Subshells

It is often convenient to make multiple temporary changes to the program state (current working directory, environment etc.) in a subshell such that the main program does not have to reset its own values.

In Idio, subshells use the {{...}} notation.

As a far too involved example, let’s create a virtualenv and run a script from it:

simple-subshell.idio

printf "%d in %s: IDIOLIB is %s\n" PID PWD IDIOLIB

{{
  tmp-dir := (libc/make-tmp-dir)

  ;; change directory in the subshell
  cd tmp-dir

  lib-dir := #S{lib/idio/${IDIO_MM_VERSION}}

  mkdir -p bin lib-dir

  ;; IDIO_EXE is the absolute path of the running executable
  ln -s IDIO_EXE bin

  ;; but IDIO_EXE is versioned, idio.M.N, and we want to use the
  ;; command idio
  ln -s #S{idio.${IDIO_MM_VERSION}} bin/idio

  ;; let's see what we did, there
  ls -l #S{${tmp-dir}/bin}

  with-output-to-file #S{${lib-dir}/tell-me.idio} (function #n {
    ;; Using an interpolated string serves no purpose, here, as there
    ;; is nothing to expand, however, it is habit-forming when
    ;; generating code
    puts #S{
;; this is the script for the virtualenv idio running in the subshell
printf "%d in %s: IDIOLIB is %s\n" PID PWD IDIOLIB
}
  })

  ;; report on the subshell
  printf "%d in %s: IDIOLIB is %s\n" PID PWD IDIOLIB

  ;; change PATH in the subshell
  PATH = #S{${tmp-dir}/bin:${PATH}}

  idio tell-me

  cd HOME

  rm -rf tmp-dir
}}

printf "%d in %s: IDIOLIB is %s\n" PID PWD IDIOLIB

$ idio simple-subshell
226136 in /home/idf: IDIOLIB is /home/idf/.local/lib/idio/0.3
total 0
lrwxrwxrwx. 1 idf idf  8 Nov 15 11:47 idio -> idio.0.3
lrwxrwxrwx. 1 idf idf 29 Nov 15 11:47 idio.0.3 -> /home/idf/.local/bin/idio.0.3
226138 in /tmp/idio-tmp-EIUqVO: IDIOLIB is /home/idf/.local/lib/idio/0.3
226143 in /tmp/idio-tmp-EIUqVO: IDIOLIB is /tmp/idio-tmp-EIUqVO/lib/idio/0.3:/home/idf/.local/lib/idio/0.3
226136 in /home/idf: IDIOLIB is /home/idf/.local/lib/idio/0.3

Here, you can see the Process ID associated with the subshell is different to the parent and is different to the idio running from the subshell.

Last built at 2024-05-21T06:11:42Z+0000 from 77077af (dev) for Idio 0.3