Why does using <() in a shell script cause a syntax error?

Question

I'm writing a shell script called myShellScript.sh inside of which I have the following text:

echo *** Print out this line ****
diff <(./myProgram) <(./otherProgram)

However, when I run sh myShellScript.sh I get the following error:

-bash-4.2$ sh myShellScript.sh 
myShellScript.sh **** Print out this line **** myShellScript.sh
myShellScript.sh: line 2: syntax error near unexpected token `('
myShellScript.sh: line 2: `diff <(./myProgram) <(./otherProgram)'

Answer 1

Process substitution with the <(...) operator is a bash feature. You're getting that error message because your script is getting executed as something else (for example dash), or an older version of bash, or bash running in POSIX-compatibility mode with non-POSIX features like process substitution disabled (thanks, @chepner!)

If you want to execute the script with a full-featured bash, you have two options:

1. Run the script with bash:

   bash myShellScript.sh 
   

2. Set the first line of the script to #!/bin/bash (or whatever is the path to bash in your system), and run the script like this:

   ./myShellScript.sh 
   

Answer 2

You need to execute your script with bash, not with sh.

You are using process substitution, which is not a standard POSIX shell feature. sh is a POSIX-compatible shell, so it does not support language extensions like process substitution. Bash will run with POSIX compatibility enabled if it is invoked as sh.

Therefore, you should execute scripts that require Bash-specific features using bash.