Q:

What is the purpose of the colon after the flag in some shell command options?

See this example, taken from the O'Reilly book Classic Shell Scripting:
sort -t: -k1,1 /etc/passwd

Why is there a : after the t? It doesn't seem to be needed, nor documented in man, but I keep seeing it in examples.

A:

It's just the argument to the -t option, specifying that fields are separated by colons in the input file.

