shell 脚本中的变量赋值约定

Question

In bash script it seems to not allow a space on either side of the = sign. For example:

# bad
a= "ok"
# bad
a ="ok"
# bad
a = "ok"
# good
a="ok"

Is this the same across all shell languages, or bash-specific? Out of curiosity, why does it not allow a space next to the assignment operator?

Answer 1

For specific shells, you'll need to look up the documentation for that shell. For bash , it's a requirement that there be no spaces in that form (note that there are no spaces on either side of the = ):

A variable may be assigned to by a statement of the form name=[value] .

One reason why this may be the case is that you can use command-temporal assignments where a variable is set only for the duration of a command:

pax:~> x=314159 ; echo $x
314159
pax:~> x=42 bash -c 'echo $x'
42
pax:~> echo $x
314159

Allowing spaces in the assignment would make it a little difficult to figure out where the assignment finished and where the command started. For example:

x= echo echo hello

Should this set x to echo then run echo hello or should it set x to "" and then run echo echo hello ?

If you want your assignment to be nicely formatted with spaces, you can use the ((...)) arithmetic evaluation:

pax:~> (( val = 117 / 3 )) ; echo $val
39