Compare content of 2 files using diff – no such file or directory problem

May I know how can I compare content of two files? I tried these codes below but I get “sh: diff: No such file or directory” message.

plink.exe 10.3.0.121 -l admin -pw abc123 -batch sh diff -s “/share/MD0_DATA/BackupData/test1.txt” “/share/MD0_DATA/BackupData/test2.txt”

plink.exe 10.3.0.121 -l admin -pw abc123 -batch sh diff -s /share/MD0_DATA/BackupData/test1.txt /share/MD0_DATA/BackupData/test2.txt