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Consider the following expression evaluations:
$ echo .
.
$ echo .[]
.[]
$ echo .[].
.[].
$ echo .[].[]
.. # <- WAT??
$ echo .[].[].
.[].[].
$ echo .[].[].[]
.[].[].[]
Can someone explain why .[].[] has this special behavior?
(Tested in bash 3.2.57(1)-release (x86_64-apple-darwin18) and 4.4.23(1)-release (arm-unknown-linux-androideabi).
I suspect it has something to do with .. being a valid "file" (the parent directory). But then why doesn't e.g. .[]. produce the same result?
650
You can use expr utility to evaluate expressions in both of the command line or shell script. This may not be the best way to find out if it is Friday, but it seems to work. It's more of an exercise in xargs.
$() means: "first evaluate this, and then evaluate the rest of the line". Ex : echo $(pwd)/myFile.txt. will be interpreted as echo /my/path/myFile.txt. On the other hand ${} expands a variable.
Bash expression is the combination of operators, features, or values used to form a bash conditional statement. Conditional expression could be binary or unary expression which involves numeric, string or any commands whose return status is zero when success.
That's because [] creates a character class. Normally, ] must be escaped (backslashed) to be included in the class, but you don't have to escape it if it immediately follows the opening bracket. Therefore, [].[] in fact means [\].[] which matches any of the characters ., ], and [.
You can verify it by creating files named .] and .[.
touch .\[ .\]
echo .[].[] # .. .[ .]
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