Repeat a Unix command every x seconds forever

command linescripting

There's a built-in Unix command repeat whose first argument is the number of times to repeat a command, where the command (with any arguments) is specified by the remaining arguments to repeat.

For example,

% repeat 100 echo "I will not automate this punishment."

will echo the given string 100 times and then stop.

I'd like a similar command – let's call it forever – that works similarly except the first argument is the number of seconds to pause between repeats, and it repeats forever. For example,

% forever 5 echo "This will get echoed every 5 seconds forever and ever."

I thought I'd ask if such a thing exists before I write it. I know it's like a 2-line Perl or Python script, but maybe there's a more standard way to do this. If not, feel free to post a solution in your favorite scripting language.

PS: Maybe a better way to do this would be to generalize repeat to take both the number of times to repeat (with -1 meaning infinity) and the number of seconds to sleep between repeats.
The above examples would then become:

% repeat 100 0 echo "I will not automate this punishment."
% repeat -1 5 echo "This will get echoed every 5 seconds forever."

Best Answer

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

Related Solutions

Shell – How to execute a command repeatedly with different arguments

You can simply use xargs

xsel | xargs -n1 echo mycommand

-n1 means one arg for mycommand, but it's just dry run, it will show what going to be run, to run it remove echo

For constant Argument

xsel | xargs -I {} -n1 echo mycommand "constantArgument" {}

Bash – Running a command multiple times with arguments (filenames) from a file

Conveniently, xargs -I does exactly what you want:

$ xargs <my_file_list.txt -I filename ./my_command "filename"

-I replace-str
    Replace occurrences of replace-str in the initial-arguments with names read
    from standard input.
    Also, unquoted blanks do not terminate input items;
    instead the separator is the newline character.
    Implies -x and -L 1.

This means it takes exactly one newline-delimited input line and invokes your command with it as a single argument. Nothing apart from newline is treated as a delimiter, so spaces and other special characters are fine.

Note that if you wanted to allow newlines in your filenames, the nul terminator (as per Wildcard's answer) will also work in a file.

Best Answer

Related Solutions

Shell – How to execute a command repeatedly with different arguments

Bash – Running a command multiple times with arguments (filenames) from a file

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