Bash script gives weird output

Tinkster · 02-08-2012, 01:51 PM

Quote:

Originally Posted by Cedrik

man bash:

Code:

...
 [[ expression ]]
...
              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.  If the  shell
              option  nocasematch  is  enabled, the match is performed without
              regard to the case of alphabetic characters.  The  return  value
              is  0 if the string matches (==) or does not match (!=) the pat-
              tern, and 1 otherwise.  Any part of the pattern may be quoted to
              force it to be matched as a string.
              ...
              An  additional  binary operator, =~, is available, with the same
              precedence as == and !=.  When it is used,  the  string  to  the
              right  of the operator is considered an extended regular expres-
              sion and matched accordingly (as in regex(3)).  The return value
              is 0 if the string matches the pattern, and 1 otherwise.
...
       Pattern Matching

       Any character that appears in a pattern, other than the special pattern
       characters described below, matches itself.  The NUL character may  not
       occur  in  a pattern.  A backslash escapes the following character; the
       escaping backslash is discarded when  matching.   The  special  pattern
       characters must be quoted if they are to be matched literally.
              
       The special pattern characters have the following meanings:
              
       *      Matches  any  string, including the null string.
...

So I was right, this is pattern matching:

Code:

if [[ "$CheckMainServerOutput" == *ms* ]] || [[ "$CheckMainServerOutput" == *32.43.26* ]]; then

Of course it is ... and so is =~ (coincidentally described in the same bit
of bash's man-page that you quoted), which you claimed, by way of different example

Quote:

BTW, a pattern matching looks like:

wasn't pattern matching.

Cheers,
Tink

Cedrik · 02-08-2012, 01:56 PM

Ok, I admit I wasn't clear

jonaskellens · 02-12-2012, 08:25 AM

Hello,

so far I was helped very well with problems that occurred.

Now I have a third question :

Code:

echo "before"
$StartProg=$(/sbin/service prog start)
echo $StartProg
echo "after"

gives output :

Quote:

before
bashscript.sh: line 14: =Starting: command not found

But I notice that the service "prog" has in fact started !

Why does bash report "command not found" ?
Is there something wrong with my syntax ?

When I execute /sbin/service prog start on CLI, then output is normal :

Quote:

Starting prog: [ OK ]

catkin · 02-12-2012, 08:49 AM

When bash processes $StartProg=$(/sbin/service prog start), it:

Runs /sbin/service prog start (starting the service) and substitutes its output into the line leaving $StartProg=Starting <whatever>
Substitutes the value of $StartProg which is nothing, leaving =Starting <whatever>
Runs that as a command, resulting in the error message you see.

jonaskellens · 02-12-2012, 09:55 AM

But I read all over the internet that the syntax $(command) is the correct way of executing a command.

How do I then get normal output of the command so that the rest of the bashscript can continue ?

Dark_Helmet · 02-12-2012, 12:05 PM

Quote:

Originally Posted by jonaskellens

But I read all over the internet that the syntax $(command) is the correct way of executing a command.

It is, but the problem is not with the $( ... ) construct.

Re-read point #2 in catkin's message.

In bash, when you want to use the value of a variable, you prefix the variable name with a '$'.

Code:

echo "This is the value: $variableValue"

When you want to assign a value to a variable, you do not prefix the variable name with a '$'.

Code:

variableValue="This is the assigned value"

This is different from other languages (e.g. PHP) where the '$' prefixes every variable reference (assignment or substitution).

jonaskellens · 02-12-2012, 01:19 PM

OK I see. I did not notice this before. In the rest of my script I did not make this mistake.

The error-message "=Starting: command not found" is gone, that's good !

However, even with StartProg=$(/sbin/service prog start) I still get no output and the script is not further executed.

catkin · 02-13-2012, 01:46 AM

Quote:

Originally Posted by jonaskellens

But I read all over the internet that the syntax $(command) is the correct way of executing a command.

It is the correct way to capture the output of a command when modified to my_variable=$(command 2>&1) (the 2>&1 captures the stderr output as well as the stdout).

The way to run a command is simply command

This makes sense when you think of a bash script being a automated command prompt session. At the command prompt, you run a command by typing it and pressing Enter.

catkin · 02-13-2012, 01:49 AM

Quote:

Originally Posted by jonaskellens

However, even with StartProg=$(/sbin/service prog start) I still get no output and the script is not further executed.

As long as the command terminates you should see the output of

Code:

echo $StartProg
echo "after"

Please post your latest script if you do not see that output.

jonaskellens · 02-18-2012, 04:58 AM

Thank you for your help. I notice that execution of the command hangs... I think it is related to the service that is being started. That's why I now execute another bash script which executes this command. This way my "main" bash script is further executed. It works well.

Another question :

suppose CheckNumber="[space][space][space]7"

I want to get the last 2 ciphers of this string. If there is only one cipher, I want to delete white spaces. I do the following :

Number=${CheckNumber:0:2} # get last 2 characters
${Number//[[:space:]]} # delete white spaces

if [[ "$Number"=="0" ]]; then
echo "number 0"
else
echo "number more than 0"
fi

I notice that the variable $Number is always empty.
So my if-then-else also fails.

See what I'm doing wrong ?

catkin · 02-18-2012, 05:04 AM

If the command does not terminate you could put it in the background by appending & to the command.

catkin · 02-18-2012, 05:09 AM

Quote:

Originally Posted by jonaskellens

suppose CheckNumber="[space][space][space]7"

I want to get the last 2 ciphers of this string. If there is only one cipher, I want to delete white spaces. I do the following :

Code:

Number=${CheckNumber:0:2} # get last 2 characters
${Number//[[:space:]]} # delete white spaces

if [[ "$Number"=="0" ]]; then
	echo "number 0"
else 
	echo "number more than 0"
fi

I notice that the variable $Number is always empty.
So my if-then-else also fails.

See what I'm doing wrong ?

Add the command set -xv just before the problem section to better see what the script is doing.

Bash wants spaces either side of comparison operators such as == and you want the numeric comparison operator -eq, not the string comparison operator ==.

jonaskellens · 02-18-2012, 05:53 AM

Code:

CheckNumber="   7"
+ CheckNumber='   7'
echo $CheckNumber
+ echo 7
7
Number=${CheckNumber:0:2}
+ Number='  '
echo "   Number: $Number"
+ echo '   Number:   '
   Number:   
${Number//[[:space:]]}
echo "   Number: $Number"
+ echo '   Number:   '
   Number:   
if [[ "$Number"=="0" ]]; then
	echo "number 0"
else 
	echo "number more than 0"
fi
+ [[ -n   ==0 ]]
+ echo 'number 0'
number 0

catkin · 02-18-2012, 06:06 AM

The comment does not match the code in ...

Code:

Number=${CheckNumber:0:2} # get last 2 characters

... the code gets the first two characters as the set -xv trace shows.

If you want to use echo to display the value of variables you must double quote them or bash will strip leading/training whitespace and convert all embedded whitespace sequences to a single space. It is also helpful to put markers at beginning and end so you can see any leading/training whitespace:

Code:

echo "Number: '$Number'"

The else echo in ...

Code:

if [[ "$Number"=="0" ]]; then
	echo "number 0"
else 
	echo "number more than 0"
fi

... is wrong. The else case is triggered when $Number is not equal to the string 0 -- or would be if the == in the test expression had space either side of it so it was a comparison operator and not a component of the string "$Number"=="0" which is actually "==0" at run time as the set -xv trace shows.

jonaskellens · 02-18-2012, 06:40 AM

${CheckNumber: -2} does now what I want it to do...

Number=${Number/ /} removes white spaces...

However, still looking for a correct if-then-else