Save your time, have these advanced Linux commands in your cheatsheet!

🐚 I describe Shell with words omnipresence or has ubiquity. Quick filtering or command chaining is like magic and gives us more powers, with Pipe ("|") and CLI utilities like sed, awk, grep, etc.

📓 Hence, if you are related to Linux and Server troubleshooting in any way, and that involves your day to day work. Here is some quick reference to command-line know-hows:

Note: This is Part 1, just a quick reference guide that I've had in my cheatsheet for server troubleshooting. The list I have is exhaustive, so I will keep things simple and continue to share insights in a phased-out manner.

---Super-powers with:
ps | SSH | Removing files | Grep | Find | Sudo | SSL
---

# SSL

With curl's insecure option we can filter the output and get only the Server certificate information:

~ $ curl --insecure -v https://www.sachcode.com 2>&1 | \
> awk 'BEGIN { cert=0 } /^\* Server certificate:/ { cert=1 } \
>  /^\*/ { if (cert) print }'

* Server certificate:
*   subject: CN=*.sachcode.com
*   start date: Mar 22 04:34:05 2020 GMT
*   expire date: Jun 20 04:34:05 2020 GMT
*   common name: *.sachcode.com
*   issuer: CN=Let's Encrypt Authority X3,O=Let's Encrypt,C=US
* Connection #0 to host www.sachcode.com left intact

# ps(processes status)

Sort by the highest CPU utilization in ascending order:

~ $ ps -aux --sort -pcpu | less

Sort by the highest Memory utilization in ascending order:

~ $ ps -aux --sort -pmem | less

Print all processes running as Root:

~ $ ps -U root -u root

Show process tree of all PIDs:

~ $ ps auxwf

Show all process info and hierarchy:

~ $ ps -efH

Find top running processes by highest memory and CPU usage:

~ $ ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem | head

or

~ $ ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%cpu | head

# SSH

Generate generic ssh key pair:

~ $ ssh-keygen -q -t rsa -f ~/.ssh/<name> -N '' -C <name>

# Removing files

Remove files over 30 days old:

~ $ find . -mtime +30 | xargs rm -rf

Remove files older than 7 day starting with 'backup':

~ $ find . -type f -name "backup*" -mtime +7 -exec rm {} \;

# Grep

Look through all files in current dir for word “foo”:

~ $ grep -R "foo” .

View last ten lines of output:

~ $ grep -i -C 10 "invalid view source” /var/log/info.log

Display line number of message:

~ $ grep -n “pattern” <file>

# Find

Exclude directories in find:

~ $ find /tmp -not \( -path /tmp/dir -prune \) -type p -o -type b

# Sudo

To check sudo access for a user!

Method 1:

~ $ sudo -l -U sachcode 
User sachcode may run the following commands on host:
    (ALL) ALL

Method 2: Another way to find out if a user has sudo access is by checking if the said user is member of the sudo group.

~ $ groups sachcode
sachcode : sachcode wheel docker

Method 3: list all sudo users of your system

~ $ genet group sudo

Read more about me: https://sachcode.com/
Reference on my website: https://sachcode.com/tech/linux-troubleshooting-cheatsheet/

Comments

[Corentin Bettiol]

Great article :)

For generating ssh key pair I use ed25519 algorithm, like this:

ssh-keygen -t ed25519 -C "[e-mail]"

(documentation)

[Waylon Walker]

Such a great list of examples. Often these are far too basic or complicated. This is a very useful set of non-trivial examples.

[AATHITH RAJENDRAN]

Remove files over 30 days old: from date of creation? or from date of last modified? Asking this because I can't find the file creation time for any of my files.

[Chathumal Jayasinghe]

❤