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CheatSheet

date

Convert unixtime to human

date -r 1692541552253

watch

Periodically Execute a Command

The watch command in Linux allows you to execute a command periodically, displaying the output in the terminal. It's particularly useful for monitoring system status and changes in real-time.

  • Basic Usage

To run a command every 2 seconds:

bash watch -n 2 "some action"

  • -n 2: Specifies the interval (in seconds) between command executions.

SSH and SCP Utilities

Copying SSH Key to a Remote Host

To copy your SSH public key to a remote server for passwordless authentication:

ssh-copy-id -i ~/.ssh/id_rsa.pub YOUR_USER_NAME@IP_ADDRESS_OF_THE_SERVER
  • Batch Copying SSH Keys to Multiple Servers

If you have a list of servers in a file and want to copy your SSH key to all of them:

bash #!/bin/bash for ip in `cat /home/list_of_servers`; do ssh-copy-id -i ~/.ssh/id_rsa.pub $ip done

This script reads a list of IP addresses from /home/list_of_servers and copies the SSH key to each server.

Accelerating File Transfers with SCP

To speed up file transfers with SCP using the Blowfish encryption algorithm:

scp -c blowfish root@host:/home/itsecforu/* /home/itsecforu
  • Copying Files to/from a Remote Host

Basic usage of scp to copy files between hosts:

bash scp [other options] [source username@IP]:/[directory and file name] [destination username@IP]:/[destination directory]

Example:

bash scp user@192.168.1.1:/home/user/file.txt /local/directory/

  • Running Commands on Multiple Hosts

To run a set of commands on multiple hosts listed in a file:

bash for HOST in $(cat hosts.txt); do ssh -f $HOST "uptime; df -h"; done

This command executes uptime and df -h on each host listed in hosts.txt.

Managing Sudo Privileges

Granting a User Sudo Access Without a Password

To allow a user to execute sudo commands without being prompted for a password, add them to the /etc/sudoers file:

  1. Manually Edit /etc/sudoers:

Add the following line:

bash username ALL=(ALL) NOPASSWD:ALL

  1. Use a Command to Automate This:

Alternatively, you can automate this process with:

bash echo "username ALL=(ALL) NOPASSWD:ALL" | sudo tee /etc/sudoers.d/username

This command adds the necessary line to a new file in /etc/sudoers.d/.

OpenSSL

Viewing SSL Certificate Data

To view detailed information about an SSL certificate:

openssl x509 -in CERTPATH -text -noout
  • CERTPATH: The path to the SSL certificate file.

To initiate a TLS/SSL connection to a server and diagnose SSL issues use: 

openssl s_client -connect myservername.domen.my:443 -servername myservername.domen.my
  • openssl s_client: This tool connects to a remote server using SSL/TLS and helps in diagnosing SSL connections.
  • -connect myservername.domen.my:443: Specifies the server (myservername.domen.my) and port (443) to connect to. Port 443 is the default for HTTPS.
  • -servername myservername.domen.my: Sends the Server Name Indication (SNI), which allows the server to present the correct SSL certificate if it's hosting multiple domains.

This command is useful for checking SSL certificate details, supported ciphers, and diagnosing connection issues.

Performance Testing

Apache Benchmarking

To test the performance of a website using the Apache ab (Apache Benchmark) tool:

  • Usage Example:

bash ab -n 1000 -c 10 http://example.com/

  • -n 1000: Number of requests to perform.

  • -c 10: Number of multiple requests to make at a time.

  • Resource: Apache Benchmarking Guide