Re: Different variable assignments

On 2024-10-11 2:11 p.m., Frank Winkler wrote:

Hi there !
 
Consider the following commands:
 
 
$ var1=`uname -sr`
$ echo $var1
Darwin 24.0.0
$ read var2 <<< `uname -sr`
$ echo $var2
Darwin 24.0.0
$ uname -sr | read var3
$ echo $var3
 
$ uname -sr | read -p var3
$ echo $var3
 
$
 
While the first two ones behave like expected, I wonder why the latter
ones fail. What's the difference behind the scenes?

I don't know about other shells, but in Bash each command in a pipeline
is run in a subshell.  (See the "Pipelines" section of the Bash man
page.)  Thus you're doing the 'read var3' part in a different shell than
where 'echo $var3' runs.  That's why it is empty when you echo it.

And even more confusing, why does this familiar one work anyway?
 
$ sw_vers | while read line; do echo $line; done
ProductName: macOS
ProductVersion: 15.0.1
BuildVersion: 24A348

Here the subshell runs everything between 'while' and 'done' so the read
and echo commands are in the same (sub)shell this time.