GNU date does not support YYYY-MMM-DD. However, it does understand DD-MMM-YYYY. So if you really have to handle dates of this format you can do it with something like this, which simply swaps the arguments around to a format that date expects:

ymd='2015-Jul-13'

dmy=$(echo "$ymd" | awk -F- '{ OFS=FS; print $3,$2,$1 }')
if date --date "$dmy" >/dev/null 2>&1
then
    echo OK
fi

mikeserv suggested a better all shell solution. Breaking it down, the IFS=- instructions tells the shell to split a forthcoming command line by hyphen - instead of whitespace. The set $ymd parses the $ymd variable as for a command line, but now splits by hyphen, assigning values to the parameters $1, $2 et seq. The echo "$3-$2-$1" trivially outputs the three captured values in reversed order.

ymd='2015-Jul-13'

dmy=$(IFS=-; set $ymd; echo "$3-$2-$1")
if date --date "$dmy" >/dev/null 2>&1
then
    echo OK
fi

GNU date does not support YYYY-MMM-DD. However, it does understand DD-MMM-YYYY. So if you really have to handle dates of this format you can do it with something like this, which simply swaps the arguments around to a format that date expects:

ymd='2015-Jul-13'

dmy=$(echo "$ymd" | awk -F- '{ OFS=FS; print $3,$2,$1 }')
if date --date "$dmy" >/dev/null 2>&1
then
    echo OK
fi

Here's an all shell solution. Breaking it down, the IFS=- instructions tells the shell to split a forthcoming command line by hyphen - instead of whitespace. The set $ymd parses the $ymd variable as for a command line, but now splits by hyphen, assigning values to the parameters $1, $2 et seq. The echo "$3-$2-$1" trivially outputs the three captured values in reversed order.

ymd='2015-Jul-13'

dmy=$(IFS=-; set $ymd; echo "$3-$2-$1")
if date --date "$dmy" >/dev/null 2>&1
then
    echo OK
fi

If you really have to handle YYYY-MMM-DD you can do it with something like this, which simply swaps the arguments around to a format that date expects:

ymd='2015-Jul-13'

dmy=$(echo "$ymd" | awk -F- '{ OFS=FS; print $3,$2,$1 }')
if date --date "$dmy" >/dev/null 2>&1
then
    echo OK
fi

GNU date does not support YYYY-MMM-DD. However, it does understand DD-MMM-YYYY. So if you really have to handle dates of this format you can do it with something like this, which simply swaps the arguments around to a format that date expects:

ymd='2015-Jul-13'

dmy=$(echo "$ymd" | awk -F- '{ OFS=FS; print $3,$2,$1 }')
if date --date "$dmy" >/dev/null 2>&1
then
    echo OK
fi

mikeserv suggested a better all shell solution. Breaking it down, the IFS=- instructions tells the shell to split a forthcoming command line by hyphen - instead of whitespace. The set $ymd parses the $ymd variable as for a command line, but now splits by hyphen, assigning values to the parameters $1, $2 et seq. The echo "$3-$2-$1" trivially outputs the three captured values in reversed order.

ymd='2015-Jul-13'

dmy=$(IFS=-; set $ymd; echo "$3-$2-$1")
if date --date "$dmy" >/dev/null 2>&1
then
    echo OK
fi