How do I get the first day of week (Sunday or Monday) for the current locale in bash?

Some exemplary locales with their first day of week: [1]

• Sunday: en_US, pt_BR (Brazil)
• Monday: en_GB, es_AR (Argentina)
• Saturday: ar_AE (United Arab Emirates)

If possible, I would prefer a platform-independent solution for UNIX-like systems.

I know that it's possible to get information about the current locale via the locale(1) command, which is part of glibc. But the documentation (locale(5)) confuses me a little, and maybe there's a better solution than using glibc.

How do I get the first day of week (Sunday or Monday) for the current locale in bash?

Some exemplary locales with their first day of week: [1]

• Sunday: en_US, pt_BR (Brazil)
• Monday: en_GB, es_AR (Argentina)
• Saturday: ar_AE (United Arab Emirates)

If possible, I would prefer a platform-independent solution for UNIX-like systems.

I know that it's possible to get information about the current locale via the locale(1) command, which is part of glibc. But the documentation (locale(5)) confuses me a little, and maybe there's a better solution than using glibc.

• bash
• shell
• localization

Share Improve this question Follow edited Jan 29 at 17:19 myrdd asked Jan 29 at 12:02 myrddmyrdd 3,88233 gold badges2525 silver badges2525 bronze badges 1

• 1 I don't know, but most sensible programs let their users decide, because lots of people don't agree with the locale value that supposedly applies to them. – xpusostomos Commented Feb 2 at 16:23

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1 Answer 1

Sorted by: Reset to default 5

The solution I present requires glibc >= 2.2 and GNU Coreutils (date).

In glibc, there are two locale keywords for defining the first day of week: week-1stday (which is the second value of the week keyword) and first_weekday.

The value of week-1stday determines the meaning of first_weekday: [2]

$(locale week-1stday)	$(locale first_weekday)
19971130, which was a Sunday	⇒ 1=Sunday ... 7=Saturday
19971201, which was a Monday	⇒ 1=Monday ... 7=Sunday

Note that the value of week-1stday can theoretically be any date. First I will present a solution that covers all theoretically possible cases, then a simplification.

Solution

Get the first day of week as a number: (0=Sunday, 1=Monday, ...)

num_first=$(( ( $(date -d $(locale week-1stday) +%w)
                + $(locale first_weekday) - 1
              ) % 7 ))

without line-breaks: $(( ( $(date -d $(locale week-1stday) +%w) +$(locale first_weekday) -1 ) % 7 ))

Get its name: (”Sunday“, ”Monday“, ...)

# localized name using the current locale (e.g. French: ”dimanche“, ”lundi“, ...)
date -d "19971130 +$num_first days" +%A
# or
date -d "$(locale week-1stday) +$(($(locale first_weekday)-1)) days" +%A

To get the name in a specific language, simply prepend LC_TIME=...:

# English name
LC_TIME=en_US.UTF-8 date -d "19971130 +$num_first days" +%A
# or
LC_TIME=en_US.UTF-8 date -d "$(locale week-1stday) +$(($(locale first_weekday)-1)) days" +%A

However, the corresponding locale (e.g. en_US) must be installed.

---

FYI: Get the the first day of week of some reference date:

reference="today"
date -d "$reference -$(date -d $reference +%w) days +$num_first days"

Simpler solution?

The notes section of locale(5) states:

For compatibility reasons, all glibc locales should set […] [week-1stday] to 19971130 (Sunday) […].

In fact, as far as I know, all relevant glibc locale definition files meet this criterion. I'm not sure if this is always the case, though. ”ISO 8601 conforming applications should use […] 19971201“ [3].

If you can assume that week-1stday is 19971130 (Sunday) on all systems you're addressing, you may simplify above commands to simple one-liners:
Get the day number: num_first=$(( $(locale first_weekday) -1 ))
Get the day's name: date -d "19971130 +$(($(locale first_weekday)-1)) days" +%A
But to be on the safe side, take the longer solution.