Valid shell script input date format

Question

I have used the method posted for the following question, but I still have some issues.

$ ./check_date.sh 20190109
Input date format 20190109        # this should report error!

$ ./check_date.sh 2019-0109
[ERROR]: Invalid input date format 2019-0109

$ ./check_date.sh 2019-01-09
Input date format 2019-01-09

$ cat ./check_date.sh
#!/usr/bin/bash
date '+%Y-%m-%d' -d $1 > /dev/null 2>&1
if [ "$?" -ne 0 ]; then
    echo "[ERROR]: Invalid input date format $1"
    exit 1
else
    echo "Input date format $1"
fi

As you can see, I expect the input 20190109 will cause the script to report ERROR but instead it works without errors.

In summary, I need to validate the input date string so that it strictly follows the format YYYY-MM-DD.

Question> What should I do to get the script work as expected?

Thank you

Do you want to validate the format, or do you want to validate the format and the date? — kvantour
– kvantour, Commented Jan 10, 2019 at 15:29
You should send error messages to stderr: echo "[ERROR]: Invalid input date format $1" >&2 — William Pursell
– William Pursell, Commented Jan 10, 2019 at 15:35
The format option for the date command will not check / enforce the input date string in any way, it's used solely for formatting the output. A format like 20190109 is accepted by the date command as input to the the -d, --date=STRING switch so you won't be able to use this approach to validate your format requirement. — marekful
– marekful, Commented Jan 10, 2019 at 15:44
Also, just as a side note: the format does not need to be quoted if it has no spaces (since it's literal and won't change), however, it's better to quote the argument should a date string with spaces be passed in date +%Y-%m-%d -d "$1" — marekful
– marekful, Commented Jan 10, 2019 at 15:47

kvantour · Accepted Answer · 2019-01-10 15:47:13Z

3

The following test should work:

if [[ "$1" =~ [0-9]{4}(-[0-9]{2}){2} ]] && date -d "$1" >/dev/null 2>&1; then
   success
else
   error
fi

It first checks if the format is correct NNNN-NN-NN, and then used date to check if the date is valid.

Why is your original code not working: You wrote:

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

You "assumed" that the string '+%Y-%m-%d' defined the input format, but it actually defines the output format. Date is capable to take a various collection of input strings and convert them accordingly:

$ date -d "yesterday" '+%F'
$ date -d "2019-01-01 + 10 days" '+%F'
$ date -d "5 fortnight ago" '+%F'
$ date -d "12/13/1099" '+%F'

You cannot force date to assume the input date string to have a given format.

edited Jan 10, 2019 at 15:47

answered Jan 10, 2019 at 15:36

kvantour

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