18

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?

Improve this question
0

2 Answers 2

Reset to default
28

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.
Improve this answer
1
  • 21
    It should be noted that `...` 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).
    – Stéphane Chazelas
    Commented Jan 29, 2014 at 11:46
15

Try export X="$(echo "abc"; echo "def")"

Improve this answer
2
  • Thanks this does work, but requires more editing than the other solution.
    – Harold
    Commented Jan 29, 2014 at 11:37
  • 2
    +1 for including the quotes that are needed in most POSIX shells (ksh and bash being the only exceptions).
    – Stéphane Chazelas
    Commented Jan 29, 2014 at 11:40

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .