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quck question about expr and (( ))

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# 1  
Old 04-04-2009
quck question about expr and (( ))
Hi

just a quick question, i have always used

Code:
var=`expr $var + 1`

to increment a variable or to perform arrithmetic. however, i found another script which did this

Code:
(( var = var + 1 ))

which worked exactly the same ? ... can anybody tell me where I can read up on this, and where I can find docs ? i tried googling and i dont even know where to start ...what is this method called ?

likewise.. whats the deal with using double square brackets on the ' test' command .. ie

Code:
   if [[ $account = "*hello*" ]] ; then

i looked at the man page for test but couldnt find anything about the use of double brackets ?


Can anybody help me grasp these two double bracket methods or point me to some docs where i can read about where and how to use them ?

Any help would be great
# 2  
Old 04-04-2009
Quote:
Originally Posted by hcclnoodles
Hi

just a quick question, i have always used

Code:
var=`expr $var + 1`

to increment a variable or to perform arrithmetic. however, i found another script which did this

Code:
(( var = var + 1 ))

which worked exactly the same ? ... can anybody tell me where I can read up on this, and where I can find docs ? i tried googling and i dont even know where to start ...what is this method called ?

likewise.. whats the deal with using double square brackets on the ' test' command .. ie

Code:
   if [[ $account = "*hello*" ]] ; then

i looked at the man page for test but couldnt find anything about the use of double brackets ?


Can anybody help me grasp these two double bracket methods or point me to some docs where i can read about where and how to use them ?

Any help would be great



The double brackets are called let command.
And you can find lot of documentation on Internet
# 3  
Old 04-04-2009
sorry which one is the let command the (( )) or the [[ ]] ?

Cheers
# 4  
Old 04-04-2009
Quote:
Originally Posted by hcclnoodles
sorry which one is the let command the (( )) or the [[ ]] ?

Cheers

(( ))
# 5  
Old 04-04-2009
thanks siquadri

ok from a bit of further investigation on the square bracket usage

it seems to enable the use of regular expressions / wildcards inside the test condition

i.e

Code:
if [[ $account = "*hello*" ]] ; then

will be true if "account" = "helloworld" or "wellhello"

but, with single brackets only absolute matches to the condition will work and you cannot use regexes, so this will not work if "account" = "helloworld"

Code:
if [ $account = "*hello*" ] ; then

but this will
Code:
if [ $account = "helloworld" ] ; then

I still have no idea what this method is called, but hey .. i know what it does Smilie
# 6  
Old 04-04-2009
Further to hccnoodles.
All of your questions are answered in "man ksh".
Tip: To find a suitable search key for your "man" pager, try using grep to give a decent phrase to search on.

man ksh|grep "\[\["
man ksh|grep "(("


The "man test" page is equivalent to the condition between the single square brackets [ ... ] . Most modern shells have "test" built-in so if in doubt read the shell manual.

Some regular expressions between double square brackets [[ ... ]] actually have the same syntax as "test".
# 7  
Old 04-04-2009
Quote:
Originally Posted by hcclnoodles
thanks siquadri

Code:
if [[ $account = "*hello*" ]] ; then

will be true if "account" = "helloworld" or "wellhello"
Quick word about this, and this is true of Bash, so I'm not sure if it is true in ksh...

You would need to use the "=~" operator, not just "=" to use the regular expressions.

Code:
[[ "stuff" =~ ".*tuff" ]] && echo yes
yes

Notice it is standard regular expressions, not shell wildcards (meaning .* matches any character, not *)
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