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Calling external tools is a natural way to solve problems in Bash: call out to a specialized tool, capture the output, and process it.
Using GNU date to find the date of the day after February 28:
year=$1
next_day=$(date -d "$year-02-28 + 1 day" '+%d')
if [[ $next_day == 29 ]]; then
echo true
else
echo false
fi
Or, more concise but less readable:
[[ $(date -d "$1-02-28 + 1 day" '+%d') == 29 ]] \
&& echo true \
|| echo false
Working with external tools like this is what shells were built to do.
From a performance perspective, it takes more work (than builtin addition) to:
- copy the environment and spawn a child process,
- connect the standard I/O channels,
- wait for the process to complete and capture the exit status.
Particularly inside of a loop, be careful about invoking external tools as the cost can add up. Over-reliance on external tools can take a job from completing in seconds to completing in minutes (or worse).
Take care about using parts of dates in shell arithmetic. For example, we can get the day of the month:
day=$(date -d "$some_date" '+%d')
next_day=$((day + 1))
That looks innocent, but if $some_date is 2024-02-08, then:
$ some_date='2024-02-08'
$ day=$(date -d "$some_date" '+%d')
$ next_day=$((day + 1))
bash: 08: value too great for base (error token is "08")
Bash treats numbers starting with zero as octal, and 8 is not a valid octal digit.
Workarounds include using %_d or %-d to avoid the leading zero, or specify base-10 in the arithmetic (the $ is required in this case).
next_day=$(( 10#$day + 1 ))
