Commands

One liners

Monitor folder size

Monitors a folder and checks if it increases or decreased since the last minute.

Linux

while true; do size=$(du -sh /root/ | awk '{print $1}'); echo "$size ($(echo $size - $prev | bc) change)"; prev=$size; sleep 60; done

TMUX

Search current pane

press Ctrl-b [ = to enter copy mode
press Ctrl-s = to start search

vim

ls

Finds what directory someone ran a command in

Linux

ls -la /proc/$PID | grep cwd

ps

Process tree

A process tree shows how processes on the system are linked to each other; processes whose parents have been killed are adopted by the init (or systemd).

Linux

ps -aef --forest

journalctl

Viewing Logs:

• journalctl: Displays logs from the current boot session.

• journalctl --boot=-1: Displays logs from the previous boot session.

• journalctl --since "2024-05-01 00:00:00" --until "2024-05-02 23:59:59": Displays logs within a specific time range.

Filtering Logs:

• journalctl _SYSTEMD_UNIT=unit_name.service: Displays logs for a specific systemd unit (service).

• journalctl _PID=1234: Displays logs for a specific process ID (PID).

• journalctl -u unit_name.service: Displays logs for a specific systemd unit (service).

• journalctl -p err: Displays logs with a priority level of "err" (error) or higher.

Output Formatting:

• journalctl -o json: Outputs logs in JSON format.

• journalctl -o short: Outputs logs in a compact, human-readable format.

• journalctl -o verbose: Outputs logs with additional metadata.

Real-time Logging:

• journalctl -f: Displays logs in real-time, similar to tail -f.

Displaying Additional Information:

• journalctl --disk-usage: Displays disk usage statistics of the journal.

• journalctl --list-boots: Lists all available boot sessions and their respective IDs.

• journalctl --list-unit-files: Lists available systemd unit files.

Other Options:

• journalctl --vacuum-size=1G: Removes old journal files until the total disk space used by the journal is reduced to 1GB.

• journalctl --rotate: Forces rotation of the journal files.

Useful:

• journalctl -u sshd --since "2024-05-08 16:00:00" --until "2024-05-08 18:30:00": Check who logged in between two times

• journalctl -u Splunkd --since "2024-05-08 17:00:00" --until "2024-05-08 18:30:00": Check Splunk logs between two times

Ansible

Run Adhoc Command on multiple servers

inventory needs to be a list of hosts.

Linux

ansible -i inventory all -u $USERNAME --become -k -K -m shell -a 'sudo grep somestring /path/to/file/$FILE || echo "Not found"'

tar

Archive Folder

Linux

tar -zcvf $tarName.tar.gz $folderName

du

Show biggest to smallest folders

Below shows top 6 folders

Linux

 du -ah /path/to/folder | sort -rh | head -6

ln

The difference between a hard-link and a symbolic-link (soft-link)

Hard Link

Hard link is a mirror copy of the original file. Deleting the original file will not impact anything, because the hard link file, will act as a mirror copy of the original file.

Symbolic Link

A symbolic link, also known as symlink or soft link, is a special type of file that points to source file or directory in Linux. It is like a shortcut in Windows which contains the path of the original file and not the contents. In general, Symbolic links are used to link libraries and log files & folders on mounted NFS (Network File System) shares.

Creates links to files and folders

Create a symbolic link to a file (or folder)

Linux

ln -s path/to/file path/to/symlink

Overwrite an existing symbolic to point to a different file

Linux

ln -sf path/to/new_file path/to/symlink

Create a hard link to a file

Linux

ln path/to/file path/to/hardlink

grep

Grep OR Statement

Linux

grep "Eps=| HT=" /opt/splunk/superconnector/current/logs/agent.out.wrapper.log 
grep "Eps=|HT=|C=" /opt/arcsight/smartconnector/2pcc_wuc_2k12_srv_64bit_01/current/logs/agent.out.wrapper.log

GREP Number Range

Linux

grep -E 'M1CacheDropped=[0-9]{3}' agent.log*

GREP with Square Brackets in string

Linux

grep -i "No Smb Transports found for the host \[someText\]"

Grep Recursive and Ignore Folders

Linux

grep -R --exclude-dir=node_modules 'some pattern' /path/to/search

dig

Check if DNS is resolving host

Linux

dig @$DNS_Server_You_Want_To_Test $Server_Your_Looking_Up

scp

Receive folder from external machine to local machine

Linux

scp $USERNAME@$LOCAL_IP:$/$FOLDER/ $FILE

Send local folder on local machine to external machine

Linux

scp $LOCAL_FOLDER $USERNAME@$EXTERNAL_IP:$FOLDER/

git

Squash commits before making merge request

Linux

git rebase -I HEAD~4 = 4 commits
git rebase -I sdds8e3esd 

telnet

Testing if SMTP works

telnet $SMTP_SERVER 25

ehlo mydomain.com 
mail from:<[e-mail]> 
rcpt to:<[e-mail]> 
data 
This is a test, please do not respond 
. 
quit