传递2个参数时tr函数的不良行为

Question

I would like to change a parameter 1 by a parameter 2 but the output is not correct

 #!/bin/sh 
    getline="hello mr XXX";    
    name="NAME";
    echo $getline  | tr "XXX" "$name" ;

the output is : "hello mr MMM"

do you have an idea ?

Answer 1

tr command maps character to character on a 1-to-1 basis ie X in input is mapped to M in replacement (the last mapping).

to replace XXX with the value of variable NAME you can use sed or parameter substitution like this:

$ sed 's/XXX/'"$name"'/g' <<< "hello mr XXX"
hello mr NAME

OR

$ echo ${getline//XXX/$name}
hello mr NAME

Answer 2

tr expects a 1:1 mapping between the input/ouput sets:

tr XXX NAME
   123 1234

Since you have THREE identical chars in the input, only the last one is used for the mapping, and X #3 maps to character #3 in the "replacements" parameter, which happens to be M

If you'd expand the command out a bit more:

$ echo hello mr XXXYZ|tr "XXXY" "NAME"
hello mr MMMEZ

Y maps to E , because they're both the 4th char in the in/out sets.