How (...) and {...} interact with creating subshell using pipes in bash?

Question

I am trying to understand the difference between invoking a shell subprocess using parentheses and curly braces. I thought that curly braces do not start the subprocess, but it seems that they start the subprocess.

#!/bin/sh a=1 b=1 ( a=2; ) | ( a=3; ) { b=2; } | { b=3; } echo "a=$a" echo "b=$b" 

This script prints

 a=1 b=1 

So, it seems that all calls are made inside subprocesses. Is there any difference between the two in this context? I understand that if I used && and || , then {..} will not start the subprocess, but I'm trying to understand how pipes work.

Answer 1

To demonstrate that this is the pipeline itself creating the subshell, and that the curly braces will not change anyway:

 #!/bin/bash echo "Base: $BASHPID" ( echo "In (): $BASHPID" ) # This will differ from the base { echo "In {}: $BASHPID"; } # This will match the base # In bash, these will both differ from the base echo "Pipeline, default config:" { echo " X: $BASHPID" >&2; } | { echo " Y: $BASHPID" >&2; } # This is exactly the same without the {}s echo "Pipeline, no {}s, default config:" echo " X: $BASHPID" >&2 | echo " Y: $BASHPID" >&2 # Only the former will differ from the base if running a new enough bash shopt -s lastpipe echo "Pipeline, lastpipe enabled:" { echo " X: $BASHPID" >&2; } | { echo " Y: $BASHPID" >&2; } 

By doing this locally with bash 4.3, I get:

 Base: 82811 In (): 82812 In {}: 82811 Pipeline, default config: X: 82813 Y: 82814 Pipeline, no {}s, default config: X: 82815 Y: 82816 Pipeline, lastpipe enabled: Y: 82811 X: 82817 

Please note that since all components of the pipeline are executed simultaneously, there is no specific order from which the first output from X or Y will be output; however, when lastpipe enabled lastpipe last component of the pipeline is called in the already running and running shell ( fork() not required from the main process), which slightly changes the probability of who writes the first to stdout.

Answer 2

{ ... } does not spawn a sub-shell. What you see is related to what you use | between two curly list commands.

This will be apparent with this test:

 $> b=1 $> echo $BASHPID 4401 $> { echo "X. $BASHPID"; b=2; } | { echo "Y. $BASHPID"; b=3; } Y. 46902 $> echo $BASHPID 4401 $> declare -pb declare -- b="1" 

You can see that { echo "Y. $BASHPID"; b=3; } { echo "Y. $BASHPID"; b=3; } { echo "Y. $BASHPID"; b=3; } is executed in another sub-shell, so changes made to b are not reflected in the current shell, where b is still 1 .

Answer 3

Parentheses execute commands in a subshell. Curly braces execute commands in the current shell:

 $ echo $BASHPID; (TEST=test; echo "sub:$TEST;$BASHPID"; exit 1);\ > { echo "current:$TEST;$BASHPID;$?"; } 2920 sub:test;3700 current:;2920;1 

Another difference is that curly braces require spaces between them and the attached commands and a semicolon after the last, while parentheses do not execute.

All of this is documented on the bash man page, "Compound Commands."

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The only exception (more precisely, another aspect) is the use of channels:

 command1 | command2 

In this case, both commands are executed in separate subshells - regardless of what they are (and which include compound commands). The Piping section of the manual page documents this.