Bash - 08.1 - bc and date

The date command in Unix-like operating systems is used to display or set the system date and time.

date [OPTION]... [+FORMAT]

date [-u|--utc|--universal] [MMDDhhmm[[CC]YY][]]

OPTION : Various options to customize how the date is displayed. FORMAT : The format in which you want to display the date. (it has to follow +)

date Format Specifiers

• $ date +%a : Abbreviated weekday name (e.g., Sun, Mon)
• $ date +%A : Full weekday name (e.g., Sunday, Monday)
• $ date +%b : Abbreviated month name (e.g., Jan, Feb)
• $ date +%B : Full month name (e.g., January, February)
• $ date +%m : Month as a number (01 to 12)
• $ date +%d: Day of the month (01 to 31)
• $ date +%H: Hour in 24-hour format (00 to 23)
• $ date +%I: Hour in 12-hour format (01 to 12)
• $ date +%r: 12-hour clock time (hh:mm:ss AM/PM)
• $ date +%R: 24-hour format time without seconds (hh:mm)
• $ date +%T: Time in 24-hour format with seconds (hh:mm:ss).
• $ date +%Y : Year with century (e.g., 2024)
• $ date +%j : Day of the year (001 to 366)
• $ date +%s : Seconds since the Unix epoch (January 1, 1970)

Customizing Date Output

Combine multiple format specifiers to display a custom formatted date.

Combining two Formats

$ date +%a:%A

Sun:Sunday

$ date "+%a, %b %d, %Y - %r"

Sun, Dec 31, 2024 - 02:00:00 PM

Making a Date String: The -d or --date option allows to specify a date string, which date will interpret and display.

$ date --date='@2147483647'
Wed Feb 16 06:46:07 UTC 2038

This will display the date corresponding to the Unix timestamp 2147483647.

UTC/Universal Time: The -u, --utc, or --universal option allows you to display the date and time in UTC.

$ date -u
Tue Dec 31 23:59:59 UTC 2024

bc (Basic Calculator)

bc is an arbitrary precision calculator language. It’s used for performing arithmetic operations, and it supports a variety of functions and operations, including basic math, variable assignments, and more.

Interactive Mode or Command-Line Usage
Simply typing bc will start an interactive mode, where we can input mathematical expressions and get results.
We can also use bc for calculations directly from the command line.

$ echo "3 + 4" | bc
7

Some Key Operators in bc:

• Arithmetic: +, -, *, /, ^ (exponentiation)
• Comparison: ==, !=, <, >, <=, >=
• Logical: &&, ||
• Bitwise: &, |, ^, ~, <<, >>

Options

-l (Standard Math Library) : loads the standard math library, which enables functions like sine, cosine, sqrt, exp, etc.

$ echo "scale=2; sqrt(2)" | bc -l
1.41

scale : The scale variable determines the number of decimal places in the output.

$ echo "scale=3; 22/7" | bc
3.142

-q (Quiet Mode): Suppresses the welcome message that is normally displayed.

$ bc -q

-s (Single Precision Mode) : Forces bc to use single precision for floating-point calculations (often leading to fewer decimal places).

$ echo "scale=3; 1/3" | bc -s
0.333

Math Functions Available with -l :

• Trigonometric: s(), c(), a(), etc. (for sine, cosine, arctangent, etc.)
• Logarithmic: l(), e(), etc.
• Exponential: exp(), etc.

Example Commands:

• Addition and Division:

$ echo "5 + 3" | bc
8
$ echo "10 / 4" | bc
2

• Using scale for Decimal Precision:

$ echo "scale=4; 7/3" | bc
2.3333

• Using Functions with -l:

$ echo "scale=5; sqrt(100)" | bc -l
10.00000

• Assigning Variables:

$ echo "a=5; b=3; a*b" | bc
15