Basic Linux Commands

2.2.2. using iptables-restore and iptables-save to edit iptables rules

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:

6. Cron Job

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)

7. Create SSH Trust Relationship between two Linux Machines

Become root:
sudo su – 

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 admin@<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]

8. 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 

9. Find Out My Linux Distribution Name and Version

10. Troubleshooting Linux System Issue with Vmstat Command

The ‘procs’ field has 2 columns:
    r – The number of processes waiting for run time.
    b – The number of processes in uninterruptible sleep (blocked processes).

The ‘memory’ field has 4 columns: (see with vmstat -a)
    swpd – The amount of used swap space(virtual memory) used.
    free – The amount of idle memory(free RAM).
    inact – The amount of inactive memory.
    active – The amount of active memory.

The ‘swap’ field has 2 columns:
    si – Amount of memory swapped in from disk (/s).
    so – Amount of memory swapped to disk (/s).

The ‘io’ field has 2 columns:
    bi – Blocks received from a block device (blocks in).
    bo – Blocks sent to a block device (blocks out).

The ‘system’ field has 2 columns:
    in – The number of interrupts per second, including the clock (System interrupts).
    cs – The number of context switches per second (Process context switches).

The ‘cpu’ field has only 4 columns:
    us: Time spent running non-kernel code. (user time, including nice time).
    sy: Time spent running kernel code. (system time).
    id: Time spent idle.
    wa: Time spent waiting for IO.

CPU slow1:
    r has numbers in it constantly, threads/tasks waiting to be processed by your gimp cpu
CPU slow2:
    in is high, you are handling too many interrupts (likely from disk activity, but could be bad driver)
Processes:
    us or sy is high? Some process is being a cpu hog, use top -n 1 to find it, and kill -9 the PID if needed
Disk Subsystem Overloaded:
    wa is high? If you are waiting for IO then you need to upgrade your disk subsystem
Not Enough RAM:
    si and so are high, swapping disk too much. You really shouldn’t swap at all for high performance. If these are high, in will be high too. Upgrade your RAM.
Low Memory2:
    cs is high? The kernel is paging memory in and out of context. Likely you need more RAM, but it could be other issues too such as damaged hardware or pitiful software.
Out of Memory:
    I ignore free, inact, active because it’s not as useful and understanding the actual reasons. Ie: if you are out of memory, you’ll know that, but unless you look at cs, so, si, etc you won’t know why. So it’s redundant.

11. 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 –