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Linux Commands to know by DevOps Engineer Cheat Sheet (DRAFT) by

This is a draft cheat sheet. It is a work in progress and is not finished yet.

System/OS related commands

To know the OS type: 
$ uname -o
To know the CPU archit­ecture: 
$ uname -m
To check the kernel version: 
$ uname -r
To get the OS name, release, version: 
$ cat /etc/o­s-r­elease
To list the system hardware: 
$ lshw
To get the CPU details: 
$ lscpu
To check system memory: 
$ free -h
To check the virtual memory stats: 
$ vmstat -S m
Free memory cache, dentries and inode (with root): 
$  echo 3 > /proc/­sys­/vm­/dr­op_­caches
To print the process specific memory utiliz­ations: 
$ ps aux --sort­=-%mem
To search packages for instal­lation: 
$ apt search <pa­ckage name> 
 e.g.: 
 $ apt search python­-boto
To installed package: 
$ sudo apt-get install <pa­ckage name>
To uninstall package:
$ sudo apt-get remove <pa­ckage name
To list the mounted disk drives: 
$ df -kh
To mount the volume:
(create the directory first to mount volume) 
$ mkdir -p <di­rectory path e..g /mount­-vo­l> 
 $ sudo mount <src path> <above created dir path>
To list biggest files from directory (biggest 5): 
$ sudo du -a /dir/ | sort -n -r | head -n 5
Find the file (search for a file): 
$ find <dir path> -name <fi­len­ame> -print 
 e.g. to find app.log in /var directory
$ find /var -name app.log –print`
Search the text string in a directory and print filename containing that string: 
$ file /var -type f -print | xargs grep <search text>
File the text string from a given directory: 
$ grep -rIn <search text> <di­rectory path>

User admin Commands

To know the group/user exists on the system: 
$ getent group <group name> 
 $ getent passwd <user name>
Check user added or not into system: 
$ id <us­ern­ame>  
 e.g. $ id clouduser1
To create a new group: 
$ sudo groupadd <group name> 
 e.g. $ sudo groupadd training
Modify existing user, add user to group: 
$ sudo usermod -aG <group name> <us­ern­ame> 
 e.g. $ sudo usermod –aG sudo clouduser1
To delete the existing group: 
$ sudo groupdel <group name> 
 e.g. $ sudo groupdel training
Add user’s home directory (example for cloudu­ser1): 
$ sudo mkdir -p /home/­user1 
 $ sudo chown cloudu­ser­1:c­lou­duser1 /home/­user1 $ ls -l /home 
 drwxr-xr-x  2 clouduser1     clouduser1      4096 Nov 18 12:13 user1 
 $ sudo usermod -d /home/­user1 clouduser1 
 $ id clouduser1 
 uid=10­02(­clo­udu­ser1) gid=10­03(­clo­udu­ser1) groups­=10­03(­clo­udu­ser­1),­27(­sudo) 
 $ su - clouduser1 
 $ pwd 
 /home/­user1
Print the groups to which the current user is associ­ated: 
$ groups
Delete existing user with all files associated with user: 
$ sudo userdel -r clouduser1 
 $ id clouduser1 
 id: ‘cloud­user1’: no such user
Change the group name: 
$ sudo groupmod –n <new group name> <old group name> 
 e.g. I want to change the groupname ‘training’ to ‘cloud­admin’ 
 $ sudo groupmod -n cloudadmin training
Add user to system:
$ sudo adduser <user name> 
 e.g. add clouduser1 to system 
$ sudo adduser clouduser1

Editor­/Text manipu­lation commands

awk command for pattern scanning & proces­sing:
1. Convert text from upper case to lower case 
 $ echo "­SAMPLE TEXT" | awk '{print tolowe­r($0)}' 
 2 Print the next word after found a pattern 
 e.g. print the next work after ‘reach:’ appear in syslog file 
 $ awk '{for(­i=1­;i<­=NF­;i+­+)i­f($­i==­"­rea­ch:­"­)print $(i+1)}' /var/log/syslog
 3. Trim the white spaces  
 echo '    aws <co­mma­nd> help ' | awk '{gsub(/^ +| +$/,"")}1' 
 4. Print the selected columns from command output. 
 E.g. from df command interested in only filesystem and use% column data 
 $ df -kh |awk '{print $1 " " $5}' 
 5. use regex as a field separator, 
 e.g input field separator as / or = e.g.  
 $ awk -F"=­|:" '{print $2}' 
 input text as ‘dnsco­nf=­/et­c/r­eso­lv.c­onf’ or ‘dnsco­nf:­/et­c/r­eso­lv.c­onf’ for both same command will work
diff, get the differ­ences by comparing files line by line 
 $ diff file1.txt file2.txt
cut, cutting out the sections from lines: 
$  cut -d “delim­iter” -f <fi­eld> <fi­le.t­xt> 
 a) cut the line on space and print 1st to 4th field 
 $ echo "my phone number is 887389­3" | cut -d " " -f 1-4 b) 
 change the delimiter space with column 
 $ echo "­hello world" | cut -d " " -f 1-2 --outp­ut-­del­imi­ter=%
Uniq, is a command that filter out the duplicates 
a) fetch repeat­ed/­dup­licate lines from a file 
 $ uniq -d <fi­le.t­xt> 
 b) get the count of uniq lines in a file {nl}} $ uniq -c <fi­len­ame>
Sort is to sort file, records, lists etc: 
a) sort file contents of text file (-r option to reverse sorting) 
 $ sort file.txt 
 b) sort based on column number 
 $ df -kh | sort -k 5
tr is to translate or delete characters 
a) translate all lowercase letters to upper case in a file 
 $ cat filename | tr “[:low­er:]” “[:upp­er:]”  
 b) translate white spaces to tabs 
 $ cat filename | tr [:space:] ‘\t’ 
 c) remove all digits from string 
 $ echo "my mob number 880392­23" | tr -d [:digit:] 
 d) Just get  the digits from string 
 $ echo "my mob number 880392­23" | tr -cd [:digit:]
tee, is a command which reads the standard input and write into standard output and also to a file. This is used to redirect logs or data to a file: 
 a) let we have two log files, file1.log & file2.log and we need to append file1.log to file2.log 
 $ cat file1.log | tee -a file2.log 
 b) redirect the command output to a log file 
 $ du --max-­depth=1 -h | sort -hr 2>&1 | tee du.log
sed - stream editor, it is used for filtering and transf­orming text 
a) Find and replace text  
 $ echo 'Unix is multi-user OS' | sed 's/Uni­x/L­inux/' 
 b) delete particular line from a file (e.g. 5th line) 
 $ sed '5d' file.txt 
 c) delete 5th to 10th line from a file 
 $ sed '5,10d' file.txt 
 (check more details in a separate block)

Network related commands

nslookup, Query internet domain name server 
a) find the IP from fqdn 
 $ nslookup google.com 
 b) check the fqdn from ip address 
 $ nslookup 172.21­7.1­67.174
netstat, print the network stats, listening ports etc 
a) print all listening ports 
 $ netstat -plunt 
 b) check if server is listening on port 8080 or not 
 $ netstat -plunt | grep 8080 
 c) list stats of all ports 
 $ netstat -s 
 d) display pid of listening ports 
 $ netstat -pt 
 e) list network interfaces 
 $ netstat -i
scp, secure copy from remote host 
a) copy file from remote host 
  (syntax) scp -i <pem file> <us­ern­ame­>@<­remote ip>­:<f­ile­pat­h> <local destin­ation dirpat­h> 
 e.g. $ scp id_rsa.pem rakesh­@19­2.1­68.5­6.120:/home­/ra­kes­h/d­ata.txt . 
 b) copy local file to remote host 
 $ scp -i id_rsa.pem data.txt rakesh­@19­2.1­68.5­6.120:/home­/rakesh
nmap, check open ports on server, generally used as network explor­ation tool 
a) check open ports on remote host 
 $ nmap 172.21­7.2­7.206 
 b) list out all machines from network that responds to ping 
 $ nmap -sP 192.16­8.5­6.0/24 
 c) scan and print ports, os & other details about remote host 
 $ sudo nmap -sS -A -T4 192.16­8.5­6.150
lsof, list open files by processes 
a) list open files by specific user 
 lsof -u <us­ern­ame> 
 b) find processes running on specific port 
 $ lsof -i TCP:9090
netcat, debug and invest­igate network 
a) start a dummy listening server on port 8080 
 $ netcat -l 8080 
 b) send data over some port to server 
 $ netcat <remote server ip> <po­rt> e.g. 
 $ netcat 192.16­8.5­6.120 8080 
 (press EOF CNTR+D at end)
curl ifconf­ig.co, get the public ip of the machine 
$ curl ifconf­ig.co
route, show/=­man­ipulate IP routing table 
a)  show current routing table 
 $ route -n 
 b) add route to particular network e.g. 
 make 10.10.7­6.0/24 accessible via gw 10.10.76.1 
 $ route add -net 10.10.76.0 netmask 255.25­5.255.0 gw 10.10.76.1
ufw, manage firewall 
a) check firewall status 
 $ sudo ufw status 
 b) enable­/di­sable firewall 
 $ sudo ufw enable­/di­sable
hostname, provides hostname of a machine 
a) get hostname 
 $ sudo hostname

sed - stream editor

Sed - perform basic transf­orm­ations on an input stream i.e. a file or a stream input from a pipeline.

Example: replace all occurr­ences of TCP to UDP in networ­k.log file
$ sed 's/TCP­/UDP/' networ­k.log > modifi­ed-­net­wor­k.log


Common sed command line options
-i : edit in place i.e. sed -i 's/TCP­/UDP/' networ­k.log
-n <line number­>p e.g. print on line no 30 from networ­k.log sed -n '30p' networ­k.log
-e : expression e.g. sed -e 's/TCP­/UDP/' networ­k.log
[ here 's' stand for substitute ]

Basic regular expression overview
. : (dot) matches any single character
* : matches a sequence of zero or more instances e.g.
 $ echo 'hostn­ame­=lo­cal­hos­t.m­yor­g.com' | sed 's/l.l/myap­pse­rver/'
*
^ : indicates the beginning of the line
$ : indicates the end of the line
[list] or [^list] :matches any single char in a list. e.g. [1-9] matches any digit from 1 to 9
\+ : As *, matches any single or multiple instances of chars
\?: As *, matches any zero or one instances of chars
\{i\}: matches exactly i sequences 'i is between 0 to 255'
\{i,\} : matches more than or equal to i sequences
regex1­|regex2 : matches regular expression 1 or regular expression 2
[a-z0-­9A-z]: matches any ASCII chars

======­===­===­===­===­===­===­===­===­=======
Examples
------­---­---­-------
# find and replace any os name with Ubuntu
e.g.
1.
input: osname: centOS7
output: osname: Ubantu
2.
input: winOS: Windows-10
output: osname: Ubantu
3.
input: MacOS:­Mac10
output: osname: Ubantu

Solution:
key=
echo "­<input string­>" | cut -d "­:" -f 1

echo "­<input string­" | sed -e 's/^$k­ey:\s.$/$key: Ubantu/g'

first store the key i.e. left side label
^ - start of line
\s* - zero or more space charaters
.* - any zero or multiple charaters
$ - end of the line
------­---­---­---­---­---­---­---­---­-------
Extract the line containing IP address from a file
sed -rn '/([0-­9]{­1,3­}\.)­{3­}[0­-9]­{1,­3}/p' /etc/hosts