Shell scripting input redirection oddities

Question

Can anyone explain this behavior? Running:

#!/bin/sh echo "hello world" | read var1 var2 echo $var1 echo $var2 

results in nothing being ouput, while:

#!/bin/sh echo "hello world" > test.file read var1 var2 < test.file echo $var1 echo $var2 

produces the expected output:

hello world 

Shouldn't the pipe do in one step what the redirection to test.file did in the second example? I tried the same code with both the dash and bash shells and got the same behavior from both of them.

Answer 1

A recent addition to bash is the lastpipe option, which allows the last command in a pipeline to run in the current shell, not a subshell, when job control is deactivated.

#!/bin/bash set +m      # Deactiveate job control shopt -s lastpipe echo "hello world" | read var1 var2 echo $var1 echo $var2 

will indeed output

hello world 

Answer 2

#!/bin/sh echo "hello world" | read var1 var2 echo $var1 echo $var2 

produces no output because pipelines run each of their components inside a subshell. Subshells inherit copies of the parent shell's variables, rather than sharing them. Try this:

#!/bin/sh foo="contents of shell variable foo" echo $foo (     echo $foo     foo="foo contents modified"     echo $foo ) echo $foo 

The parentheses define a region of code that gets run in a subshell, and $foo retains its original value after being modified inside them.

Now try this:

#!/bin/sh foo="contents of shell variable foo" echo $foo {     echo $foo     foo="foo contents modified"     echo $foo } echo $foo 

The braces are purely for grouping, no subshell is created, and the $foo modified inside the braces is the same $foo modified outside them.

Now try this:

#!/bin/sh echo "hello world" | {     read var1 var2     echo $var1     echo $var2 } echo $var1 echo $var2 

Inside the braces, the read builtin creates $var1 and $var2 properly and you can see that they get echoed. Outside the braces, they don't exist any more. All the code within the braces has been run in a subshell because it's one component of a pipeline.

You can put arbitrary amounts of code between braces, so you can use this piping-into-a-block construction whenever you need to run a block of shell script that parses the output of something else.