====== Some useful bash snippets: ======

:!: If you declare variables, use lower case names!

===== Using math =====
<code>
expr 30 + 10           # 30 + 10
expr 30 - 10           # 30 - 10
expr 30 / 10           # 30 / 10
expr 30 \* 4           # 30 x 4, needs escape!
</code>


===== Checking if file exists: =====
<code>
if [[ -f ~/.ssh/id_rsa ]]
then
        echo "id_rsa exists"
else
        echo "id_rsa does not exist"
fi
</code>

===== Checking if directory exists: =====
<code>
if [[ -d ~/.ssh/ ]]
then
        echo "directory .ssh exists"
else
        echo "directory .ssh does not exist"
fi
</code>

===== Checking if variable equals 200 =====
<code>
num = 200
if [[ $num -eq 200 ]]
then
        echo "$num equals 200"
else
        echo "$num does not equal 200"
fi
</code>

===== Ask for variable input: =====
<code>
# Read Password
echo -n Password:
read -s password
echo
# Run Command
echo $password
</code>

===== Ecnrypting / Decrypting file: =====
A useful tool for encrypting/decrypting files is //ccrypt//. Using this you can make use of //git// containing things like SSH keys:

<code>
apt-get install ccrypt -y                                  #install ccrypt
ccencrypt ~/.ssh/id_rsa*                                   #encrypt private key
ccdecrypt id_rsa.cpt -P "Enter password to decrypt file: " #decrypt private key
mv id_rsa ~/.ssh/ && chown -R user: ~/.ssh                 #move private key and set ownership
chmod 600 ~/.ssh/id_rsa                                    #set private key permissions
</code>

===== Install a list of programs: =====
<code>
progs=(
	mc
	iotop
	htop
	tmux
	wget
	ccrypt
	openssh-server
)
for program in "${progs[@]}"; do
	if ! command -v "$program" >/dev/null 2>&1; then
		apt-get install "$program" -y
	fi
done
</code>

===== A simple menu (ssh): =====
<code>

#!/bin/bash
clear

echo """

##################################################################################################
#                                                                                                #
#                               Welcome to sshmenu                                               #
#                                                                                                #
#                        Which server would you like to connect to?                              #
#                                                                                                #
#                           1) wekan (kanban)                                                    #
#                           2) wiki                                                              #
#                           3) nextcloud                                                         #
#                           4) dockervm1                                                         #
#                           5) dockertestvm                                                      #
#                                                                                                #
##################################################################################################

input number: 
"""
read -s server
echo $server
sleep 1
if [ $server = "1" ]; then
    clear
    echo "Setting up connection to wekan server..."
    sleep 2
    ssh 192.168.1.118
elif [ $server = "2" ]; then
    clear
    echo "Setting up connection to wiki server..."
    sleep 2
    ssh 192.168.1.117
elif [ $server = "3" ]; then
    clear
    echo "Setting up connection to nextcloud server..."
    sleep 2
    ssh 192.168.1.119
elif [ $server = "4" ]; then
    clear
    echo "Setting up connection to dockervm1 server..."
    sleep 2
    ssh 192.168.1.116
elif [ $server = "5" ]; then
    clear
    echo "Setting up connection to dockertestvm server..."
    sleep 2
    ssh 192.168.1.120
else
    echo "Incorrect choice, programm will now terminate..."
    sleep 3
    clear
    exit
fi
clear
exit</code>

===== Conditional while loop: =====

<code>
if [ ! -f /usr/bin/zsh ]
then
	while true; do
		read -p "Would you like to install 'zsh'? (y/n) " yn
		case $yn in
			[Yy]* ) echo "OK, installing zsh...";
			while read program; do
				sudo apt install -y "$program"
			done <zsh
			break;;
    	    [Nn]* ) echo "ZSH will not be installed"; break;;
        	* ) echo "Please answer yes or no.";;
    	esac
	done
fi</code>


===== Replace all files in a directory with empty files with the same name =====
<code>
#!/bin/bash

# Get the directory path
echo "Enter the path to the directory: "
read dir_path

# Check if the directory exists
if [ ! -d "$dir_path" ]; then
  echo "Error: Directory does not exist."
  exit 1
fi

# Replace all files in the directory with empty files
for file in "$dir_path"/*; do
  if [ -f "$file" ]; then
    > "$file"
  fi
done

echo "All files in the directory have been replaced with empty files."
</code>

===== Find SSH logins =====

:!: This script assumes that you have the sshpass package installed on your system. You can install it with the following command: sudo apt-get install sshpass (for Ubuntu) or brew install http://macappstore.org/sshpass/ (for macOS).

<code>
#!/bin/bash

servers_file="servers"
users_file="users"
passwords_file="passwords"
logins_file="logins.txt"

# Check if the input files exist
if [ ! -f "$servers_file" ] || [ ! -f "$users_file" ] || [ ! -f "$passwords_file" ]; then
  echo "Error: One or more of the input files do not exist."
  exit 1
fi

# Read the servers, users, and passwords into arrays
servers=($(cat "$servers_file"))
users=($(cat "$users_file"))
passwords=($(cat "$passwords_file"))

# Loop through all combinations of servers, users, and passwords
for server in "${servers[@]}"; do
  for user in "${users[@]}"; do
    for password in "${passwords[@]}"; do
      # Try to ssh into the server with the current combination of user and password
      ssh_output=$(sshpass -p "$password" ssh "$user"@"$server" -o StrictHostKeyChecking=no -o ConnectTimeout=3 2>&1)

      # Check if the ssh login was successful
      if [ "$?" -eq 0 ]; then
        echo "Successful ssh login to $server using account $user and password $password"
        echo "$server $user $password" >> "$logins_file"
      else
        echo "Failed ssh login to $server using account $user and password $password"
      fi
    done
  done
done
</code>

===== Remove duplicate and empty lines from file =====
<code>
#!/bin/bash

# Remove duplicate lines from url.txt and save to temp file
sort -u url.txt > temp.txt

# Remove empty lines from temp file and save back to url.txt
grep -v "^$" temp.txt > url.txt

# Remove temp file
rm temp.txt
</code>

===== Download files based on URLs in a file =====
<code>
#!/bin/bash

# read list of URLs from file
while read url; do
  # download file using wget
  wget "$url"
done < url.txt
</code>

Do the same but check if file exists. If it does compare sizes and ask what to do.

<code>#!/bin/bash

# Read URLs from the url.txt file and download the files
while read url; do
    filename=$(basename "$url")
    if [ -e "$filename" ]; then
        local_size=$(wc -c <"$filename")
        remote_size=$(curl -sI "$url" | awk '/Content-Length/ {sub("\r",""); print $2}')
        if [ "$local_size" = "$remote_size" ]; then
            echo "$filename already exists and has the same file size. Skipping download."
            continue
        else
            read -p "A file with the name $filename already exists. Do you want to save the new file with a different name? (y/n) " response
            if [ "$response" = "y" ]; then
                read -p "Enter the new file name: " new_filename
                curl -L -o "$new_filename" "$url"
                echo "File saved as $new_filename."
            else
                echo "Skipping download of $filename."
            fi
        fi
    else
        curl -L -o "$filename" "$url"
        echo "Downloaded $filename."
    fi
done < url.txt
</code>