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.

Answers


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.


200>/var/lock/mylock; only creates mylock but that's all. For instance,

$ 200>/var/lock/mylock;
$ echo "hello" >&200
bash: 200: bad file descriptor.

doesn't work. If you want this to run, add exec :

$ exec 200>/var/lock/mylock;
$ echo "hello" >&200
$ cat /var/lock/mylock
hello

Now about subshells : when writing

(
   ...
) 200>/var/lock/myfile

bash creates a child using fork(), then the child redirects 1 to 200 using dup2, then parses and executes the code inside ( ... ).

Thus if you want to create the file before the subshell, a solution is

exec 200> /var/lock/file
(
  ...
) >&200

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