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bashio-redirectionsubshell

Sub-shell precendence

Yesterday I got some clarification around using flock and some simple concepts are becoming clear to me.

My question now is around sub-shells. Take a look at this block

(
 flock -s 200 
 # ... commands executed under lock ... 
) 200>/var/lock/mylockfile

My understanding is 200>/var/lock/mylockfile runs before flock -s 200, yet if I try something like this

( echo This is a sub-shell ) command

I get an error from BASH

-bash: syntax error near unexpected token `command'

Introducing a semi-colon

( echo This is a sub-shell ); command

fixes the error, but causes command to run after the sub-shell returns.

So I'm wondering how the initial example causes the redirect to run first. Is it something to do with precedence of the > over sub-shell (list) notation? Looking in the BASH manpage I find

Operators are evaluated in order of precedence. Sub-expressions in parentheses are evaluated first and may override the precedence rules above.

Solution

  • 200>/var/lock/mylockfile is, in this context, information about how the subshell should be configured (how its redirection is supposed to be set up). It's not a separate command.

    This is true in the same way that

    echo "foo" >bar.txt

    ...always does the redirection first, as does

    >bar.txt echo "foo"

    In neither of these cases does order matter at all.

    A subshell is similar in this respect -- it's just a compound command, and like any other command, it can have redirections specified either at its beginning or its end.