Escape sequences in bash scripts?

[Kosmaj]

`foo` is the stdout of running function "foo". It's consumed by "if".
It's the same like
z=`foo` # nothing is being printed, z='aa'

To get the "aaUnequal" output, execute foo once more:
foo
if [ `foo` = 'aa' ]; then
...

Originally Posted by CRGreathouse

It's not actually relevant anymore -- I redesigned the script -- but here's a testcase.

foo () {
	echo -n 'aa'
}
if [ `foo` = 'aa' ]; then
	echo Equal
else
	echo Unequal
fi
exit 0

It displays "Equal" instead of "aaUnequal".

[CRGreathouse]

Originally Posted by Kosmaj

1) To catch the return value from a function, refer to $? immidiately after calling it.

Great, that simplifies a lot of things.

Originally Posted by Kosmaj

2) In bash you use single "=" to compare strings. That's why your code works. You can also use "==" but there is something tricky about it, it has different meaning in single brackets and in double brackets (will find you a reference later). It's the best to avoid it for the time being.

I know about single and double brackets (I use both, correctly, in my script). I'm not sure why you call out my use of = for string comparison: my task clearly calls for string comparison and I use the string comparison operator... is it that hard to believe I'd use it correctly?

Because I didn't know how to access the return value I was using the backtick hack (passing the whole comparison rather than the result of the comparison), but that won't be an issue anymore.

Originally Posted by Kosmaj

3) Binary valules in echo can confuse bash, that's why it's better to use "od" and do string comparisons. Even better, execute "file" and perform string comparisons on its output.

Honestly I still don't understand how to use od. And I'm not sure what you mean by string comparisons, since I'm doing string comparisons now.

[Kosmaj]

(2) was directed to Alex in post #9, not to you.

(3) Try "od -aN4 a.png", it returns "P N G" with spaces. You can remove spaces with bash (or sed) and get "PNG". Ditto for jpg (look for JFIF), with "-aN10". You will have to select a suitable max number of bytes to find the magic number. But that's exactly what "file" does:
z=`file $input_file`

and check $z for all doc types you want to handle. Type "man file" for details. Magic numbers are kept in another file that you can modify if you want to add new file types. No need to rediscover the wheel. :-)

[CRGreathouse]

Originally Posted by Kosmaj

(2) was directed to Alex in post #9, not to you.

Ah! No wonder I was confused. I pick up programming languages much faster than human subtleties (doubly so on forums where there's no intonation).

Originally Posted by Kosmaj

(3) Try "od -aN4 a.png", it returns "P N G" with spaces. You can remove spaces with bash (or sed) and get "PNG". Ditto for jpg (look for JFIF), with "-aN10". You will have to select a suitable max number of bytes to find the magic number.

Hmm, okay. I can see how that would be handy.

Originally Posted by Kosmaj

But that's exactly what "file" does:
z=`file $input_file`

and check $z for all doc types you want to handle. Type "man file" for details. Magic numbers are kept in another file that you can modify if you want to add new file types. No need to rediscover the wheel.

I found file easy enough to understand (sed and od still have me a bit confused). But actually my project is far overdesigned for what I actually need -- the main purpose was just to write something nontrivial as an exercise. I think I've basically done that, though I may play with my toy a bit more.

Any suggestions on where to go from here? Any other fun toys I should learn? I've been using regular expressions longer than linux, so don't say grep. But as you might imagine, most things are still mysterious to me. I have two decades of experience on DOS/Windows but only one month of experience on Ubuntu...


Edit: I don't think I told you that I appreciated your efforts to explain things to me. Thanks: it all makes sense now!

[jrk]

See if you can make a Mersenne prime-prover using only Bash built-in commands.

[akruppa]

Originally Posted by Kosmaj

2) In bash you use single "=" to compare strings. That's why your code works. You can also use "==" but there is something tricky about it, it has different meaning in single brackets and in double brackets (will find you a reference later). It's the best to avoid it for the time being.

Right... the man page lists "==" as the comparison operator with "=" a synonym for Posix compliance, but the "test" command understands only "=".

Note to self:

an introductory text like Kosmaj posted would probably be worthwhile.

Alex