Bash: Assign output of pipe to a variable

Question

I am trying to get the output of a pipe into a variable. I tried the following things:

echo foo | myvar=$(</dev/stdin)
echo foo | myvar=$(cat)
echo foo | myvar=$(tee)

But $myvar is empty.

I don’t want to do:

myvar=$(echo foo)

Because I don’t want to spawn a subshell.

Any ideas?

Edit: I don’t want to spawn a subshell because the command before the pipe needs to edit global variables, which it can’t do in a subshell. Can it? The echo thing is just for simplification. It’s more like:

complex_function | myvar=$(</dev/stdin)

And I don’t get, why that doesn’t work. This works for example:

complex_function | echo $(</dev/stdin)

Answer 1

The correct solution is to use command substitution like this:

variable=$(complex_command)

as in

message=$(echo 'hello')

(or for that matter, message=hello in this case).

Your pipeline:

echo 'hello' | message=$(</dev/stdin)

or

echo 'hello' | read message

actually works. The only problem is that the shell that you're using will run the second part of the pipeline in a subshell. This subshell is destroyed when the pipeline exits, so the value of $message is not retained in shell.

Here you can see that it works:

$ echo 'hello' | { read message; echo "$message"; }
hello

... but since the subshell's environment is separate (and gone):

$ echo "$message"

(no output)

One solution for you would be to switch to ksh93 which is smarter about this:

$ echo 'hello' | read message
$ echo "$message"
hello

Another solution for bash would be to set the lastpipe shell option. This would make the last part of the pipeline run in the current environment. This however does not work in interactive shells as lastpipe requires that job control is not active.

#!/bin/bash

shopt -s lastpipe
echo 'hello' | read message
echo "$message"

Answer 2

Use command substitution:

myvar=`echo foo`

or

myvar=$(echo foo)

Answer 3

In ksh93, you can use:

var=${
  my command that updates global variable
}

That's a form of command substitution that doesn't spawn a subshell. For commands that are builtin commands, instead of having them writing their output to a pipe (for which you'd need different processes to read and write on the pipe to avoid dead-locks), ksh93 just makes them not output anything but gather what they would have been outputting in to make up the expansion.

$ ksh -c 'a=${ b=123; echo foo;}; echo "$a $b"'
foo 123

fish's command substitution also behaves like that:

$ fish -c 'set a (set b 123; echo foo); echo $a $b'
foo 123

In most other shells, you'd use a temporary file:

my command that updates global variable > file
var=$(cat file) # can be optimised to $(<file) with some shells

On Linux, and with bash or zsh (that use temp files for <<<), you can do:

{ my command that updates global variable > /dev/fd/3 &&
  var=$(cat<&3); } 3<<< ''

Answer 4

Here is a simple way to do it in bash -

complexFunction(){
cat <<_endOfHereFile_
The lazy dog
jumped over 
the moon
_endOfHereFile_
}

{ read -d '' message; }< <(complexFunction)
echo "${message}"

The result of echo "${message}" is

The lazy dog
jumped over 
the moon

The complex function could be any function emitting to stdout. It happens to use a here file in this example, but that is not important.

The call read -d '' reads stdin up to the delimiter, which since it is the empty character `` means reading until the end of input.

The syntax {lhs;}< <(rhs) redirects the stdout of rhs to the stdin of lhs, where lhs enjoys the shared variable namespace so that echo ${message} works as desired.

Answer 5

I am not really an expert, but have you tried the following?

echo "hello world" \
| { echo_out=$(< /dev/stdin); echo "echo output is: $echo_out"; } \
| cut -d":" -f 2