As a matter of example : this scripts "rewrites" its final command
depending on optional arguments. Its purpose is to restore select parts
of the environment variables, uid/gid and working directory, that were
saved in a directory at some point in the past.
```
#!execline
define url example.com
s6-tcpclient $url 80
execlineb "
foreground {
fdmove 1 7 echo -en \"…\"
fdmove 0 6 cat
}
"
```
I have trouble understanding what you want to do in the general case,
because in this example, s6-tcpclient takes a whole command line, so
you do not have to
