14/180: Linux powerful Commands for System administration

Today is 14th day of my challenge of 180 days streak. Very tired today. It’s 11:30 already, just got free from office. Hectic day, but then this challenge must go on. Today I am going to discuss more on Linux commands, I have already covered Linux basic commands in my previous blog. Today we are going to learn the stuff from a system administrator perspective.

Let’s begin with few Filters / Text Processor Commands

awk: is for data extraction. Let’s say I have a file having list of names and the names consists of first and last name like

1. Navneet Ojha
2. Jeni Kaur
3. Kenny Parker …..
4. …………….and so on

awk '{print $1}' filename

The above command will print the first column in the file. i.e. the first names from the file.

//it will print the send column
awk '{print $2}' filename
// this command file print first and 3rd column of the list
ls -l | awk '{print $1, $3}' 
//print last column
ls -l | awk '{print $NF}' 
//awk can do search for you, it will give you that particular row
awk '/keyword to be searched/{print}' filename

grep/egrep: It process text line by line and show the line which matches.

grep keyword filename
grep -c keyword filename // -c will show the count of occurances
grep -i KEYword filename //-i ignore case
grep -n keyword filename //give line numbers also
grep -vi keword filename | aw '{print $1}' //will give only first col
ls -l | grep Desktop //give matched keyword from list
egrep -i "keyword1|keyword2" filename 

sort/uniq -> filters out repeated lines

sort -> sorts in alphabetical order.

sort file | uniq //alone uniq will not work, it is required to use sort with this.
sort file | uniq -c //give counts for each.

wc: Read input newline count, word count, byte count.

Linux File editor: Though there are many file editors, but I will basically talk about vi editor. Usage of vi editor:

1. Supplies command for Inserting and Deleting text
2. Replacing Text
3. Moving around the file
4. Finding and Substituting strings
5. Cutting and Pasting

Basic vi command:

1. i -> insert
2. r -> replace
3. d -> delete
4. :q! -> quit without write
5. :wq! -> quit with save

Another very powerful command is sed

“sed” command

1. Substitute command
2. Replace a string a file with new string
3. Find and Delete
4. Remove empty lines
5. To replace tab with spaces
6. Show defined lines from a file

sed 's/keyword/replacewith' filename
sed 's/keyword/replacewith/g' filename //replace globally
sed -i 's/keyword/replacewith' filename //if want to insert changes in file
sed 's/keyword/d' filename //delete all lines having keword
sed 's/^$/d' filename //remove empty lines
sed 's/\t/ /g' filename //remove tabs

User Account Management

1. useradd
2. groupadd
3. groupdel
4. usermod

Files: /etc/passwd, /etc/group, /etc/shadow

Monitor User

1. who -> to see how many people are logged in to system
2. last -> last time user logged in
3. w -> almost same as who, with some more information
4. id -> give information about the user

Talking to User

1. users ->users who are logged in
2. wall -> Write message to all users.
3. write -> write message to particular user

Linux Directory Service, linux account authentication

1. local account
2. Domain / Directory Account
3. Windows = Active Directory
4. Linux = LDAP -> protocol to authenticate against directory.
5. IDM -> Identity manager

System Utility Commands

1. date -> shows date
2. uptime -> how long system was up for
3. hostname
4. uname
5. which -> location of command
6. cal -> calender
7. bc -> binary calculator

Processes And Jobs

Application = Service

1. Script -> written in files and packages
2. Process -> when your application generates process id
3. Daemon -> also process run always
4. Threads -> Service Process -> thread 1, thread 2 …
5. systemctl or service
6. ps -> allows you to see the process
7. top -> system resources for monitoring
8. kill -> stop process by process id
9. crontab -> schedule processes
10. at -> like crontab

Process Management (bg, fg, nice)

1. bg -> background
2. fg -> forground
3. nice -> prioritize the process

System Monitoring

1. df -> disk partition info
2. df -h -> disk partition info in human readable format
3. iostat -> input out statistics {iostat 1 (refresh every 1 second)}
4. free -> gives physical memory
5. dmesg -> issues related to system hardware

Log Monitoring

Log Directory = /var/log

1. boot -> when system boots up logs are generated
2. chronyd = NTP
3. cron
4. maillog
5. secure -> logging in logout activity
6. message -> to monitor system messages
7. httpd
8. cat /proc/cpuinfo -> get cpu info
9. cat /proc/meminfo -> get memory info