Shell Programming and Scripting

Hi I have a simple question.
Is there an easy way to read a date of birth from a file and calculate how old that person is based on today's date? And would I need the make sure the birthdates are enterered in a particular format?
Thanks

You might have to take a look at datecalc script from the forum.

A sample to calculate the age with a given date, adapt it to read the date from a file or if you have a different date format.

Code:

#!/bin/sh

echo -n "Enter the birthdate (mm-dd-yyyy): "
read bdate

bmonth=${bdate:0:2}
bday=${bdate:3:2}
byear=${bdate:6:4}

cdate=`date +%m-%d-%Y`

cmonth=${cdate:0:2}
cday=${cdate:3:2}
cyear=${cdate:6:4}

if [[ "$cmonth" -lt "$bmonth" ]] || [[ "$cmonth" -eq "$bmonth" && "$cday" -lt "$bday" ]]
then
  let age=cyear-byear-1
else
  let age=cyear-byear
fi

echo "Age = "$age

Regards

The -n option to echo is not standard.

That is not sh syntax; it is limited to bash and ksh.

Simpler (and POSIX compliant) is:

Code:

eval "$( date "+cyear=%Y cmonth=%m cday=%d" )"

[[ ... ]] is not sh syntax. Use [ .. ].

let is not standard. Use POSIX arithmetic:

Code:

age=$(( $cyear - $byear - 1 ))

This will run in any POSIX shell:

Code:

## Accept input on command line, or read interactively if none
case $# in
  3) bday=$1; bmonth=$2; byear=$3 ;;
  *) printf "Enter the birthdate (day month year): "
     read bday bmonth byear
     ;;
esac

## Ensure 2 digits for $bmonth and $bday
case $bmonth in ?) bmonth=0$bmonth;; esac
case $bday in ?) bday=0$bday;; esac

eval "$( date "+cyear=%Y cmonth=%m cday=%d" )"

[ $bmonth$bday -lt $cmonth$cday ] &&
 age=$(( $cyear - $byear )) ||
  age=$(( $cyear - $byear - 1 ))

echo "Age = $age"

]
Afraid it is supported by IEEE 1003.1:2004. Under shell command language reserved words section:

Reserved words are words that have special meaning to the shell
.....
The following words may be recognized as reserved words on some implementations (when none of the characters are quoted), causing unspecified results:

[[ ]] function select
......

No, It is neither supported nor required. In fact, it's behaviour is not even specified.

A POSIX shell is not required to support it, and if it does, it could do anything at all, including erase the contents of you hard drive.

I agree with fpmurphy. [[ ]] is supported in Posix and required though I personally have only found it useful in the context of "while [[ condition ]]".

[[ and ]] are keywords with strict syntax rules. A keyword starts a command and should not be used in the wrong context or be quoted (or you can get unpredictable results).

[ and ] are commands. (On early unix they were executables).

More importantly.
What goes between [[ and ]] is a "conditional expression" with one set of syntax.
What goes between [ and ] is a simple "test" with a simpler syntax. See "man test" where it you'll see that [ ] is an equivalent to "test".

The two syntaxes are covered in the man page for a Posix shell. As it happens some of the basic syntax is common between the two types of condition giving the (wrong) impression that the syntax is always interchangable.

The big syntax difference is with boolean expressions.


BTW. fpmurphy script gives syntax errors with both Posix sh and ksh because the substring variables are from bash. I'm not convinced that the line starting "if [[" will give the right result either (not tested).