    

    
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
    

    
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
    

    
It is also possible that a host key has just been changed.
    

    
The fingerprint for the ECDSA key sent by the remote host is
    

    
SHA256:V9o8sh2Vxm56r+2q42minB4D88a8yTanlAPs4h/VAMY.
    

    
Please contact your system administrator.
    

    
Add correct host key in /Users/user/.ssh/known_hosts to get rid of this message.
    

    
Offending ECDSA key in /Users/user/.ssh/known_hosts:4
    

    
ECDSA host key for 111.67.205.98 has changed and you have requested strict checking.
    

    
Host key verification failed.
    

    


    

    
users-Mac-mini:~ user$ vim ~/.ssh/known_hosts 
    

    
and hit
    

    
dd
    

    
then save it.
    

    

    

    

    

    
Reference:
    

    

    


    

    

    

    

    

    

    


    

    


    

    

    


    

      



    

    

    

    


    


    

    

    

    


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