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I am trying to validate the date entered by the user(mmddyyyy),used the below but its not working, please correct me where i am wrong.

 date  "+%m%d%Y" -d "02012019" > /dev/null  2>&1
        echo $?
10
  • How does the user enter the date? by a simple read command? Feb 15 at 17:19
  • via command line he will pass ,ex: sh xx.sh -f 02122029
    – Renga
    Feb 15 at 17:23
  • 1
    I don't think it's possible to use date in that way (at least, not the GNU implementation) - it parses the string given via -d using its own built-in rules, then if successful outputs the resulting date in the specified format. AFAIK it does not attempt to use the output format to parse strings given as an inputs via -d (as, for example, strptime would). Feb 15 at 17:23
  • 1
    @Renga in your comment you use sh. The script must be interpreted by /bin/sh, or could be /bin/bash? Feb 15 at 18:05
  • 1
    Also the point that the user can enter the input date in several alternative valid formats, like @1613412105 or 'now + 23 days' or 'last tuesday', which all return a zero status. Feb 15 at 18:09
2

If I understand you correctly, this is a possible approach:

#!/bin/bash

d="$1"

[[ ! "$d" =~ ^[0-9]{8}$ ]] && \
  echo "Illegal number of characters, or not numbers." && \
  exit

d="${d:4}${d:2:2}${d:0:2}"

date  "+%m%d%Y" -d"$d" > /dev/null 2>&1

[ $? -ne 0 ] && echo "Invalid date." || echo "Valid date."

$ ./xx.sh 0201201x
Illegal number of characters, or not numbers.
$ ./xx.sh 0201201900000
Illegal number of characters, or not numbers.
$ ./xx.sh 32022019
Invalid date
$ ./xx.sh 02022019
Valid date.

Now 01022019 would be a valid date, but if the user typed the day first and then the month, you would need another set of validations for your specific needs.

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  • Thanks alot,i just edited the parsing section(d="${d:4}${d:0:2}${d:2:2}") and working fine .
    – Renga
    Feb 15 at 18:59
3

With zsh:

#! /bin/zsh -
date=${1?No date specified}

zmodload zsh/datetime || exit
if
  strftime -rs t %m%d%Y "$date" 2> /dev/null &&
  strftime -s canonical_date_for_t %m%d%Y "$t" 2> /dev/null &&
  [[ $date = $canonical_date_for_t ]]
then
  print -r -- $date is valid
else
  print -ru2 -- ${(q+)date} is invalid
  exit 1
fi

strftime -r calls strptime() which parses the input $date according to %m%d%Y. While strftime without -r converts back to a %m%d%Y formatted date using strftime().

strptime() would accept a 02292019 (and treat it like 03012019), but once converted back with strftime(), that becomes 03012019 so would be rejected. Similarly 12310001 (0001-12-31) would be rejected as 12311 would be expected instead (%Y for year 1 AC is 1, not 0001; replace %Y with %4Y in the second strftime invocation, if you'd rather 0001 be the expected representation of year 1).

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