Shellscripting.md

Shellscripting

A collection of the most used shellscripting tricks.

wc -l file.txt counts the number of lines

Delete old files of a folder

Useful when you want to free old data/logs.

Using alias

I use them to quick connect my ssh servers

alias ssh-prod-dc-app1-server1='ssh user@server1'

Shellscript functions

#!/bin/bash
function_name(){
  param1=$1
  param2=$2
  echo $param1
  echo "this is param 2 $param1"
 }
 
 function_name hello world

Find a file in a folder

find ./ -name filename.txt

Run a command for ever very xx seconds

while true; do date; ps aux|grep ssh;sleep 5;done

Forensic Analysis

uptime - Get server uptime. last - Get who was login into the system.

The history log

You can cat the history log from cat ~/.bash_history

Tools

Analize trafic with TcpDump

To dump everything from iface enp0s3

tcpdump -w dump.pcap -i enp0s3

To dump all comes and goes to an specific ip. (Look that with the -i any we can listen all the interfaces)

tcpdump -w dump.pcap -i any host 1.1.1.1

The following comand save dump files of 10 mb, and rotate it in 5 files, so we will have as much 10mb x 5 files of dump.

tcpdump -W 5 -C 10 -w capfile

OpenSSL

Get .pem certificate info

 openssl.exe x509 -in cert.pem -noout -text

Get public key from .pem certificate

 openssl x509 -pubkey -noout -in Cert.pem

Generate selfsigned cert and private key

openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -sha256 -days 365

Try to connect to a host using MTLS

openssl s_client -connect <host>:<port> -cert <client_cert>.pem -key <client_cert_key>.key -showcerts -cert_chain <client_cert_chain>.pem