Validating user fed time in bash with regex pattern [duplicate]

Question

Assume a user fed time :

read -p "Enter the start time you want to start(hh:mm:ss) " user_start_time

and the regex :

timePattern="[0-2][0-3]:[0-5][0-9]:[0-5][0-9]"

if i do :

if [ "$user_start_time" == "$timePattern" ] 
#or if [ "$user_start_time" =~ "$timePattern" ]
then
         echo "in if"
else
         echo "in else"
fi

It doesn't validates.....being newbie, i understand comparison of int or string but how we go about time (date is there too, but date -d solves lot of problems)

There is this question too but couldn't understand the accepted answer!!

Your problem is using == and =~ within [ ... ]. You need to use double square brackets. Refer to the linked question. — devnull
– devnull, Commented Feb 17, 2014 at 6:23
@devnull : yeah, referred it the moment u posted....thankx mate!! :) — NoobEditor
– NoobEditor, Commented Feb 17, 2014 at 6:24
@devnull : a quick question, will it be a good practice if i always use [[ in validations rather than [ just to be on safer side?? — NoobEditor
– NoobEditor, Commented Feb 17, 2014 at 6:25
Moreover, avoid regex for validating date/time. You might want to refer to this answer for an alternate approach. — devnull
– devnull, Commented Feb 17, 2014 at 6:26
You can, it's just that [[ isn't as portable! If you're sure where your script would be running, I see no harm. — devnull
– devnull, Commented Feb 17, 2014 at 6:27

anubhava · Accepted Answer · 2014-02-17 06:11:36Z

2

For regex matching you need to use =~ operator instead of == in BASH. So use it like this:

[[ "$user_start_time" =~ $timePattern ]] && echo "in if" || echo "in else"

Also you need to remove quotes around regex variable $timePattern

answered Feb 17, 2014 at 6:11

anubhava

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5 Comments

NoobEditor Over a year ago

+1 : so my code has 2 mistakes....Pattern should not be in quotes and double square brackets ....mmm.....when should the double square brackets and single square brackets be used?? i mean is there any criteria for it???

anubhava Over a year ago

As a thumb rule always used [[ and ]] while using BASH. As you have noticed [ and ] don't even support =~ operator. [ was actually a command which is still supported for backward compatibility purpose but for all practical purposes in BASH use [[ and ]] as it is much more feature rich and efficient.

NoobEditor Over a year ago

didn't this thing...thanks!! :)

anubhava Over a year ago

@MortezaLSC: Yes sure you can do: if [[ "$user_start_time" =~ $timePattern ]]; then echo "in if"; else echo "in else"; fi

MLSC Over a year ago

I removed my comment... I found it yes thank you :)

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