3

This is my case scenario:

luis@Balanceador:~$ echo ${array[@]}
a b
luis@Balanceador:~$ echo ${array[1]}
a
luis@Balanceador:~$ echo ${array[2]}
b
luis@Balanceador:~$ parallel echo ${array[]} ::: 1 2
-bash: ${array[]}: bad substitution
luis@Balanceador:~$ parallel echo ${array[{}]} ::: 1 2
-bash: {}: syntax error: operand expected (error token is "{}")
luis@Balanceador:~$ parallel echo ${array[{1}]} ::: 1 2
-bash: {1}: syntax error: operand expected (error token is "{1}")
luis@Balanceador:~$ parallel echo ${array[{#}]} ::: 1 2
-bash: {#}: syntax error: operand expected (error token is "{#}")

How can I reference the individuals elements of some array on GNU Parallel?

Sure this is an easy one, but I have not been able to find it on the manual.

This question has been made to answer this other, but, after asking it, I considered they were two different questions.

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4
  • parallel echo ::: "${array[@]}" Commented Jun 26, 2016 at 22:11
  • Well, thanks you for answering, @don_crissti , but my original idea was a bit more complex. I have edited the original question to reflect, but you can see the main idea here: unix.stackexchange.com/questions/292275/… Commented Jun 26, 2016 at 22:29
  • I saw your other question. Assuming you were able to do that, gnu parallel starts sub-shells... how do you expect to access those variables if they're defined in sub-shells ? Run seq 3 | parallel echo $$ (this is the shell that runs parallel) and then run seq 3 | parallel echo \$\$ (these are sub-shells started by parallel)... Commented Jun 26, 2016 at 22:41
  • I think you are right, @don_crissti . Maybe the only way will be to store the results inside files. Thanks you anyway. Commented Jun 26, 2016 at 22:44

1 Answer 1

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2

While it looks easy, it is really very hard.

Jobs started by GNU Parallel are not started inside the same shell as GNU Parallel is run from. So it looks like this:

bash[1]---perl(running parallel)---bash[2]

$array is defined in bash[1] but you want to use it in bash[2]. It is impossible to do completely (i.e. if you want write access to the array), but we can make a copy of $array available:

env_parallel 'echo ${array[{}]}' ::: 1 2

env_parallel (introduced in GNU Parallel 20140822) copies the entire environment of bash[1] to bash[2] (so your environment has to be kinda small) where it is initiated before the job is run.

env_parallel is quite unstable, so if you find bugs, please report them.

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8
  • What is env_parallel? How does it copy arrays (you can't pass arrays through the environment directly)? Commented Jun 27, 2016 at 21:28
  • @Giles env_parallel is a fairly recent development and is still not production quality. Please read man env_parallel and man parallel_design and get back if that does not answer your question. Commented Jun 28, 2016 at 6:31
  • No manual entry for env_parallel. I see, not yet in Debian unstable. I'm still confused as to what it does: you say that it copies the environment but then your examples show it copying shell variables, aliases and functions. I guess this is a documentation bug report: “environment” already has a technical meaning in this context and it's really confusing to use it with a different meaning. I suggest calling this “context”, and explaning what is considered context (what about shell options, for example?). Commented Jun 28, 2016 at 12:34
  • Maybe you are right about the wording. Can you come up with an authoritative source that defines the difference between shell environment and context? The goal is to copy the whole environment, but that will of course never be possible 100% (e.g. $$ will never work), but shell options for Bash will be supported in next version. Commented Jun 28, 2016 at 16:37
  • “Environment” in this context appears to refer to a very specific technical meaning: the set of environment variables. “Context” is a more conceptual term that would fit and that has no existing specific meaning in Unix shells and processes. This needs an explanation either way, but at least with “context” there's no contradiction between meanings, only a clarification. Commented Jun 28, 2016 at 17:33

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