Bash – Scheduling command/script by specifying the exact second

bashcommand linescriptingshell

I'm using bash and wondering how I can execute a command/script at a later specified time, with the accuracy of a second?

I read the man page for the at-command, but as far as I could understand it's only possible specify minutes (and not seconds). Right now I first use at and then sleep to get it to execute the right second. To demonstrate, if I would like to run my_script.sh at 22.21.05 I would do:

echo "sleep 5; my_script.sh" | at 22.21

But it would be much nicer to have a command with it built in. Something like at 22.21.05.

Best Answer

I don't believe that this is possible. cron is only granular down to the minute and the at utility hooks into it to do it's work. I think you have the right solution--sleeping for x seconds before executing.

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Repeat a Unix command every x seconds forever

Try the watch command.

Usage: watch [-dhntv] [--differences[=cumulative]] [--help] [--interval=<n>] 
             [--no-title] [--version] <command>`

So that:

watch -n1  command

will run the command every second (well, technically, every one second plus the time it takes for command to run as watch (at least the procps and busybox implementations) just sleeps one second in between two runs of command), forever.

Would you want to pass the command to exec instead of sh -c, use -x option:

watch -n1 -x command

On macOS, you can get watch from Mac Ports:

port install watch

Or you can get it from Homebrew:

brew install watch

Bash – echo of a string with an exclamation point

In command:

echo "!$"

!$ is expanded by bash before passed to echo. Inside double quotes, if enabled, history expansion will be performed unless you escape !, using backslash \. bash had done the expansion, echo does nothing here, it just print what it got.

!$ refer to the last argument of preceding command, which is string '!$'.

In your second example:

$ echo '!$'
!$
$ echo "!echo "!$""
echo "echo '!$' "'!$'""
echo '!$' !$

Command echo "echo '!$' "'!$'"", arguments passed to echo are divided in three parts:

First: "echo '!$' ", expanded to string echo '!$'.
Second: '!$', expanded to string !$.
Third: "", expanded to empty string.

Best Answer

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Repeat a Unix command every x seconds forever

Bash – echo of a string with an exclamation point

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