音译行为（shell 中的 tr 程序）

Question

I understand the basic behavior of tr but am confused when sets and sequences become involved. Some examples:

$ tr 'a' '[0]'
aaaaaa
[[[[[[
$ tr 'a-z' '[0-9]'
abcdef
[01234
$ tr 'a-z' '[*]'
abcde
[*]]]
$ tr 'a-z' '[\n]'
abcdef
[
]]]]
$ tr 'a-z' '[\n*]'
abcdef






$ tr 'a-z' '\n'
abcdef

It seems to me that in some cases (like with the [\n*] ) it correctly interprets the set, but in other cases, it seems to stall at the closing bracket and continuously output that. I had thought that sets [] mean "any one of the enclosed characters", and that * would not be treated as a special char within the set. It seems like this holds true in some cases but not others. What is going on here?

Also, it seems to me that tr 'az' '\n' performs the same action as tr 'az' '[\n*] . Is there some nuance I'm missing?

Answer 1

[ only have special meaning if you are providing the set in the form [CHAR*] , [CHAR*REPEAT] , [:POSIXCLASS:] , or [=CHAR=] .

From your examples, only tr 'az' '[\n*]' is interpreted as set ("in SET2, copies CHAR until length of SET1"); all other examples are interpreted literally.

So tr 'af' '[*]' is identical to writing tr 'abcdef' '[*]' and will translate a→[, b→*, c→], d→], e→], f→].

tr 'af' '[\n*]' however is identcal to writing tr 'abcdef' '\n\n\n\n\n\n' and will replace every character with a newline. For a single character, this is already the default with tr , so you could have simply written tr 'af' '\n' as you have found out with your last example.