[SOLVED] [bash] if statement options.

RaptorX · 08-14-2009, 05:37 PM

I am reading right now Advanced Bash Scripting.

in there there is a code that looks like this:

Code:

if [ -z "$1" -o ! -r "$1" ]
then
  directory=.
else
  directory="$1"
fi

what i dont understand is the option "-r"

and when i look for "bash if tests" or "bash if comparison" I get nothing specific for that option.

Can you guys clarify the purpose of "-r" to me, and give me a list of the if options if possible.

bigearsbilly · 08-14-2009, 05:59 PM

Code:

 -r FILE
	      FILE exists and read permission is granted

it's actually man test you want.

[ ] is a synonym for /usr/bin/test

blackhole54 · 08-14-2009, 06:20 PM

Quote:

Originally Posted by bigearsbilly

it's actually man test you want.

[ ] is a synonym for /usr/bin/test

I never realized before that /usr/bin/test existed. But test is also a bash builtin. It is described in bash's man page under SHELL BUILTIN COMMANDS. The description for test describes how to combine "primaries" and further advises the reader:

Quote:

Expressions are composed of the primaries described above under CONDITIONAL EXPRESSIONS.

So in the bash man page you would need to look under CONDITIONAL EXPRESSIONS for the information you seek. Admittedly, there is so much information in the bash man page that it can be a real pain to navigate.

EDIT: I probably should have mentioned that bigearsbilly's answer to what -r means in this case is still correct.

David the H. · 08-14-2009, 06:31 PM

Actually, bash uses its own built-in test (try "type test" or "type ["). But the operation of it is almost exactly the same as the external tool, so the man page still works as a reference.

blackhole54 · 08-14-2009, 07:21 PM

As a builtin, you can also use: help test

RaptorX · 08-15-2009, 05:51 AM

Thanks guys for all your replays... I did read in abs that ____[dont remember what now.] was synonymous of "test" but I didnt realize that it was EVERYTHING between []...

now all makes sense!

Thanks for the help, now i have much to read.

pixellany · 08-15-2009, 07:56 AM

No....it is the "[" that is synonymous with "test". IIRC, the closing "]" is not required.

The stuff between "[]" is the arguments to test...

Yes, I am a nitpicker........

catkin · 08-15-2009, 08:09 AM

Hello pixellany

I'm glad you brought that up because it has been an unresolved uncertainty for me for a long time; unresolved because IIRC [[ ]] has advantages over [ ] leaving no reason to use [ ] and hence no reason to understand it. The pragmatism of a young man in a hurry!

The closing ] is necessary, though. According to the Gnu bash Reference for test, "When the [ form is used, the last argument to the command must be a ]". That's kludgy!

blackhole54 · 08-15-2009, 06:07 PM

Quote:

Originally Posted by catkin

... unresolved because IIRC [[ ]] has advantages over [ ] leaving no reason to use [ ] and hence no reason to understand it.

I am not sure if there is anything [...] can do that [[...]] cannot or not (a lot of nuances that would need to be checked out). If you have something that works for you, fine. But people who read this thread, particularly those that are just starting out, should realize that [[...]] is not a drop-in replacement for [...]. Of the top of my head, the == operator is interpreted differently and the 'logical and" and "logical or" operators are expressed differently. There may be other differences.

chrism01 · 08-17-2009, 12:30 AM

For [[ ]] vs [ ] see here: http://tldp.org/LDP/abs/html/testcon...ml#DBLBRACKETS
The former is more robust/allows operators that will have syntax errs in the latter.
Handles null args better.