diff

# To view the differences between two files:
diff -u <file-1> <file-2>

# To view the differences between two directories:
diff -ur <dir-1> <dir-2>

# To ignore whitespace:
diff -ub <file-1> <file-2>

# To ignore blank lines:
diff -uB <file-1> <file-2>

# To ignore the differences between uppercase and lowercase:
diff -ui <file-1> <file-2>

# To report whether the files differ:
diff -q <file-1> <file-2>

# To report whether the files are identical:
diff -s <file-1> <file-2>

# To diff the output of two commands or scripts:
diff <(command1) <(command2)

# To generate a patch file from two files:
diff -Naur <file-1> <file-2> > <patch-file>