CODEWITHSUNDEEP
pipeshposixpipelineio-redirection
kjh6b6a68
kjh6b6a68

Reputation: 509

Is it guaranteed that POSIX shell always open files for redirection before executing any commands in pipeline?

For example, consider invoking this shell command in an empty directory:

ls | cat | cat | cat > file; cat file

When I actually tested, the result was always:

file

But is this behaviour guaranteed by POSIX? I read section 2.7 redirection and 2.9.2 pipeline from the opengroup POSIX standard page, but I couldn't find anything mentioned about this.

And there was one statement that seems relevant, but I couldn't really understand what it means since I'm not native English user. The statement was this:

The standard input, standard output, or both of a command shall be 
considered to be assigned by the pipeline before any redirection 
specified by redirection operators that are part of the command (see 
Redirection).

Upvotes: 2

Views: 169

Answers (1)

oguz ismail
oguz ismail

Reputation: 50795

No, it's not.

Pipeline components are expanded, redirected, and executed in parallel. In your example, ls may outrun cat > file and retrieve the list of files before file is created, and vice versa. You can't rely on cat > file winning the race every time.

In order to ensure that the redirection is performed before any component of the pipeline is executed, you need to enclose the pipeline in braces, and move the redirection outside:

{ ls | cat | cat | cat; } > file
cat file

Upvotes: 3

Related Questions