contents -

:1.Know your machine 💻

:2.Monitoring 🔧

:3.Tweaks for better Administration 👨🏻‍💻

:4.Networking 📡

:5.Searching 🔍

:6.Automation ⚙️

:7.RHEL Web Console 👾

:8.File Exploring 📁

:9.VI Editor 📝

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1. Know your machine 🖥🖥️

1.Display Linux system information-

uname -a

hostnamectl

2.Show operating version & name-

cat /etc/os-release

3.Show host name-

hostname

hostname -I

(to change hostname- $sudo hostname new_hostname)

3.Show users currently logged in- W

w

4.Display CPU information-

cat /proc/cpuinfo

5.Display memory/RAM -

cat /proc/meminfo

6.Display RAM usage (redable, in MBs, in GBs)

free -h

free -m

free -g

7.Displays all environment variables running on the system.

env

8.Display DMI/SMBIOS (hardware info) from the BIOS

dmidecode

9.To List all installed packages on machine -

rpm -qa

10.To list active services -

systemctl list-units --type=service

11.Displays kernel-related messages

dmesg -HTx

12. List installed packages-

yum list installed

monitoring

To check running processes with more details-

ps aux

To get PID of running processes by name-

pgrep process_name

lists all active services managed by systemd

systemctl list-units --type=service

1.WATCH - shows periodic updates in output

watch df -h

watch -n 5 -d '/bin/free -m'

Monitor open netstat connections-
watch -n 1 "netstat -tpanl | grep ESTABLISHED

2.Monitor all traffic on port

tcpdump -i device 'port port_n0'

tcpdump -i enp0s3 'port 9090'

3.Display the last 100 syslog messages

tail -100 /var/log/messages

head -100 /var/log/messages

4.reboot History

last reboot

5.History - to check previous commands.

history 100

6.NMON - 'TOP' alternative for monitering-

nmon

NMON can be used as a better 'top' alternative.

1.CPU utilization,
2.Memory
3.Disks
4.Network utilization & more
In a better viewe where refresh rate can be adjusted by pressing +, - buttons.
Stats can be saved to CSV for later analysis & graphing.

exploring

File Exploring

List only directories-

ls -d */

List files with subdirectories-

ls *

Show mountpoints in tree-

findmnt

lsblk

List files recursively-

ls -R

List files with their sizes-

ls -s

List files in long format-

ls -lrt

List files in long format with readable file sizes-

ls -lh

List files including hidden files-

ls -a

List files and sort by file size-

ls -S

-t <- time

-r <- reverse

compare files - diff | comm | cmp

to check difference in 2 files-

diff file1 file2

to check difference by lines & size-

cmp file1 file2

to display unique lines in 2 files-

comm file1 file2

to check size of current directory-

du -sh

to find the largest log file-

find / -type f -name "*log*" | xargs ls -lSh | more

rsync - scp alternative. For copying and synchronizing files and directories remotely and locally, Can be used to mirror data on 2 machines.-

rsync [OPTIONS] /SOURCE /DESTINATION

-v -display transfer details
-P –displaying detailed information
-r –copies data recursively
-a –copies data & preserves file permissions, user & group ownerships,
and timestamps.
-z –Compress files during transfer to reduce network usage.
-h –output transfer numbers in a human-readable format.

Copying files

cp -r source_folder /path/to/destination_folder/

TAR - compress & Extract files -
To compress files-

tar -czvf file_name.tar.gz *

To view files in tar-

tar -tvf file_name.tar.gz

To extract files-

tar -xzvf file_name.tar.gz

MC - Midnight Commander - To manage files grahically on terminal-
can be managed using input from mouse. can be used for- Copy, Delete, Rename , Move, make Directory , edit files & change permissions. Press TAB to swich between the panes of active directories. Bottom menu can be accessed using Function keys F1-F10.

mc

Check out : VI Editor tips & Tricks

tweaks

for better Administration

1.Record Terminal session -

to start recording session-

script script_name.txt

to stop session-

exit

to view the recorded session-

cat script_name.txt

2.Increase SSH timeout -

vi /etc/ssh/sshd_config
insert/assign values as per following to set timeout to 1 hour -

ClientAliveInterval 1200
ClientAliveCountMax 3

3. long running command in background -

long running commands can be sent to background by simply using '&' after the command. Ampersand instructs the shell to execute the command as a separate background process.
for e.g checking unreadable blocks on disk took hours to get finished, So this job will be sent to background-
badblocks -s /dev/sda &
to check status - jobs

4. nohup

nohup is extension to '&' , is used to keep the process running in background even after user logs out,

nohup your_command &

e.g. nohup sh script_runner.sh &
to check status - jobs

5. TMUX -

to enable second terminal pane for multitasking without opening a second session.

1. Type tmux
2. To open second pane - Ctrl + b then Shift + 5(%)
3. To switch pane - Ctrl + b then left arrow or right arrow to switch the curser.
4. To close pane - Ctrl + b followed by d
5. type tmux attach to open the earlier pane. (after closing the pane session continues running in bg)

Check byobu , screen & Zellij for more enhanced terminal multiplexing.

Check out : VI Editor tips & Tricks

6. DISOWN -

disown command removes the given process from running terminal shell & runs in background until completion. this can be used for long running jobs on time limited sessions.

7. to save command output in a file-

your_command > file1.txt

to append more commands on same file -

your_2nd_command >> file1.txt

8. Solving space crunch -

1.find largest file on server by-

find / -type f -name "*log*" | xargs ls -lSh | more

2.empty the file using echo trick instead of using rm-

echo "" > access.log

9. wall - Send a msg to all logged in users -

wall -n hi all, system reboot will be initiated in 10 mins

To send msg to a specific user group-

wall -g [group-name] [message-text]

9.To find recent errors in system using journalctl-

journalctl --no-pager --since today \ --grep 'fail|error|fatal' --output json|jq '._EXE' | \ sort | uniq -c | sort --numeric --reverse --key 1

10.Search specific files fast-

grep -R 'import' --include='*.java' --color MySourceCodeDir

11.cfg2html- bash script

A powerfull bash script Useful in recovery like situations, exports necessary system configuration files and system setup in html & txt format.

git clone https://github.com/cfg2html/cfg2html

./cfg2html

12. BCC (BPF Compiler Collection)

BCC is a toolkit for creating efficient kernel tracing and manipulation programs, and includes several useful tools and examples. It makes use of extended BPF (Berkeley Packet Filters),
url - https://github.com/iovisor/bcc

networking

1.Monitor all traffic on port

tcpdump -i device 'port port_n0'

tcpdump -i enp0s3 'port 9090'

To check open ports on server-

netstat -pnltu

netstat -nutlp

Check connectivity & port opening status

ssh -vvv <IP> -p <PORT>

ssh -vvv 192.168.186.42 -p 9090

connection succesful -

connection failure -

NetCat -

nc -z -v -w 5 <IP> <PORT>

To get DNS information-

dig dns_name

Display the top 10 IP addresses hitting a webserver -

cat /var/log/nginx/access.log | cut -f 1 -d ' ' | sort | \ uniq -c | sort -hr | head -n 10

To change hostname -

sudo hostname new_name

To create ssh tunnle

ssh -f -L 9000:TARGET_SERVER_IP:8088 root@IP_ADDR -N

To Print routing-

traceroute google.com

nc -vw5 google.com 80

Sniff network traffic on a network interface-

sudo tcpdump -i wlan0 -n ip | awk '{ print gensub(/(.*)\..*/,"\\1","g",$3), $4, gensub(/(.*)\..*/,"\\1","g",$5) }' | awk -F " > " '{ print $1" "$2}'

searching

1.AWK- data extraction from files, text processing, report generation

Syntax-
awk options 'selection _criteria {action }' input-file > output-file

Examples -
1.Sorted print - login name of all users-

awk -F ":" '{ print $1 | "sort" }' /etc/passwd

2.Calculate size of directory-

ls -al | awk '{total +=$5};END {print "Total size: " total/1024/1024 " Mb"}'

3.To count size of specif files in dir-

ls -l *.sh *.yaml | awk '{sum+=$5} END {print sum}'

4.Print all lines of a file prefixed with a line number-

awk '{print NR, $0}' [FILENAME]

5.To extract e-mail address from a file-

awk '/[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}/ { print }' file.txt

6.To extract FQDN,IP, URL from a log file-

awk '$6~/GET/{split($7,url,"/"); print url[3]}' /App_logs/log/sysout.log

7.To print error lines-

awk '$3 == "Error:"' /App_logs/log/sysout.log

2. GREP- Global Regular Expression Print - To look for things in files

Syntax-
grep [options] pattern [files]

Examples-
1.Search for lines matching 'pattern' in file1.txt-
grep 'pattern' file.txt

2.Case-insensitive search for 'pattern'-
grep -i 'pattern' file.txt

3.Print lines not containing 'pattern'-
grep -v 'pattern' file.txt

4.Count lines containing 'pattern'-
grep -c 'pattern' file.txt :

5.Print lines with line numbers-
grep -n 'pattern' file.txt

6.Recursively search in directory dir/
grep -r 'pattern' dir/

7.Highlight the matched pattern-
grep --color 'pattern' file.txt

8.Extended regex for multiple patterns-
grep -E 'pattern1|pattern2' file.txt

9.List files containing 'pattern'-
grep -1 'pattern' dir/*

10.Match whole words only-
grep -w 'pattern' file.txt

11.Print only the matched parts of the line-
grep -o 'pattern' file.txt

12.Print 3 lines after the matched line-
grep -A 3 'pattern' file.txt

13.Print 3 lines before the matched line-
grep -B 3 'pattern' file.txt:

14.Print 3 lines around the matched line-
grep -C 3 'pattern' file.txt

15.Use patterns from file for matching-
grep -f patterns.txt file.txt

16.Search only in .txt files within dir/-
grep --include '*.txt' 'pattern' dir/

17.Exclude log files from search-
grep --exclude '*.log' 'pattern' dir/

18.Quiet mode, returns 0 if pattern is found-
grep -q 'pattern' file.txt

19.Use pattern for matching-
grep -e 'pattern' file.txt

20.Recursively search with symbolic links-
grep 'pattern' -R dir/

1. Print details about processes- ps -ef | grep java prints all processes of java ps -ef| grep pmon prints all DBs currently running

3. SED -

sed OPTIONS... [SCRIPT] [INPUTFILE]

Examples-
replace a string on one or more files
sed -i 's#ORIGINAL_VALLUE#NEW_VALUE#g' myfile1 myfile2
``

Check out : VI Editor tips & Tricks

console

RHEL Web Console-
A web-based interface can be used for managing and monitoring RHEL systems in realtime. Follow the commands to enable & use RHEL web console-

Enable web console -
systemctl enable --now cockpit.socket

if cockpit package not installed -
dnf install cockpit

open port 9090 for web console -
firewall-cmd --add-service=cockpit --permanent

open browser & type URL -
https://IP or hostname:9090
e.g. https://192.168.186.42:9090/system

login page-

System Overview-

Install tools -

Critical Logs -

Terminal access -

Storage-

Network details-

users & groups

[------

Automation

1.at command - an easier alternative for cron.
Task scheduling for shorter times or to automate simpler jobs.

at

echo "hello world" | at 1:30 AM

To view at schedule-

atq

To remove a job-

atrm job_number

Time expressions for at-

• now
• midnight
• noon
• teatime (4 PM)
• AM
• PM
• minutes
• hours
• days
• weeks
• months
• years

Examples-

echo "rsync -av /source/files /destination" | at 3:30 AM tomorrow

echo "mv filename filename2" | at 3:30 AM 08/01/2022

echo "./script_runme.sh" | at now + 3 days