check if argument is a valid date time in shell script - linux

Question

I am writing a bash shell script in Linux, this program will accept a date 08-FEB-18 11.45.18.844 AM as a parameter.
I am wondering if there is a simply way to check if the date time is valid?

Answer 1

You can get a bit creative since you have bash and map the date string into an array which can then be easily parsed with date -d (and the help of another associative array). Once the date/time is mapped to array elements and converted to seconds since epoch with date -d, you simply check the return of the date command to determine if the conversion succeeded or failed. Handle the return appropriately:
#!/bin/bash
[ -n "$1" ] || { ## validate one argument given
printf "error: insufficient input\nusage: %s dd-mmm-yy hh.mm.ss.ms\n" \
"${0##*/}"
exit 1
}
oifs="$IFS" ## save original Internal Field Separator
IFS=$' \t\n-.'; ## set IFS to break on - or .
dt=( $(echo $1) ) ## separate date into indexed array
[ "${#dt[#]}" -lt '7' ] && { ## check all 7 components present
printf "error: date doesn't match dd-mmm-yy hh.mm.ss.ms format\n"
exit 1
}
IFS="$oifs" ## reset original IFS
## create associative array mapping months to numerics
declare -A mo=(
[JAN]=1
[FEB]=2
[MAR]=3
[APR]=4
[MAY]=5
[JUN]=6
[JUL]=7
[AUG]=8
[SEP]=9
[OCT]=10
[NOV]=11
[DEC]=12
)
## any date after 30 considerd 1930, else considered 2000
[ "${dt[2]}" -gt '30' ] && dt[2]=$((${dt[2]} + 1000)) || \
dt[2]=$((${dt[2]} + 2000))
## use date to convert array contents to seconds since epoch
epochsec=$( date -d "${dt[2]}-${mo[${dt[1]}]}-${dt[0]} \
${dt[3]}:${dt[4]}:${dt[5]}.${dt[6]}" +%s )
if [ "$?" -ne '0' ]; then ## check if last return was error
printf "error: invalid date.\n"
else ## output good date
printf "date: %s\n" "$(date -d #$epochsec)"
fi
Example Use/Output
$ bash chkcustomdt.sh "08-FEB-18 11.45.18.844"
date: Thu Feb 8 11:45:18 CST 2018
There are a lot of ways to approach this, this was just the first that came to mind.