From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mark Galeck <mark_galeck@pacbell.net>
Subject: why does dash save, dup, and restore redirected descriptor in the
 parent, rather than redirect in the child?
Date: Tue, 31 Jan 2017 20:00:34 +0000 (UTC)
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Hello,

a dash script like: 

date > foobar.txt 
date 

is (as an SSCCE) handled like this: 


int fd; 
int saved; 

fd = open64("foobar.txt", O_WRONLY|O_CREAT); 
saved = fcntl(1, F_DUPFD, 10); 
dup2(fd, 1); 
if (!fork()) { 
    execl("/bin/date", "date", (char *)NULL); 
} 

dup2(saved, 1); 
if (!fork()) { 
    execl("/bin/date", "date", (char *)NULL); 
} 



This is strange. Why save, dup and dup again to restore, descriptors in the parent, when it would be much simpler to just dup in the child, and not have to save and restore. This is simpler and I checked it works the same: 


int fd; 

if (!fork()) { 
    fd = open64("foobar.txt", O_WRONLY|O_CREAT); 
    dup2(fd, 1); 
    execl("/bin/date", "date", (char *)NULL); 
} 

if (!fork()) { 
    execl("/bin/date", "date", (char *)NULL); 
} 



I am sure there must be a good reason and I am not understanding something deeper. What is it?


Thank you,

Mark