In bash, read after a pipe is not setting values

With ksh I’m using read as a convenient way to separate values:

$ echo 1 2 3 4 5 | read a b dump
$ echo $b $a 
2 1
$

But it fails in Bash:

$ echo 1 2 3 4 5 | read a b dump
$ echo $b $a 
  
$

I didn’t find a reason in the man page why it fails, any idea?

Asked By: Emmanuel

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bash runs the right-hand side of a pipeline in a subshell context, so changes to variables (which is what read does) are not preserved — they die when the subshell does, at the end of the command.

Instead, you can use process substitution:

$ read a b dump < <(echo 1 2 3 4 5)
$ echo $b $a
2 1

In this case, read is running within our primary shell, and our output-producing command runs in the subshell. The <(...) syntax creates a subshell and connects its output to a pipe, which we redirect into the input of read with the ordinary < operation. Because read ran in our main shell the variables are set correctly.

As pointed out in a comment, if your goal is literally to split a string into variables somehow, you can use a here string:

read a b dump <<<"1 2 3 4 5"

I assume there’s more to it than that, but this is a better option if there isn’t.

Answered By: Michael Homer

This is not a bash bug as POSIX allows both bash and ksh behaviors, leading to the unfortunate discrepancy you are observing.

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_12

Additionally, each command of a multi-command pipeline is in a subshell environment; as an extension, however, any or all commands in a pipeline may be executed in the current environment. All other commands shall be executed in the current shell environment.

However, with bash 4.2 and newer, you can set the lastpipe option in non interactive scripts to get the expected result, eg:

#!/bin/bash

echo 1 2 3 4 5 | read a b dump
echo before: $b $a 
shopt -s lastpipe
echo 1 2 3 4 5 | read a b dump
echo after: $b $a 

Output:

before:
after: 2 1
Answered By: jlliagre
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