Scenario

I need to check if a file is a subset of another file into the CI pipeline. Thus bash script is chosen since it's performant and we don't need to install extra dependencies into the CI pipeline.

1. diff
   The first command comes to my mind is diff, which is a really powerful command telling the difference between two files.

   However it's too powerful. diff "predicts" which line needs to be changed in order to make the two files identical; which is unnecessary for my use case.

   E.g. (Example from GeeksToGeeks)
   

   $ cat a.txt
   Gujarat
   Uttar Pradesh
   Kolkata
   Bihar
   Jammu and Kashmir
   
   $ cat b.txt
   Tamil Nadu
   Gujarat
   Andhra Pradesh
   Bihar
   Uttar pradesh
   
   $ diff a.txt b.txt
   0a1
   > Tamil Nadu
   2,3c3
   < Uttar Pradesh
    Andhra Pradesh
   5c5
    Uttar pradesh
   

2. comm
   Without further digging into diff, I found another command comm which is simple and just fit in my use case.

   comm returns 3 columns:

   • first column contains names only present in the 1st file
   • second column contains names only present in 2nd file
   • the third column contains names common to both the files

   E.g. (Example from GeeksToGeeks)
   

   // displaying contents of file1 //
   $cat file1.txt
   Apaar 
   Ayush Rajput
   Deepak
   Hemant
   
   // displaying contents of file2 //
   $cat file2.txt
   Apaar
   Hemant
   Lucky
   Pranjal Thakral
   
   $comm file1.txt file2.txt
                   Apaar
   Ayush Rajput
   Deepak
                   Hemant
           Lucky
           Pranjal Thakral
   

   And to check if one file is a subset of another file, we just need the 1st column. We could just do -23 to neglect the 2nd and 3rd column. I.e.
   

   comm -23 file1.txt file2.txt
   

Conclusion

At last, I just end up with this simple bash script to check the subset condition:

#!/bin/bash    
SUBSET="<subset_file_path>"
SUPERSET="<superset_file_path>"
CHECK=$(comm -23 <(sort $SUBSET | uniq ) <(sort $SUPERSET | uniq ) | head -1)

if [[ ! -z $CHECK ]]; then
  echo "Detected extra line in $SUBSET and not in $SUPERSET."
  echo $CHECK
  exit 1
fi

Added the extra sort and uniq commands there just to make sure we're comparing two sorted and deduplicated files.