capture the stdout of a heredoc in a variable

roybal · 08-12-2010, 08:48 PM

Why doesn't this work?

Code:

cat | myvar=$(</dev/stdin) <<EOS
  This is some content
EOS

echo "$myvar"

i have also tried several variants of cat in place of the redirection.

Code:

$(cat -) 
$(cat)
$(cat /dev/stdin)

None of the variants print "This is some content"

I have a version that works:

Code:

myvar=$(cat <<EOS
  This is some content
EOS
)

echo "$myvar"

but i don't like the syntax and it doesn't play nice in my editor with code folding.

Thanks!

ghostdog74 · 08-12-2010, 09:15 PM

what is it that you are actually trying to do ? If you just want to store multiline text into a variable, then just assign to the variable

Code:

var="this 
is my 
comment"

konsolebox · 08-12-2010, 09:22 PM

@ghostdog74 He probably wants to redirect all output to a variable like a buffer stream in C++. Maybe it can work multiline on arrays with while loops,.. but not against a single variable.

You can do this:

Code:

cat <<< "$VARIABLE"

but not anything like this:

Code:

cat >>> VARIABLE

catkin · 08-13-2010, 07:52 AM

The reason it does not work is that each command in a pipeline is run in a subshell. The variable is being set as intended but in the subshell, so has no effect in the script.

roybal · 08-13-2010, 09:20 AM

@qhost: Yes, this simple assignment works as far as the example I gave. Please excuse me for not providing more context, but I thought it would confuse the issue. @konsole is correct. What I am trying to accomplish is something like a data stream. @cat I see what you mean. So what can i use that stays in the same process space, takes stdin, and makes the assignment? Is there some form of redirection that will get me there? imaginary code: (<cat -)

Thank you guys

konsolebox · 08-13-2010, 08:23 PM

For that concept you can still do something like this but it will be terrible I think. Only for bash version > 3.1 since I chose +=.

Code:

function readstream {
    __=''
    local REPLY IFS=''
    while read -n 1 -t 1 -d ''; do
        __+=$__$REPLY
    done
}

readstream < <(cat [args])

# do something with $__

Sometimes reading single characters works with -d '' only but sometimes it only works with IFS=''. The timeout is there just to make sure but it's not really used.

I guess I was wrong. It can also work with single (i meant normal) variables. I didn't think about +=. VAR=$VAR$NEWVALUE can also be used but that's already way too many malloc() and memcpy()/strcpy().