Important Linux and Unix Commands

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Linux

Necessary Linux / Unix Commands

There are a lot of articles on the web regarding Linux commands. Here I have listed some of my favorite Linux commands. These commands are little bit more advanced than those mentioned in the general blog but very useful.

- - Check top usages process
```
ps -eo pid,user,ppid,cmd,%mem,%cpu --sort=-%cpu | head
```
  - Start service at startup in CentOS/Redhat 7
```
systemctl enable httpd
```
  - Check the listening port on Linux
```
netstat -atupn | grep LISTEN
netstat -atupn | grep LISTEN | grep ":22"
lsof -i:22
```
  - Check the IO performance
```
iostat -x 5
```
    The above command continuously prints the IO stat on the screen at 5-second intervals
  - Check the Process performance Vs IO and Memory
```
#details command 
pidstat -h -U -r -d -l 10
#Short 
pidstat -h -U -r -d  10
```
    The above command continuously prints the process consuming IO on the screen at 10-second intervals
  - Check the Process performance Vs CPU
```
  pidstat -h -U -l 10
```
    The above command continuously prints the process consuming IO on the screen at 10-second intervals
  - Find all files following a pattern
```
find /home/mdn2000/ms-h/bin  -name "core*"
```
    Description: The above command finds all files prefixed with core and show them on the screen
  - Find all files following a pattern and older than a specific time
```
find /home/mdn2000/ms-h/bin  -name "core*"  -ctime +7
```
    Description: The above command finds all files prefixed with core and older than 7 day
  - Find all files following a pattern and older than a specific time and then move them specific directory
```
find /home/huawei/mdn2000/ms-h/bin -name "core.hmsserver*"  -ctime +7 -exec mv  {} /home/hms/data/c/core_backup/   \;
```
    Description: The above command finds all the files prefixed with core and older than a specific time and then move them to specific directory
  - Find all files following a pattern and rename them to another pattern
```
find ./ -name "*.repo.osms-backup" -exec sh -c 'mv "$1" "${1%.repo.osms-backup}.repo"' _ {} \;
```
    Description: The above command finds all the files prefixed with “*.repo.osms-backup” andrename them to *.repo”.
  - Compress and archive all files of a specific pattern
```
i) tar -czvf test.tar.gz *.unl
ii) tar –czvf test.tar.gz prm*
```
    Description: The first command will compress and archive all files with the suffix /file extension “.unl”. The second one will compress and archive all files with the prefix “prm” . Here gzip compress is used which is very efficient.
  - Uncompress and Untar
```
tar -xzvf  test.tar.gz
```
    Description: The above command will untar and uncompress all files that were previously archived and compressed as file name test.tar.gz by “ tar –czvf” command
  - Copy all files recursively from the web
```
wget --recursive --no-parent --no-clobber --execute robots=off http://public-yum.oracle.com/repo/OracleLinux/OL5/latest/
```
  - Check how many HTTP processes are running
```
# ps -ylC  httpd  | wc  -l
```
    Description: The above command shows how many httpd processes are running. When more requests come to the Webserver ( Apache, Nginx etc.), it spawns more processes. Generally, the process consumes more memory and CPU.
  - Boot Redhat / Oracle Linux in Single user/Rescuew mode
```
During boot time press e and then add rd.break at the end of linux kernel reference line. CTRl +X will let
you log in single use rmode. After log in run the following command
mount -o remount,rw /
chroot /sysroot/

or 
replace "ro" of kernel reference line with "rw init=/sysroot/bin/sh" and CTRL +X. After log in run
chroot /sysroot/
```
  - Rescan newly added space on an existing disk (Especially applicable on Cloud & Virtual environment)
```
dd iflag=direct if=/dev/sdg of=/dev/null count=1
echo "1" | sudo tee /sys/class/block/sdg/device/rescan
or 
partprobe
```
    Description: /dev/sdg is already present in the server. From Cloud / virtual management console disk space is increased.
  - Rescan all disks (Especially applicable on Virtual environment)
```
for device in /sys/class/scsi_disk/*; do
echo "Rescanning $device"
echo 1 > "$device/device/rescan"
done
```
  - Check total and average memory consumption of a process For httpd process:
```
# ps -ylC httpd | awk '{x += $8;y += 1} END {print "Apache Memory Usage (MB): "x/1024; print "Average Process Size (MB): "x/((y-1)*1024)}'

Apache Memory Usage (MB): 284.121

Average Process Size (MB): 10.523
```
    For mysqld process:
```
# ps -ylC mysqld  | awk '{x += $8;y += 1} END {print "MySQL Memory Usage (MB): "x/1024; print "Average Process Size (MB): "x/((y-1)*1024)}'

MySQL Memory Usage (MB): 15840.1
Average Process Size (MB): 15840.1
```
    Description: The above two commands will output how much total memory is consumed by Apache and MySQL server as well as the average memory consumption of a process. For Apache many httpd process are spawned but for MySQL only one mysqld process is spawned.
  - Increase partition szie in AIX:
```
chfs -a size=+500000 /var
```
    The above command will increase the size of /var parttion with 50 MB. /var must be part of LV that can take free sapce from its VG
  - Check which package is required to be installed for a command by yum for Centos/Redhat
```
# [root@localhost ~]# yum whatprovides netstat
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: centosmirror.go4hosting.in
 * extras: centos.ustc.edu.cn
 * updates: centos.ustc.edu.cn
net-tools-2.0-0.17.20131004git.el7.x86_64 : Basic networking tools
Repo : @base
Matched from:
Filename : /usr/bin/netstat
```
    Description: The above command will show to activate netstat command( if it is not already installed on your Linux server), you need to install net-tools package. So the next command will be
    
    #yum install net-tools
  - Activate tcp/udp port in firewall at Redhat7/Cen0S7Allow the port through firewalld:
  - ```
      firewall-cmd --permanent --add-port=21/tcp
```
  And reload the firewall:
```
      firewall-cmd --reload
```
  Description: Here we activate ftp port 21 so that the remote host can get FTP access
- Block an ip/network at Linux firewalls
```
  firewall-cmd --permanent --add-rich-rule="rule family='ipv4' source address='' reject"
  firewall-cmd --reload
```
  Remove the previous bloked IP/Network
```
  firewall-cmd --permanent --remove-rich-rule="rule family='ipv4' source address='' reject"
  firewall-cmd --reload 
```
- Split a big file into smaller ones according to the line
- ```
    split -l 200000  -d  NID_formatted.txt  nid
```
    Description: The file NID_formatted.txt contains 2000000 lines. The split command will divide it into 10 files containing each 200000 lines. -d option will add a numeric suffix (length 2) and nid is the prefix. so the splitted files name will be nid00, nid01, nid02 ……. nid19. If you do not use -d option, the suffix will be alphanumeric which will generate files like nidaa, nidbb … and so on.
  - Set up an SSH key passphrase for your environment
  - ```
  eval "$(ssh-agent -s)"
  ssh-add ~/.ssh/id_rsa 
```

Important Linux and Unix Commands

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