There appears to be a strong preference for a syntax like "~/10" instead of "*/~10". The following diff implements that instead and also supports random ranges like "1~58/10". When a high and/or low value is used with a random range, the initial offset is a random value less than eiter the step value or the difference of the high and low values (whichever is smaller).
- todd Index: usr.sbin/cron/crontab.5 =================================================================== RCS file: /cvs/src/usr.sbin/cron/crontab.5,v retrieving revision 1.41 diff -u -p -u -r1.41 crontab.5 --- usr.sbin/cron/crontab.5 18 Apr 2020 17:11:40 -0000 1.41 +++ usr.sbin/cron/crontab.5 6 May 2023 02:19:40 -0000 @@ -157,8 +157,7 @@ If either (or both) of the numbers on ei .Ql ~ are omitted, the appropriate limit (low or high) for the field will be used. .Pp -Step values can be used in conjunction with ranges (but not random ranges -which represent a single number). +Step values can be used in conjunction with ranges. Following a range with .No / Ns Ar number specifies skips of @@ -173,6 +172,18 @@ Steps are also permitted after an asteri .Dq every two hours , just use .Dq */2 . +A step value after a random range will execute the command at a random +offset less than the step size. +For example, to avoid a thundering herd at the top and bottom of the hour, +.Dq 0~59/30 +.Po +or simply +.Dq ~/30 +.Pc +can be used in the +.Ar minute +field to specify that command execution happen twice an hour at +consistent intervals. .Pp An asterisk .Pq Ql * Index: usr.sbin/cron/entry.c =================================================================== RCS file: /cvs/src/usr.sbin/cron/entry.c,v retrieving revision 1.53 diff -u -p -u -r1.53 entry.c --- usr.sbin/cron/entry.c 21 May 2022 01:21:29 -0000 1.53 +++ usr.sbin/cron/entry.c 5 May 2023 21:39:30 -0000 @@ -456,10 +456,11 @@ get_range(bitstr_t *bits, int low, int h /* range = number | number* "~" number* | number "-" number ["/" number] */ - int i, num1, num2, num3; + int i, num1, num2, num3, rndstep; num1 = low; num2 = high; + rndstep = 0; if (ch == '*') { /* '*' means [low, high] but can still be modified by /step @@ -497,7 +498,7 @@ get_range(bitstr_t *bits, int low, int h /* get the (optional) number following the tilde */ - ch = get_number(&num2, low, names, ch, file, ", \t\n"); + ch = get_number(&num2, low, names, ch, file, "/, \t\n"); if (ch == EOF) ch = get_char(file); if (ch == EOF || num1 > num2) { @@ -505,6 +506,13 @@ get_range(bitstr_t *bits, int low, int h return (EOF); } + if (ch == '/') { + /* randomize the step value instead of num1 + */ + rndstep = 1; + break; + } + /* get a random number in the interval [num1, num2] */ num3 = num1; @@ -538,6 +546,13 @@ get_range(bitstr_t *bits, int low, int h ch = get_number(&num3, 0, NULL, ch, file, ", \t\n"); if (ch == EOF || num3 == 0) return (EOF); + if (rndstep) { + /* + * use a random offset smaller than the step size + * and the difference between high and low values. + */ + num1 += arc4random_uniform(MINIMUM(num3, num2 - num1)); + } } else { /* no step. default==1. */ Index: usr.sbin/cron/macros.h =================================================================== RCS file: /cvs/src/usr.sbin/cron/macros.h,v retrieving revision 1.15 diff -u -p -u -r1.15 macros.h --- usr.sbin/cron/macros.h 12 Nov 2015 21:12:05 -0000 1.15 +++ usr.sbin/cron/macros.h 5 May 2023 21:38:19 -0000 @@ -29,6 +29,8 @@ #define MAX_TEMPSTR 100 /* obvious */ #define MAX_UNAME (_PW_NAME_LEN+1) /* max length of username, should be overkill */ +#define MINIMUM(a, b) (((a) < (b)) ? (a) : (b)) + #define Skip_Blanks(c, f) \ while (c == '\t' || c == ' ') \ c = get_char(f);