What does expression begins with dash mean in shell assignment?

Question

I tryied using JAVA_OPTS=-Xms1024m -Xmx1024m ... in a script file. It shows error like "-Xmx1024m: command not found".

After quoting the assignment in double or single quotes, the error disappear.

And I found that the error is caused by the first dash sign. Because all assignments like "a=-bb xx yy ..." will raise the same error: it tried to run the xx as a program. It seems that shell recognize the second word as a command if the first word begins with a dash sign as long as there's no quotes in the assignment expression.

So my question is: what does assignment expression begins with dash sign mean in shell? I'm using bash as default.

Answer 1

First of all, you really should be quoting your variables:

JAVA_OPTS="-Xms1024m -Xmx1024m"

Because the variable was unquoted,in your original command only JAVA_OPTS=-Xms1024m was interpreted as variable assignment,and -Xmx1024m as a command name. This is often refereed to as setting up environment variable for command. For instance,

LC_ALL=C  date +%s

would modify date's environment with setting LC_ALL=C ( that is running date with english locale, even if you have something else set). Note the difference here - LC_ALL=C doesn't have spaces, so word splitting doesn't occur, which is what's happening in case of JAVA_OPTS=-Xms1024m -Xmx1024m

Second, the dash signifies short style of options. There's also long style, for instance bash --posix or ls --format=long. Why dash ? Historical convention. It should be noted that POSIX standard requires only short version of the command-line arguments.