Shell Programming and Scripting

Code:

is_number()
 {
         echo $1|egrep '^[1-9][0-9]*$' 2>&1 1>/dev/null
         return $?
 }

why the following snippet always give an output as "no" and never "yes" whatever the parameter I give to function is_number?

Code:

if [[ `is_number 0100` ]]; then echo yes; else echo no; fi

In addition, the function is_number() is working fine as per the below command line:

Code:

linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # is_number 100
linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # echo $?
0
linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # is_number 0100
linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # echo $?
1
linux-hhaq:/apps/informatica/current/pmserver/tes/scripts #

Lose the brackets and backticks completely...

Code:

if is_number 0100; then echo yes; else echo no; fi

That's right, but why [[ ]] does not support `` ?

---------- Post updated at 09:31 AM ---------- Previous update was at 09:16 AM ----------

bro, what if I need to put multiple conditions in an if statement without using the brackets?
like:
if is_number $3 || $# -ne 3

As your function has all its outputs redirected to /dev/null, it outputs nothing.
so the test [[ `is_number 0100` ]] is interpreted as [[ ]] with nothing inside.

By the way, the function could be written: is_number() { echo $1 | egrep -q '^[1-9][0-9]*$'; }
---------- Post updated at 15:39 ---------- Previous update was at 15:35 ----------

Tthe first part doesn't need test (where the brackets stand for) bur the second, yes. Youi should write

Code:

if is_number $3 || [ $# -ne 3 ]
# or using arithmetic evaluation
if is_number $3 || (( $# != 3 ))

and the following test in bash:
can anyone could explain the differences of the behaviors?

linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # if [[ `echo` ]]; then echo yes; else echo no; fi
no
linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # if [[ `echo aa` ]]; then echo yes; else echo no; fi
yes
linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # if [[ `function_not_defined` ]]; then echo yes; else echo no; fi
yes
linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # function_return1()
> {
> return 1
> }
linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # if [[ `function_return1` ]]; then echo yes; else echo no; fi
no
linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # function_return0() { return 0; }
linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # if [[ `function_return0` ]]; then echo yes; else echo no; fi
no

linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # if [[ `echo` ]]; then echo yes; else echo no; fi
no
echo returns nothing

linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # if [[ `echo aa` ]]; then echo yes; else echo no; fi
yes
echo returns 'aa'

linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # if [[ `function_not_defined` ]]; then echo yes; else echo no; fi
yes
It returns an error on standard output (i redirected 2>/dev/null and got an output !?)
linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # function_return1()
> {
> return 1
> }
linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # if [[ `function_return1` ]]; then echo yes; else echo no; fi
no
It echoes NOTHING or don't use backticks !
linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # function_return0() { return 0; }
linux-hhaq:/apps/informatica/current/pmserver/tes/scripts # if [[ `function_return0` ]]; then echo yes; else echo no; fi
no
It echoes NOTHING or don't use backticks !

---------- Post updated at 16:01 ---------- Previous update was at 15:56 ----------

When You use backticks (it's better to use $() which can be nested and is more readable), you evaluate the ouput of the function (what it echoes on stdout) but not the return code.