check if argument is a valid date in bash shell

Question

I am writing a bash shell script in Linux, this program will accept a date (mm-dd-yyyy) as a parameter. I am wondering if there is a simply way to check if the date is valid? is there an operator and I can just use test to check?

Answer 1

The date command will parse a date given with the -d argument. If the date is invalid, an error message is printed to STDERR and date exits with an error status. If the date is valid, it prints the date on STDOUT and exits with a success status.

Because of this, date -d "$date" can be used directly in a bash if statement.

The first wrinkle is that to prevent printing a message for valid dates, you need to redirect STDOUT to /dev/null using >/dev/null.

The second wrinkle is that date accepts an empty string as a valid date without complaint. In most cases, that should mean that your user didn't enter a date when they should have. You will want to test for an empty date separately using the test [ "z$date" != "z" ]

date also accepts a variety of formats. If you are using actual bash (as opposed to dash or some of ther sh variety, you could use regular expressions against your preferred format in place of a simple check for an empty string. For example to check my preferred ISO format, I would use: [[ "$date" =~ ^[0-9]{4}-[0-9]{2}-[0-9]{2}$ ]]

date=2001-01-01
if [ "z$date" != "z" ] && date -d "$date" >/dev/null
then
  echo "VALID DATE"
fi

If you try this with an invalid date (such as 2001-01-53), it doesn't get into the if and it prints out:

date: invalid date ‘2001-01-53’

Alternately, you could check if the date is invalid and exit:

date=2001-01-01
if [ "z$date" == "z" ]
then
  echo "No date specified"
  exit 1
fi
if ! [[ "$date" =~ ^[0-9]{4}-[0-9]{2}-[0-9]{2}$ ]]
then
  echo "Expected date in YYYY-MM-DD format"
  exit 1
fi
if ! date -d "$date" >/dev/null
then
  exit 1
fi
echo "VALID DATE"

Answer 2

You can check with date -d "datestring"

So date -d "12/31/2012" is valid, but using hyphens, e.g. date -d "12-31-2012", is not valid for date.

You can also use words: date -d 'yesterday' or date -d '1 week ago' are both valid.

Answer 3

For validation of YYYY-MM-DD (ISO 8601) dates on OSX in the BASH shell, the following approach validates both the format and the date.

isYYYYMMDDdate() {
  [[ "$1" =~ ^[0-9]{4}-[0-9]{2}-[0-9]{2}$ ]] && [[ "$1" == $(date -r $(date -j -f "%Y-%m-%d" "$1" "+%s") '+%Y-%m-%d') ]] &> /dev/null; echo "$?"
}

1. It first uses a regular expression match to check the format.
2. Then, it converts the date to epoch time and then back to a date.
3. If the original and twice-converted dates match, then it is valid.

Test a valid date: 2005-11-30

$ isYYYYMMDDdate 2005-11-30
0

Test an invalid date: 2005-11-31

$ isYYYYMMDDdate 2005-11-31
1

Test a valid date formatted incorrectly: 1979-20-04

$ isYYYYMMDDdate 1979-20-04
1

Answer 4

You can extract the day, month, and year values from the input date value MM-DD-YYYY and validate it as the unambiguous (ISO) format YYYY-MM-DD instead (you can validate a DD-MM-YYY formatted date as "correct" using date, e.g. 25-12-2010, but it is not a valid MM-DD-YYY date, hence the need to change the date format first)

---

A valid date in the correct format is OK

30th November 2005 is valid:

$ DATE=11-30-2005; d=${DATE:3:2}; m=${DATE:0:2}; Y=${DATE:6:4}; echo "year=$Y, month=$m, day=$d"; if date -d "$Y-$m-$d" &> /dev/null; then echo VALID; else echo INVALID; fi
year=2005, month=11, day=30  
VALID

$ DATE=11-30-2005; if date -d "${DATE:6:4}-${DATE:0:2}-${DATE:3:2}" &> /dev/null; then echo VALID; else echo INVALID; fi
VALID

---

An invalid date in the correct format is NOT OK

31st November 2005 does not validate:

$ DATE=11-31-2005; d=${DATE:3:2}; m=${DATE:0:2}; Y=${DATE:6:4}; echo "year=$Y, month=$m, day=$d"; if date -d "$Y-$m-$d" &> /dev/null; then echo VALID; else echo INVALID; fi
year=2005, month=11, day=31  
INVALID

$ DATE=11-31-2005; if date -d "${DATE:6:4}-${DATE:0:2}-${DATE:3:2}" &> /dev/null; then echo VALID; else echo INVALID; fi
INVALID

---

A valid date in the incorrect format is NOT OK

20th April 1979 in DD-MM-YYYY format does not validate as a MM-DD-YYYY date:

$ DATE=20-04-1979; d=${DATE:3:2}; m=${DATE:0:2}; Y=${DATE:6:4}; echo "year=$Y, month=$m, day=$d"; if date -d "$Y-$m-$d" &> /dev/null; then echo VALID; else echo INVALID; fi
year=1979, month=20, day=04  
INVALID

$ DATE=20-04-1979; if date -d "${DATE:6:4}-${DATE:0:2}-${DATE:3:2}" &> /dev/null; then echo VALID; else echo INVALID; fi
INVALID

---

Alternate simpler method: use BASH variable string replace hyphens to slashes

$ DATE="04-30-2005"; [[ $(date -d "${DATE//-/\/}" 2> /dev/null) ]] && echo VALID || echo INVALID
VALID

$ DATE="04-31-2005"; [[ $(date -d "${DATE//-/\/}" 2> /dev/null) ]] && echo VALID || echo INVALID
INVALID

Answer 5

For script use, I kept it as simple as I could. Testing the date value with the date function then checking the exit code of the process.

date -d "02/01/2000" 2>: 1>:; echo $?

This will redirect the standard in and standard error to null : and using echo to return the exit code with $? allows me to check for 0=good date and 1=bad date.

Answer 6

The following worked well for me. Many thanks to my co-worker, Tyler Chamberlain, for the OSX solution.

# Validate a given date/time in Bash on either Linux or Mac (OSX).

# Expected date/time format (in quotes from the command line):  YYYY-MM-DD HH:MM:SS
# Example(s):  ./this_script "2012-02-29 13:00:00"   # IS valid
#              ./this_script "2013-02-29 13:00:00"   # Is NOT valid

START_DATETIME=$1

function report_error_and_exit
{
   local MSG=$1
   echo "$MSG" >&2
   exit 1
}

# We can use OSTYPE to determine what OS we're running on.
# From http://stackoverflow.com/questions/394230/detect-the-os-from-a-bash-script

# Determine whether the given START_DATETIME is valid.
if [[ "$OSTYPE" == "linux-gnu" ]]
then
   # Validate the date on a Linux machine (Redhat or Debian).  On Linux, this is 
   # as easy as adding one minute and checking the return code.  If one minute 
   # cannot be added, then the starting value is not a valid date/time.
   date -d "$START_DATETIME UTC + 1 min" +"%F %T" &> /dev/null
   test $? -eq 0 || report_error_and_exit "'$START_DATETIME' is not a valid date/time value. $OSTYPE"
elif [[ "$OSTYPE" == "darwin"* ]]
then
   # Validate the date on a Mac (OSX).  This is done by adding and subtracting
   # one minute from the given date/time.  If the resulting date/time string is identical 
   # to the given date/time string, then the given date/time is valid.  If not, then the
   # given date/time is invalid.
   TEST_DATETIME=$(date -v+1M -v-1M -jf "%F %T" "$START_DATETIME" +"%F %T" 2> /dev/null)

   if [[ "$TEST_DATETIME" != "$START_DATETIME" ]]
   then
      report_error_and_exit "'$START_DATETIME' is not a valid date/time value. $OSTYPE"
   fi
fi

echo "The date/time is valid."

I tested this script on a Red Hat-based system, a Debian-based system and OSX, and it worked as expected on all three platforms. I did not have time to test on Windows (Cygwin).

Answer 7

You can use the strptime() function available in Python's time or datetime modules or Perl's Time::Piece module.

Answer 8

case statements make it easy to support multiple formats and capturing date-parts, i.e.

 case ${date} in
    [0-3][0-9]-[0-1][0-9]-[0-9][0-9][0-9][0-9] )
       yr=...
       mn=...
       dy=... 
    ;;
    [0-1][0-9]-[0-3][0-9]-[0-9][0-9][0-9][0-9] )
       yr=...
       dy=... 
       mn=...
    ;;
    .... other formats
    ;;
    * )
      echo "ERROR on date format, from value=$date, expected formats ..."
      return 1
    ;;     
 esac

I hope this helps.