What are "double sh" calls?

Question

Can you explain what are (as I call it) "double sh calls", how they work and the advantages of such constructs ?

EDIT1 : Maybe an example can help you understand what I'm talking about.

$ set -- --no-resume-playback https://www.facebook.com/TBN/videos/1580372468665943/
$ sh -c 'echo $*'
$
$ sh -c 'echo $*' sh "$*"
--no-resume-playback https://www.facebook.com/TBN/videos/1580372468665943/

I couln't find any example of this in the man :

$ man bash | col -b | egrep "sh -c [\"']"
$

N.B.: If the title of my question is ambiguous, please help me change it because I don't know how to call such shell calls.

What is your question.? Are you just looking for understanding what `sh -c` is and how it works. If so your answer is at https://stackoverflow.com/q/23951346/5291015 — Inian, Apr 09 '19 at 09:06
@Inian Nope. Please take a little more time and see my EDIT1 and the example below it so as to understand what I'm talking about. — SebMa, Apr 09 '19 at 09:13
@Inian Please reconsider your down-vote and see my EDIT2 and the example below it that I've just updated, hope this helps make myself understood. — SebMa, Apr 09 '19 at 09:24

score 2 · Accepted Answer · answered Apr 09 '19 at 09:19

2

man bash describes -c and its arguments:

If the -c option is present, then commands are read from the first non-option argument command_string. If there are arguments after the command_string, they are assigned to the positional parameters, starting with $0.

In this case, the command_string is echo $*, and the positional arguments are set in this way:

$0 = sh
$1 = "$*" = '--no-resume-playback https://www.facebook.com/TBN/videos/1580372468665943/'

answered Apr 09 '19 at 09:19

choroba

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Please see my EDIT2 and the example below it that I've just updated, hope this helps make myself understood – SebMa Apr 09 '19 at 09:23
What's your question? – choroba Apr 09 '19 at 09:58
What's the second `sh` call doing after the subshell script : `sh -c 'script'` ? – SebMa Apr 09 '19 at 10:09
It's not a call, it's a value assigned to `$0`. – choroba Apr 09 '19 at 11:02
In MY example, what is the second `sh` doing after the subshell script : sh -c 'script' ? – SebMa Apr 09 '19 at 11:04
1

If you mean this: `sh -c 'echo $*' sh "$*"`, then the second `sh` is the value `$0` gets set to as I've already explained. – choroba Apr 09 '19 at 11:35
I am finally starting to understand, at last :) So the second `sh` and what follows it are just the $0, $1, ... of the first `sh -c` subscript parameters, right ? – SebMa Apr 09 '19 at 11:38
@SebMa Exactly. – choroba Apr 09 '19 at 13:36

What are "double sh" calls?

1 Answers1