Day-10: So I did continued with bash scripting today! Going more deep in bash scripts! Check out my Bash Scripting - #1 before reading the below notes!

Variables!

• In bash syntax, variables $1 to represent the first command-line argument, $2 to represent the second command-line argument, and so on. And “$@” is the variable name for all arguments passed in. Use the syntax VARIABLE_NAME=VARIABLE_VALUE

#!/bin/bash

BIRTHDATE=$1

Operating the Variables!

• The user’s input string will be in the format of “2020–05–06”, we can convert the user’s birthdate to a Unix timestamp.

#!/bin/bash

BIRTHDATE=$(date -jf “%Y-%m-%d” $1 +%s)
NOW=$(date -jf “%a %b %d %T %Z %Y” “$(date)” +%s)

DIFF_IN_SECONDS=$(expr $NOW — $BIRTHDATE)
DIFF_IN_DAYS=$(expr $DIFF_IN_SECONDS / 86400)

echo "You are $DIFF_IN_DAYS days old."

• Breaking down, The $() tells Unix to execute the command surrounded by the parenthesis and replace the variable with the output of the command. This is called "command substitution".
• Date: displays time on Unix systems!
• -jf: Convert one date format to another
• '%Y-%m-%d': Tells date that the input date string will be in the format of year-month-day!
• $1: Specifies that the input string is the first argument of the script!

• +%s: Tells date to output the date as the number of seconds that have passed since the “Unix epoch”.

• The second line command: NOW=$(date -jf “%a %b %d %T %Z %Y” “$(date)” +%s) in this converted the current time into a Unix timestamp.

• The third line command:DIFF_IN_SECONDS=$(expr $NOW — $BIRTHDATE) in this calculated the difference between the two dates in seconds. We can calculate the difference between the two timestamps by using "expr".

• The fourth line command: DIFF_IN_DAYS=$(expr $DIFF_IN_SECONDS / 86400) then converted the difference in seconds to a difference in days by dividing the time difference in seconds by 86400. 86400 Seconds = 1 day.

• The fifth line command: echo "You are $DIFF_IN_DAYS days old." used echo then to display the time difference!

Resources:

Bash Scripting Cheatsheet: https://devhints.io/bash
Bash Script examples: https://linuxhint.com/30_bash_script_examples/
Medium Blog: Bash Scripting (You need Premium subs to access this!)

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