topic Re: Conditional Expressions doubt in Operating System - HP-UX

Conditional Expressions doubt

Ester Saez — Mon, 05 Apr 2004 09:22:43 GMT

I would like to know what is the difference (if any) between using "-h" and "-L" to know if a file is a symbolic link. And also the difference between using "-a" and "-e" to know if a file exists. On line help doen't say if there is or if there is not a difference, so I like to be sure.

Re: Conditional Expressions doubt

Jeroen Peereboom — Mon, 05 Apr 2004 09:37:56 GMT

Ester,

what's the context?

If I look at 'test' manpage, I cannot find -L, -e and -a.

So, what's the language are you 'writing' in?

JP

Re: Conditional Expressions doubt

Jeff_Traigle — Mon, 05 Apr 2004 09:40:03 GMT

I know it's Monday and I might be missing something, but what are you using to test this that you see these flags in the man page? test(1) only shows -h for symbolic links and -a for binary AND operator... no -L or -e at all... file existence is -f.

Re: Conditional Expressions doubt

A. Clay Stephenson — Mon, 05 Apr 2004 09:44:56 GMT

You really have to identify the shell that you are using for a complete difference but if I assume that you are using the POSIX shell then there is no difference between -L and -h as both will test if a file is a symbolic link -- but only if using the shell's internal test operator. -L means nothing to the test command. Also, -e is only known to the shell's internal test operator and -a is actually an AND operator to the test command.

The best way to avoid all this confussion is to adopt the "double brackets" approach:

This version will always use the shell's internal test operator (and is thus more efficient):
if [[ $(a) -gt $(b) && -f "${MYFILE}" ]]
then
fi

This equivalent version will call the external test command:

if [ $(a) -gt $(b) -a -f "${MYFILE}" ]
then
fi

Note that the syntax is a little different in the two cases.

If you must run under the Bourne shell for portability then you should always use the test command and stay within its more limited set of operators but if you know you will always be ruunning on a Korn or POSIX shell or bash then I would favor the internal test operator.

Re: Conditional Expressions doubt

A. Clay Stephenson — Mon, 05 Apr 2004 09:50:03 GMT

Oops, the $a's and $b's should have had {}'s around them instead of ()'s:

if [[ ${a} -gt ${b} && -f "${MYFILE}" ]]
then
fi

Re: Conditional Expressions doubt

Ester Saez — Mon, 05 Apr 2004 09:50:53 GMT

Sorry for being so cryptic. I am talking abour Shell Script, using sh shell. "man sh-posix" sayig that -h and -L can be used to know if a file is a symbolic link nad that -a and -e can be used to know if a file exists.
Check this example:

"pru.sh" 17 lines, 183 characters
file="/tmp/pru/sbinsh"
if [ -a ${file} ]; then
echo "-a"
fi
if [ -e ${file} ]; then
echo "-e"
fi
if [ -h ${file} ]; then
echo "-h"
fi
if [ -L ${file} ]; then
echo "-L"
fi

# ll
total 2
-rwx------ 1 root sistemas 183 Apr 5 16:46 pru.sh
lrwxrwxrwx 1 root sistemas 8 Apr 5 16:43 sbinsh -> /sbin/sh
# ./pru.sh
-a
-e
-h
-L

Re: Conditional Expressions doubt

Sanjay Kumar Suri — Mon, 05 Apr 2004 09:59:42 GMT

There is no difference as your output also proves.

sks