Bash-common-checks

Common checks and validations that are a PITA to remember

Is Integer?

• Check if a var is an integer
• support negative as well as positive integer values using extended regex

VAR=123
if [[ ${VAR} =~ ^-?[0-9]+$ ]]; then
  echo "Is integer"
else
  echo "Is not integer"
fi

TCP port check

• leverage /dev
• use timeout for sanity
• Use for any kind of TCP knock
• do not use FQDN, go by IP

IP=192.168.15.100
timeout 1 bash -c "</dev/tcp/${IP}/22"
if [[ $? -ne 0 ]]; then
  echo "ssh port closed at $IP"
else
  echo "ssh port open at $IP"
fi

Find what using inodes

• use sudo if you are going against /

find / -xdev -printf '%h\n' | sort | uniq -c | sort -k 1 -n

Make a banner

skull(){
cat << EIEIO
*************************************************************
*                                                           *
*   .=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-.      *
*   |                     ______                     |      *
*   |                  .-"      "-.                  |      *
*   |                 /            \                 |      *
*   |     _          |              |          _     |      *
*   |    ( \         |,  .-.  .-.  ,|         / )    |      *
*   |     > "=._     | )(__/  \__)( |     _.=" <     |      *
*   |    (_/"=._"=._ |/     /\     \| _.="_.="\_)    |      *
*   |           "=._"(_     ^^     _)"_.="           |      *
*   |               "=\__|IIIIII|__/="               |      *
*   |              _.="| \IIIIII/ |"=._              |      *
*   |    _     _.="_.="\          /"=._"=._     _    |      *
*   |   ( \_.="_.="     '--------'     "=._"=._/ )   |      *
*   |    > _.="                            "=._ <    |      *
*   |   (_/                                    \_)   |      *
*   |                                                |      *
*   '-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-='      *
*                                                           *
*                   Abandon All Hope                        *
*************************************************************/
EIEIO
}
# abandon all hope ye who enter here
skull

Create a CRC16 against string

• use caution in creating unique values < 65k, as there can be slop and overlap
• Good quick and dirty creation that is reproducible

STRING=foo
crc16() {
    while read -r -d "" -n 1 ; do astring+=( "$REPLY" ) ; done <<< "$1"
    cnt=${#1}
    c=65535
    for ((x=0;x<$cnt;x++)); do
        char=$(printf '%d' \'"${1:$x:1}")
        e=$(((c ^ char) & 0x00FF))
        s=$(((e << 4) & 0x00FF))
        f=$(((e ^ s) & 0x00FF))
        r1=$(((c >> 8) ^ (f << 8) ^ (f << 3) ^ (f >> 4)))
        c=$r1
    done
    c=$((c ^ 0xffff))
    echo "$c"
}
crc16 ${STRING}
returns value 26676

Generate random string

• Great for creating a password
• no reserved chars get generated

strings /dev/urandom | grep -o '[[:alnum:]]' | head -n 20 | tr -d '\n'