How do different operators deal with a given string

Question

I want to read two strings as parameters to bash script, these strings are meant to be extensions, then I want to replace the extension of files with the first extension to the second extension.

The best I could think about/find so far is:

#!/bin/bash
first=1ドル
second=2ドル
for files in *.1ドル
do
 mv "$files" "${files%.1ドル}.2ドル"
done

Now, lets assume 1ドル is cpp, and 2ドル is C

My "doubts" are:

1. would .1ドル be equal to .cpp?
2. Is *.1ドル equal to say: *.cpp?
3. Is line 6 equal to mv "$files" "${files%.cpp}.C"?
4. Would the previous code handle file names with spaces (like: my file.cpp), or dots (like my.file.cpp)?

I know I could've said the first 3 question in one question, but I need to understand what is exactly happening, and how will each operator (./*/%) deal with a given string in a parameter.

Answer 1

You have two typo's writing & instead of $ for addressing variables, but otherwise it should work.

Also, filenames with spaces will work, since you quoted them correctly.

Minor detail: I'd recommend renaming the variable $files to $file, since it always contains only a single filename from the list that is looped over.

• Joseph R.: I would also add -- to mv (as in mv -- "$file" ...) to ensure it can handle file names starting with a -.
• user2750466: @ Joseph R Good point, is there any other options that you recommend to add to 'mv'? or that the way it is written can already handle all kinds of of file names?
• Joseph R.: @user2750466 I'm no expert myself but I think this should cover most edge cases.

Answer 2

Your code is correct with the minor note by Joseph R. about files containing a dash. However lines 2 and 3 seem redundant, because you don't use the variables first and second in the snippet.

To alleviate your "doubts" (assuming 1ドル is cpp, and 2ドル is C):

1. The . is not an operator here, so bash does just a parameter expansion on the 1ドル. So the answer is yes, .1ドル is equal to .cpp
2. Parameter expansion has precedence over pathname expansion (link), meaning *.1ドル is equal to *.cpp
3. Again, the line does what you suppose it does, because first the 1ドル is expanded to be handed to the pattern matching algorithm evoked by the %
4. The double quotes prevent bash from doing field splitting, which is what usually causes problems with filenames containing spaces

• user2750466: I have a quite stupid question: would the program work correctly also if 1ドル and 2ドル were replaced by first and second, I mean if I did for example: .first or *.first, that would give a similar result, right?
• Jaap Eldering: If you use $first and $second then yes. The reason is that the variable assignments from 1ドル and 2ドル no word splitting is performed either.