Novice Korn Shell WantAbe Programmer

metallica1973 · 01-11-2008, 09:56 AM

This is part of a nested if statement and what I am trying to do is say'" If there is data that has a '"v0" or a "v2" or "v3" in the variable $ver then print this or else print all other variables except $ver"

PHP Code:



 if [[ "$ver" = "v[0-2]"]] 
   then 
     printf "%-24s%-8s%-8s%-14s%-6s\n"  \ 
          $ip_address $ver $ping_able $telnet_able $log_able >> $check_log 
   else 
     printf "%-24s%-8s%-8s%-6s\n"  \ 
          $ip_address $ping_able $telnet_able $log_able >> $check_log

Or can I say," If there is anything in $ver then print the whole thing or else print everything without $ver - help

makyo · 01-12-2008, 09:38 AM

Hi.

I think your syntax is incorrect for matching. Here's an example similar to yours:

Code:

#!/bin/bash -

# @(#) s1       Demonstrate == for matching within [[, ]].

show_me()
{
  local ver="$1"
  echo " $FUNCNAME: looking at ver, $ver"
  if [[ "$ver" = "v[0-2]" ]]
  then
    echo " single \"=\", variable ver, $ver, matched."
  fi
  if [[ "$ver" == v[0-2] ]]
  then
    echo " double \"=\", variable ver, $ver, matched."
  fi
}

echo
echo " calling with v3"
show_me v3

echo
echo " calling with v0"
show_me v0

exit 0

Producing:

Code:

% ./s1

 calling with v3
 show_me: looking at ver, v3

 calling with v0
 show_me: looking at ver, v0
 double "=", variable ver, v0, matched.

When all else fails, reading the documentation:

Quote:

[[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the
conditional expression expression. Expressions are composed of
the primaries described below under CONDITIONAL EXPRESSIONS.
Word splitting and pathname expansion are not performed on the
words between the [[ and ]]; tilde expansion, parameter and
variable expansion, arithmetic expansion, command substitution,
process substitution, and quote removal are performed.

When the == and != operators are used, the string to the right
of the operator is considered a pattern and matched according to
the rules described below under Pattern Matching. The return
value is 0 if the string matches or does not match the pattern [sic],
respectively, and 1 otherwise. Any part of the pattern may be
quoted to force it to be matched as a string

-- excerpt from man bash

Note also this is an expression for 0-2, not including 3.

Best wishes ... cheers, makyo

makyo · 01-12-2008, 09:50 AM

Hi.

I just noticed that you were asking about the Korn shell, but I replied with a bash example.

I changed "bash" to "ksh" in the shebang and the results were the same as the bash example, i.e. the double "=" and omitted quotes allowed the match to succeed. I used pdksh.

Apologies for the confusion ... cheers, makyo

sundialsvcs · 01-13-2008, 07:09 PM

Y'know, suit yourself doing things like this with shell programming... but as for me, "that's what a real programming language is for." Perl, Python, and probably PHP (not to mention the special-purpose but very handy "awk") are probably all right there at your disposal.

Sure, this is my "JM2CW" and absolutely nothing more, but my take on the situation is that shell scripting is best suited to what is "a very simple command," whereas a general-purpose language like one of the above is purpose-built for "programs." When you run them, thanks to the "shebang" #! syntax, it's impossible to tell the difference.

metallica1973 · 01-14-2008, 05:26 AM

I somewhat understand your example. Can you give me another example that easier to understand. You dealing with a real novice.

PHP Code:



then 
    echo " single \"=\", variable ver, $ver, matched." 
  fi 
  if [[ "$ver" == v[0-2] ]] 
  then 
    echo " double \"=\", variable ver, $ver, matched."

single, double ?

makyo · 01-14-2008, 06:19 AM

Hi, metallica1973.

There were 2 issues with your code. First, the "==". That informs ksh that you want the right-hand-side to be interpreted as a pattern. You understood the pattern concept, because you used the character-class symbols "[]".

However, the use of quotes causes the enclosed characters to be interpreted as plain characters, not as a pattern.

So in the function I created, I first used what you had -- a single "=" and the quotes, followed by another test with "==" with no quotes. In the function calls later in the script, the first test failed, the second test succeeded.

The bottom-line is to use "==" and omit quotes in order to cause pattern-matching.

Code:

right:
[[ variable == pattern_string ]]

wrong:
[[ variable = "pattern_string" ]]

I have not checked all the differences between ksh and bash, but most people get a lot of good information from http://www.tldp.org/LDP/abs/html/index.html , and for the "[[ ]]" sequence specifically from http://www.tldp.org/LDP/abs/html/tes...ucts.html#EX11 , which discusses the "[ ]" sequence, then shows how "[[ ]]" improves on that. In fact, it notes that "[[ ]]" was adapted for bash from ksh88.

There may be some confusion between "==" and "=~" in bash3, unfortunately. The "=~" is intended to allow extended regular expressions to be used in tests. The "=~" operator does not exist in pdksh, but I don't know about ksh. The operator was designed to follow the use in perl.

Best wishes ... cheers, makyo

metallica1973 · 01-14-2008, 06:25 AM

I have a better understanding, thanks.