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'

Create CRC32 from string

• just to be complete... :)

crc32 <(echo "LongString")
1624d322

Generate HEX for string

• Note this is not able to do the reverse
• No found method to convert the output back to LongString so this is buggy

echo -n "LongString"  | cksum | cut -d " "  -f1 | xargs echo printf '%0X\\n' | sh
5751BDB2

Convert first letter to Upper case

• PROJECT=testproject

PROJECT="$(tr '[:lower:]' '[:upper:]' <<< ${PROJECT:0:1})${PROJECT:1}" 
echo ${PROJECT}
Testproject

Using tr, this will make the first letter always upper case and ignore the
remainder of the string

Make leading zeros for output

• make 2 say 02 instead
• change the % values for things such as decimal precision or more leading numbers

$(printf "%02d" 2)
02

Stupid loop I keep forgetting

Used for renaming television episodes when what you think is a season does not match what tbdb thinks is a season

 for x in $(seq 1 12); do z=$(( $x + 12 )); y=$(printf "%02d" $x) ; s2='s02e' ; s1='s01e'; if=$(ls ./*$s2$y*) ; fn=$(echo $if | sed "s/$s2$y/$s1$z/") ; mv $if $fn ; done

Function return code examples

Stack Overflow source

#!/bin/bash -e
function func1() {
   return ${1:-0}
}

function func2() {
   local RVAL=""
   RVAL=$(func1 ${1:-0} && echo 0 || echo $?)
   if [[ $RVAL != "0" ]]; then
      echo "Some logic on exit code $RVAL"
      exit $RVAL
   fi
   echo "Some logic on exit code 0"
}

func2 ${1:-0}
Some basic tests

$ ./test.sh
Some logic on exit code 0
$ echo $?
0
$ ./test.sh 128
Some logic on exit code 128
$ echo $?
128

Definition for parameter expansion

In Bash, the syntax return ${1:-0} is a way to return a value from a function, with a default value if the first argument ($1) is not provided. Here's a breakdown of the components:

    ${1:-0}:
        ${1} refers to the first positional parameter passed to the script or function.
        :- is a parameter expansion operator that provides a default value.
        0 is the default value.

    Therefore, ${1:-0} means "use the value of the first argument if it's provided; otherwise, use 0."

    return:
        This command is used within a function to return an exit status. The exit status is a numeric value (0-255) that indicates success (0) or failure (non-zero).

Putting it all together, return ${1:-0} means:

    If the function is called with an argument, return the value of that argument.
    If no argument is provided, return 0.

Count number of occurrences of a specific character

A nice simple way to count in this case the number of periods in a variable. I like this more than some of the other solutions I have seen.

VALUE="this.is.a.test.string"
DOTS="${VALUE//[^.]/}"
echo "${#DOTS}"
4