As a beginner, I did not find answered anywhere, the rules about spacing (grammar) and parsing.
For example.
Can I do
if [$# -eq 2] ;
then
llll
fi
or must I always have a blank or two between the objects, as
if [ $# -eq 2 ] ;
then
llll
fi
and the second related question is about the difference between
if [[ $# -eq 2 ]] ;
then
wafwaf
fi
The concern I have is about spacing before/after [
, ]
.
No searching has provided me with a set of rules.
Spaces are required after [
and before ]
.
[
is actually the name of a command, an alias for test
. It's not a special symbol, it's just a command with an unusual name.
$ help '['
[: [ arg... ]
Evaluate conditional expression.
This is a synonym for the "test" builtin, but the last argument must
be a literal `]', to match the opening `['.
Because it's an ordinary command name and not a special character, a space is required after the [
. If you omit the space and write [foo
the shell will search the $PATH
for a command named [foo
.
$ [ foo = foo ] && echo true
true
$ [foo = foo] && echo true
[foo: command not found
For readability's sake, [
expects its last argument to be exactly ]
. Being an ordinary command-line argument, ]
must have a space before it. If there's no space then the bracket will become the last character of the previous argument, and [
will complain about its last argument not being ]
.
$ [ foo = foo]
bash: [: missing `]'
$ [ foo = 'foo]'
bash: [: missing `]'
[[
is a bash enhancement with more features than [
, namely saner handling of unquoted variable names. It requires the a space on both ends, same as [
. However [[
is in fact special shell syntax and is parsed differently. It's not an "ordinary command" the way [
is.
For a detailed explanation of the difference between [
and [[
, see:
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