Re: Off Topic Scripting Question

From: Bill Thompson (bill.thompson@GOODYEAR.COM)
Date: Fri Jul 19 2002 - 11:30:30 EDT

My understanding of the file attribute operators from page 131 of
O'Reilly's Learning the Korn Shell was that they applied to internal Korn
shell evaluations which are accessed by using the double square brackets
([[]]).

According to page 126 when you use the single square brackets ([]) you are
calling the external command /bin/test (this can be easily proved):

unset A

if [[ -z $A ]]; then print true; fi
true

if [ -z $A ]; then print true; fi
ksh: test: 0403-004 Specify a parameter with this command.

Notice the error comes from "test".

The man page for test says "-a Binary AND operator". There is nothing in
the test man page for a simple "file exists".

My confusion is the code in question uses the single brackets for the
evaluation therefore the -a should be interpreted as a binary AND, not a
"file exists".

Is this an undocumented feature of test?

Bill Thompson
Sr UNIX Systems Administrator
The Goodyear Tire & Rubber Co.

Contains Confidential and/or Proprietary Information
May Not Be Copied or Disseminated Without Express Consent of The Goodyear
Tire & Rubber Company.

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 ----- Original Message -----
 From: Steve Thornhill
 Newsgroups: bit.listserv.aix-l
 To: aix-l@Princeton.EDU
 Sent: Friday, July 19, 2002 10:09 AM
 Subject: Re: Off Topic Scripting Question

 Excerpt from page 131 of O'reilly's Learning the Korn Shell

 Operator True if...
 -a file file exists
 -d file file is a directory
 -f file file is a regular file (i.e., not a directory or other special
 type of file)
 -r file You have read permission on file
 -s file file exists and is not empty
 -w file You have write permission on file

 ..... + a few more.

 Any help?

 Steve Thornhill - IBM CLS
 Int - 255104
 Ext - 023 92 565104
 Mob - 07778 289770
 Email/Sametime - steve_thornhill@uk.ibm.com

 Bill Thompson <bill.thompson@GOODYEAR.COM>

                            Bill Thompson
                            <bill.thompson@GOODYEAR.CO To:
                            M> aix-l@Princeton.ED
                            Sent by: IBM AIX U
                            Discussion List cc:
                            <aix-l@Princeton.EDU> Subject: Off Topic
                                                       Scripting Question

                            19/07/2002 14:38
                            Please respond to IBM AIX
                            Discussion List

 This is a Borne/Korn shell question which is not AIX specific. Sorry if
 it's somewhat off topic.

 I was reviewing an old script and found the following conditional:

 if [ -a $filename ]; then ...

 My immediate reaction is that the -a should have been a -f

 While the -f is probably the more appropriate test, I did some
 experimenting at the command line...

 if [ -a ]; then echo true; else echo false; fi
 true

 if [ -a .profile ]; then echo true; else echo false; fi
 true

 if [ -a not_a_file ]; then echo true; else echo false; fi
 false

 -a is the binary AND which is typically used between two conditionals like
 this:
      if [ $A -lt $B -a $C = "OK" ]; then ...
 Assuming the contents of "$filename" is the name of a file (or directory),
 can somebody explain why "if [ -a $filename ]" returns TRUE if the file
 exists and FALSE if the file does not exist?

 BTW:
 if [ .profile ] or if [ not_a_file ] both return TRUE (which is what I
 would expect because the strings are not null).

 TIA

 Bill Thompson
 The Goodyear Tire & Rubber Company
 Contains Confidential and/or Proprietary Information.
 May Not Be Copied or Disseminated Without Express Consent of The Goodyear
 Tire & Rubber Company.

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