Why does escaping greater than or less than work in bash string comparisons?

Question

If I try something like

if [ a < b ]

in bash it interprets it as trying to pipe one file to another. If I want to do a string comparison I know I should escape the operator like this:

if [ a \< b ]

I know I could use [[ to get around this but my question is, why does escaping like this work?

Answer 1

In essence, [ is a program that does what you want when its arguments are a, <, b, and ]. (In fact, Bash implements [ as a builtin, but it implements it as if it were a separate program, rather than treating it specially. This is for compatibility with systems where [ is literally a separate program on the path.)

So, just as writing echo a \< b or echo a '<' b or echo a "<" b lets you call echo with the arguments a, <, and b (so as to print a < b), writing [ a \< b ] or [ a '<' b ] or [ a "<" b ] lets you call [ with the arguments a, <, b, and ] (so as to test whether a is less than b).

Answer 2

[ is an ordinary command, so if you don't escape it, it is parsed as a command with 2 arguments, a and ]. The < b is processed as an input redirection and removed from the command line before [ is executed. That is, [ a < b ] and [ a ] < b are equivalent.

< is not a standard operator supported by [; bash's implementation allows for such comparisons, but if you are relying on one non-standard extension, you may as well rely on [[ instead.