Linux

Linux_CheatSheet

Logging

Monitor logs

journalctl

tail -f /var/log/messages

Kernel messages

journalctl -k

cat /var/log/kern.log

tail -f /var/log/syslog

System messages

journalctl

journalctl | grep ssh

The boot process, kernel, and all systemd services put messages into the systemd journal.

Installation logs

anaconda.log

/var/log/anaconda/anaconda.log

Logs location

/var/log

Performance monitoring

CPU mpstatMemory vmstatDisk iostatNetwork netstat

top (Display and manage the top processes)

free -m or -g

vmstat -a (active and inactive memory), d (disk stats)

mpstat 1 (processor related statistics)

iostat 1 (I/O statistics)

Process management

ps -ef to find the process ID.

kill -9 <PID> to kill the process. -9 is the priority.

Port

netstat -putan | grep :22 (check open port)

lsof -i :22 (list open port)

tcpdump -i any port 80 ( captures traffic from port 80)

ngrep -q -d any port 80 (identifying misconfigured clients)

iftop -P -f "port 80" (monitoring website load during high traffic)

tcpdump -i eth0 port 80 -w suspicious_traffic.pcap (suspects malicious traffic targeting port 80)

Cronjobs

* * * * * command_to_execute

/etc/crontab

crontab -e (edit crontab file, or create one if it doesn’t already exist)

crontab -l (crontab list of cronjobs , display crontab file contents)

crontab -r (remove your crontab file)

If your cron job didn't work, how would you check?

First check your system time

then crontab entry,

then check /var/log/messages

How to check open port on Linux system?

netstat putan | grep :22

IP

hostname -I

Process vs Thread

A process is an instance of a running program with its own dedicated memory space

A thread is the smallest unit of processing that can be scheduled by an operating system

Threads operate within a process and share its memory space.

Text manipulation

sed -i 's/OLD_TERM/NEW_TERM/g' file.txt