Bash-Script: How to test if hour number is even?

Question

In a bash script (run every 30min), I want to do an action every even hour (and minutes 0), so I want to test the hour number if it's an even number or not. So I use this:

        HEURE=$(date +"%H")
        MINUTES=$(date +"%M")
        HEURE_PAIRE=""
        MINUTES_ZERO=""
        [[ $((HEURE % 2)) -eq 0 ]] && HEURE_PAIRE="OUI" || HEURE_PAIRE="NON"
        [[ $MINUTES -eq 0 ]] && MINUTES_ZERO="OUI" || MINUTES_ZERO="NON"
        if [[ "${HEURE_PAIRE}" == "OUI" ]] && [[ "MINUTES_ZERO" == "OUI" ]]; then
            # My code
        fi

But I get an error for this line (n°170 in my script) [[ $((HEURE % 2)) -eq 0 ]] && HEURE_PAIRE="OUI" || HEURE_PAIRE="NON" when it was 08:00, 08:30, 09:00, 09:30, but not for 10:00 or 10:30. See the error I get:

/volume4/docker/_Scripts-DOCKER/driver-pkgctl-r8152-restart-reload.sh: line 170: 08: value too great for base (error token is "07")
/volume4/docker/_Scripts-DOCKER/driver-pkgctl-r8152-restart-reload.sh: line 170: 09: value too great for base (error token is "09")

I expected to have it working like this: For 08:00, the 08%2 should return 0 as a result, proving hour is even. For 09:00, the 09%2 should return 1 as a result, proving hour isn't even. Not having an error

I assume it's the 0 before the 8 or 9... But I don't know how to fix this.

I found a workaround with this question by adding this:

HEURE=${HEURE#0}

It seems to be working.

My question: is there a better way to achieve what I want?

Thanks in advance.

Answer 1

Corrected script:

forcing base 10¹:

#!/bin/bash

heure=10#$(date +"%H") minutes=10#$(date +"%M")
if (( heure % 2 == 0 && minutes == 0 )); then
    # My code
fi

---

Another way using POSIX parameter expansions

#!/bin/bash

heure=$(date +"%H") minutes=$(date +"%M")
if (( ${heure#0} % 2 == 0 && ${minutes#0} == 0 )); then
    # My code
fi

---

another way using crontab

*/30 * * * * /path/to/script

---

Don't use UPPER case variables

---

((...)) and $((...)) are arithmetic commands, which returns an exit status of 0 if the expression is nonzero, or 1 if the expression is zero. Also used as a synonym for "let", if side effects (assignments) are needed. See http://mywiki.wooledge.org/ArithmeticExpression

---

¹ In arithmetic operations with bash, is there's a leading 0 like 08, it's treated as an octal number. To force base10, I use 10# prefix on hour and minutes

---

No need to quote in [[ ... ]] syntax, see http://mywiki.wooledge.org/BashFAQ/031 and http://mywiki.wooledge.org/BashGuide/TestsAndConditionals

Answer 2

An alternative method without using arithmetic expressions:

#!/bin/bash

hm=$(date +%H%M)
if [[ $hm = ?[02468]00 ]]; then
    # do stuff
fi

or, in POSIX shell:

#!/bin/sh

hm=$(date +%H%M)
case $hm in
    ?[02468]00) do_stuff ;;
esac

or, with Bash arithmetic in a single operation:

#!/bin/bash

if (( 10#$(date +%H%M) % 200 == 0 )); then
        echo "It is time" 
fi

Answer 3

Setting the base is a good method for bash.

For POSIX shells that don't support that, there are other options.

Prefix with a non-zero digit:

heure=$(date +1%H)
...

Strip the leading zero:

heure=$(date +%H)
heure=${heure#0}
...

Compare textually:

heure=$(date +%H)
...
case $heure in ?[02468]) ... ;; esac
...

Answer 4

First this is bad:

HEURE=$(date +"%H")
MINUTES=$(date +"%M")

The time can switch e.g. from 10:59 to 11:00 in between. And then you get the wrong 10:00.

Use either

time=$( date +%H:%M )
HEURE="${time%:*}"
MINUTES="${time#*:}"

... or ...

time=$( date +%s )
HEURE=$( date -d@$time +%H )
MINUTES=$( date -d@$time +%M )

In both variants you get the time only once. In the second example you'll get the epoche-time and then calculates hour an minute from this time. Thew first example seems to be more efficient.

Then replace [[ $((HEURE % 2)) -eq 0 ]] by (( !( 10#$HEURE % 2 ))) or (( ( 10#$HEURE % 2 ) == 0 )). The prefix 10# forces the evaluation as decimal number.

Last note: In general, use lower case letters or mixed case for local var names.

Answer 5

In numeric context, strings starting with a zero are considered octal. Therefore, 08 is in error.

However, I wouldn't do artithmetic to test for your condition. Instead, I would do something like

if [[ $(date +%HM) == ?[02468]00 ]]
then
  ....
fi