Bash set subtraction

Question

How to subtract a set from another in Bash?

This is similar to: Is there a "set" data structure in bash? but different as it asks how to perform the subtra

Answer 1

comm -23 <(command_which_generate_N|sort) <(command_which_generate_M|sort)

comm without option display 3 columns of output: 1: only in first file, 2: only in second file, 3: in both files. -23 removes the second and third columns.

$ cat > file1.list
A
B
C
$ cat > file2.list
A
C
D
$ comm file1.list file2.list 
        A
B
        C
    D
$ comm -12 file1.list file2.list # In both
A
C
$ comm -23 file1.list file2.list # Only in set 1
B
$ comm -13 file1.list file2.list # Only in set 2
D

Input files must be sorted.

GNU sort and comm depends on locale, for example output order may be different (but content must be the same)

(export LC_ALL=C; comm -23 <(command_which_generate_N|sort) <(command_which_generate_M|sort))