The system() function in awk run the command and return its status. You got 0 because the command is success. The output of date +'%s' was printed separately.

To get the output of external command, you would use getline():

$ echo foo bar baz | awk '{"date +%s" | getline d; printf "%s %s\n", d, $0}'
1491147643 foo bar baz

There's nothing to do with OFS, because it's a space by default.

IOW, you can do it alone with date:

date +'%s foo bar baz'

The system() function in awk run the command and return its status. You got 0 because the command is success. The output of date +'%s' was printed separately.

To get the output of external command, you would use getline():

$ echo foo bar baz | awk '{"date +%s" | getline d; printf "%s %s\n", d, $0}'
1491147643 foo bar baz

There's nothing to do with OFS, because it's a space by default.

With gawk or mawk, you can also do:

echo foo bar baz | gawk '{printf "%s %s\n", systime(), $0}'

The systime() return the number of seconds since epoch, not counting leap seconds

IOW, you can do it alone with date:

date +'%s foo bar baz'

The system() function in awk run the command and return its status. You got 0 because the command is success. The output of date +'%s' was printed separately.

To get the output of external command, you would use getline():

$ echo foo bar baz | awk '{"date +%s" | getline d; printf "%s %s\n", d, $0}'
1491147643 foo bar baz

There's nothing to do with OFS, because it's a space by default.

IOW, you can do it alone with date:

date +'%s foo bar baz'

The system() function in awk run the command and return its status. You got 0 because the command is success. The output of date +'%s' was printed separately.

To get the output of external command, you would use getline():

$ echo foo bar baz | awk '{"date +%s" | getline d; printf "%s %s\n", d, $0}'
1491147643 foo bar baz

There's nothing to do with OFS, because it's a space by default.

IOW, you can do it alone with date:

date +'%s foo bar baz'