I have seen a crontab record in system.
0-55/5 * * * * root <command>
I read the crontab -e example files and I know the first position stands for minute. But I cannot figure out the meaning of / (slash) there. Could anyone explain the meaning to me?
The forward slash is used in conjunction with ranges to specify step values.
0-55/5 * * * * means your command will be executed every five minutes (0, 5, 10, 15, ..., 55).
0-55/5 is the same as */5.
5 * * * * will run at five minutes after the top of the hour, every hour (i.e., once an hour). */5 * * * * will run every five minutes (i.e., twelve times an hour)
I've found an useful info from the official documentation:
Step values can be used in conjunction with ranges. Following a range with
/<number>specifies skips of the number's value through the range. For example,0-23/2can be used in the hours field to specify command execution every other hour (the alternative in the V7 standard is0,2,4,6,8,10,12,14,16,18,20,22). Steps are also permitted after an asterisk, so if you want to say every two hours, just use*/2.―
man 5 crontab- 4th Berkeley Distribution - 19 April 2010
In short, all of these are valid syntax:
0-55/5 * * * *
\ \ \ \ \- every day of week
\ \ \ \-- every month
\ \ \--- every day of month
\ \---- every hours
\----------- from minute 0 to 55, using a step of 5 minutes
That means minute 0, 5, 10, 15, ..., 45 and 55 included.
man 5 crontabhas explanations too.man? I read throughman crontaband through it was the end.man crontabbrings up the first entry forcrontab, which is for thecrontabcommand in section 1. Towards the end of that manpage, it saysSEE ALSO crontab(5). That tells you that you can useman 5 crontabto read thecrontabentry in section 5, which describes the format of thecrontabfile.(3)thing in theSEE ALSOsection!