I have some old scripts that I try to update. Some of the code condenses to:
export X=`(echo "abc"; echo "def")` echo $X which gives the expected output:
abc def Now the internet tells me backticks are out $() is what I need to use, but when I try:
export X=$((echo "abc"; echo "def")) X is not set and I get the error:
bash: echo "abc"; echo "def": syntax error: invalid arithmetic operator (error token is ""abc"; echo "def"") What am I doing wrong?
The $(( … )) syntax is an arithmetic expression.
What is missing is a space between the $( and the following (, to avoid the arithmetic expression syntax.
The section on command substitution in the shell command language specification actually warns for that:
If the command substitution consists of a single subshell, such as: $( (command) ) a conforming application shall separate the "`$(`" and '`(`' into two tokens (that is, separate them with white space). This is required to avoid any ambiguities with arithmetic expansion. 
`...` and $(...) start a subshell anyway, so the inner (...) are not needed (waste a process). You'd need the space in things like $( (...); (...) ) for instance (where the inner subshells may be needed). Try export X="$(echo "abc"; echo "def")"
ksh and bash being the only exceptions).