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---+!! Pub Talk Part IV<!--pula duas linhas-->


%TOC% 

------

'My fellow!!! How have you been, mr. bin? Have you done the exercise I asked you to?'

'I surely did! When programming is the topic, if you don't practice, you don't learn. You've asked me a simple script that tells whether a user is logged in or not. I have made the following script:'

%TERMINAL_INI%
$ cat logado%OUT_INI%
#!/bin/bash
# searches whether a user is logged in or not
if who | grep $1
then
    echo $1 is logged
else
    echo $1 can't be seen anywhere around
fi%OUT_FIM%
%TERMINAL_FIM%

'Easy boy!! You look quite excited, but let's just ask for some beer first. John, two beers, please! And please, pour mine with no foam...'

'OK! Now that we've had our drinks, let's take a look:'

%TERMINAL_INI%
$ logado jneves%OUT_INI%
jneves   pts/0        Oct 18 12:02 (10.2.4.144)
jneves is logged%OUT_FIM%
%TERMINAL_FIM%

'Well, it really works! I used my login as parameter and it told me I was logged in. Nevertheless, it told me something I didn't want to: a line of the who command. In order to avoid it, it's just necessary to send that line to the black hole called /dev/null. Check it out:'

%TERMINAL_INI%
$ cat logado%OUT_INI%
#!/bin/bash
# searches whether a user is logged in or not (version 2)
if who | grep $1 > /dev/null
then
    echo $1 is logged
else
    echo $1 can't be seen anywhere around
fi%OUT_FIM%
%TERMINAL_FIM%


'Now, let's test:'

%TERMINAL_INI%
$ logado jneves%OUT_INI%
jneves is logged
$ logado chico
chico can't be seen anywhere around%OUT_FIM%
%TERMINAL_FIM%

%ATTENTION_INI%
Remember the trick: most of the commands have a standard output and an error output (=grep= is one of the few exceptions because it shows no error message when it doesn't find a string) and we should pay attention when it is necessary to send them to the black hole.
%ATTENTION_FIM%

'But let's change the subject. The last time we met, I was showing you some conditional commands and, when I was thirsty as hell, you asked me how one can test conditions. Let's see the test command, then.'

---++ The test Command

'Well, we are all acquainted to the use of =if= testing conditions (which are always _greater than_, _less then_, _greater or equal_, _less or equal_, _equal_ and _not equal_). When using Shell to test conditions, we can use the =test= command, but it is a lot more powerful than we can imagine. Let me first show you the main options to test files in a disc:'

<center>
%TABLE{ databg="#ffffff" headerrows="1" }%
|  *Options of the test command to files*  ||
|  &nbsp; *Option* &nbsp;  |&nbsp; *True if:* &nbsp;|
|  &nbsp; =-e file= &nbsp;  | &nbsp; =file= does exist &nbsp;|
|  &nbsp; =-s file= &nbsp;  | &nbsp; =file= does exist and is bigger than zero &nbsp;|
|  &nbsp; =-f file= &nbsp;  | &nbsp; =file= does exist and is a regular file &nbsp;|
|  &nbsp; =-d file= &nbsp;  | &nbsp; =file= does exist and is a directory &nbsp;|
|  &nbsp; =-r file= &nbsp;  | &nbsp; =file= does exist with reading grant &nbsp;|
|  &nbsp; =-w file= &nbsp;  | &nbsp; =file= does exist with writing grant &nbsp;|
|  &nbsp; =-x file= &nbsp;  | &nbsp; =file= does exist with execution grant &nbsp;|
</center>

'Now, the main testing options for character chains:'

<center>
%TABLE{ databg="#ffffff" headerrows="1" }%
|  *Options of the test command for character strings*  ||
|  &nbsp; *Option* &nbsp;  |&nbsp; *True if:* &nbsp;|
|  &nbsp; =-e string= &nbsp;  |&nbsp; =string= size is zero &nbsp;|
|  &nbsp; =-n string= &nbsp;  |&nbsp; =string= size is bigger than zero &nbsp;|
|  &nbsp; =string= &nbsp;  |&nbsp; the string =string= is bigger than zero &nbsp;|
|  &nbsp; =c1 = c2= &nbsp;  |&nbsp; strings =c1= & =c2= are identical &nbsp;|
</center>

'Thinking it's over? So sorry!!! Now the part you are not acquainted to, comparisons with numbers. Check out the table below:'

<center>
%TABLE{ databg="#ffffff" headerrows="1" }%
|  *Options of the test command for numbers*  ||
|  &nbsp; *Option* &nbsp;  |&nbsp; *True if* &nbsp;|
|  &nbsp; =n1 -eq n2= &nbsp;  |&nbsp; =n1= equals =n2= &nbsp;|
|  &nbsp; =n1 -ne n2= &nbsp;  |&nbsp; =n1= and =n2= are different &nbsp;|
|  &nbsp; =n1 -gt n2= &nbsp;  |&nbsp; =n1= is greater than =n2= &nbsp;|
|  &nbsp; =n1 -ge n2= &nbsp;  |&nbsp; =n1= is greater or equals than =n2= &nbsp;|
|  &nbsp; =n1 -lt n1= &nbsp;  |&nbsp; =n1= is less than =n2= &nbsp;|
|  &nbsp; =n1 -le n2= &nbsp;  |&nbsp; =n1= is less or equals than =n2= &nbsp;|
</center>

'Furthermore, consider the following operators'

<center>
%TABLE{ databg="#ffffff" headerrows="1" }%
|  *Operators*  ||
|  &nbsp; *Operator* &nbsp;  |&nbsp; *Purpose* &nbsp;|
|  &nbsp; Parenthesis =( )= &nbsp;  |&nbsp; Grouping &nbsp;|
|  &nbsp; Exclamation =!= &nbsp;  |&nbsp; Denying &nbsp;|
|  &nbsp; =-a= &nbsp;  |&nbsp; logical _AND_ &nbsp;|
|  &nbsp; =-o= &nbsp;  |&nbsp; logical _OR_ &nbsp;|
</center>

'Wow! As you've seen, there is a lot of stuff here, and as I told you, our =if= is much more powerful than others's. Let's see some examples of how it works, first, testing the existence of a directory:'

Example:

<verbatim>
    if  test -d lmb
    then
        cd lmb
    else
        mkdir lmb
        cd lmb
    fi
</verbatim>

'That example tests if there is a =lmb= directory, or else, it creates such directory. I know you will probably question this logic saying the script is not optimized. I know, but I wanted you to understand it the way it is, so that we can use an exclamation point =(!)= to deny the test command. Check it:'

<verbatim>
    if  test ! -d lmb
    then
        mkdir lmb
    fi
    cd lmb
</verbatim>
    
'This way, the =lmb= directory would be created if (and only if) it did not exist. This denial comes from the exclamation point =(!)= that preceeds the option =-d=. At the end of the execution of this piece of script, you would certainly be in the =lmb= directory.

Let's see other two examples to check the difference  between numbers and chains.'

<verbatim>
    str1=1
    str2=01
    if  test $str1 = $str2
    then
        echo The variables are equal.
    else
        echo The variables are not equal.
    fi
</verbatim>

'Running the piece of software above, the answer would be:'

<verbatim>
    The variables are not equal.
</verbatim>

'Now, let's change it in order to have a numerical comparison:'

<verbatim>
    str1=1
    str2=01
    if  test $str1 -eq $str2
    then
        echo The variables are equal.
    else
        echo The variables are not equal.
    fi
</verbatim>

'And let's run it again:'

<verbatim>
     The variables are equal.
</verbatim>

'As you have seen above, we've had two different results because the string =01= is quite different from the chain string =1=, though they are the same when considered numerically, since the number =1= is equivalent to the number =01=.'

Examples:

'In order to show the use of conectors =-o= (_OR_) and =-a= (_AND_), check this example made at the prompt:'

%TERMINAL_INI%
$ Familia=felinae
$ Genero=cat
$ if  test $Familia = canidea -a $Genero = lobo -o $Familia = felino -a $Genero = lion
&gt; then
&gt;     echo Be aware
&gt; else
&gt;     echo Can wave it
&gt; fi%OUT_INI%
Can wave it%OUT_FIM%
%TERMINAL_FIM%

%TIP_INI%
The angle brackets =(>)= in the begining of the internal lines of the =if=, are the continuation prompts (that are defined as =$PS2=) and when our friend Shell identifies that a command will have a continuation in the following line, it automatically places it until the end of the command.
%TIP_FIM%

'Let's change the example to check if it still works:'

%TERMINAL_INI%
$ Familia=felino
$ Genero=cat
$ if  test $Familia = felino -o $Familia = canideo -a $Genero = lion -o $Genero = lobo
&gt; then
&gt;     echo be aware
&gt; else
&gt;     echo Can wave it
&gt; fi%OUT_INI%
Be aware%OUT_FIM%
%TERMINAL_FIM%

'The operation has, obviously, generated an error. That has happened because the option =-a= preceeds the option =-o=. This way, what was firstly evaluated was the expression:'

<verbatim>
    $Familia = canideo -a $Genero = lion
</verbatim>

'That expression was evaluated as false, so that the answer was:'

<verbatim>
    $Familia = felino -o FALSE -o $Genero = lobo
</verbatim>

'Solved, it would be:'

<verbatim>
    TRUE -o FALSE -o FALSE 
</verbatim>
     
'Since all the conectors are =-o=, the final expression has resulted as =TRUE= (considering that, when in a series of logical expressions connected by logic _OR_, only one of those expressions must be true to the result become true) and the =then= was wrongly executed. To make it work properly (again), let's try the following procedure:'

%TERMINAL_INI%
$ if  test \($Familia = felino -o $Familia = canideo\) -a \($Genero = lion -o $Genero = lobo\)
&gt; then
&gt;     echo Be aware
&gt; else
&gt;     echo Can wave it
&gt; fi%OUT_INI%
Can wave it%OUT_FIM%
%TERMINAL_FIM%

'This way, using the parentheses, the expressions are grouped with the connector =-o=, which priorizes the execution and results in:'

<verbatim>
    TRUE -a FALSE
</verbatim>
     
'The result of expressions connected by the operator =-a= is true when all the expressions that it connects are true (which is not the case above). This way, the result was =FALSE= and the =else= was correctly executed.'

'If we decide to read a CD with songs of different singers, we could be temptated to use an if with a connector =-a=, but it is better to bare in mind that bash provides us many resources, and it could be done more easily with a single =grep= command, as in the following example:'

%TERMINAL_INI%
$ grep Musician1 musics | grep Musician2
%TERMINAL_FIM%

'Similarly, if we pick a CD with songs of an =Musician1= and of an =Musician2=, it is not necessary to use an =if= with the connector =-o=. The =egrep= (or =grep -E=, which is more proper for that situation) can also solve this problem. Check it out:'

%TERMINAL_INI%
$ egrep (Musician1|Musician2) musics
%TERMINAL_FIM%

'Or (specifically for that case) =grep= itself could help us out:'

%TERMINAL_INI%
$ grep Musician[12] musics
%TERMINAL_FIM%

'Above, a regular expression was used. The vertical bar =(|)= works as a logical _OR_ and the parentheses are used to limit that _OR_. On the next =grep=, on the other hand, the word =Artista= must be followed by one of the values of the list between square brackets (=[]=), that means, =1= or =2=.'

'OK! I accept it when you tell me that shell's if is much more powerful than other's. But let me tell you one thing: that syntax =if test...= is quite creepy... han?'

'Yeah, I think you're right. I don't like it either... no one does, I guess. I think that's why Shell has another syntax to substitute the =test= command.'

Examples

'In order to do so, we'll use that example to switch directories. It was like that:'

<verbatim>
    if  test ! -d lmb
    then
        mkdir lmb
    fi
    cd lmb
</verbatim>

'Using a new syntax, it will be:'

<verbatim>
    if  [ ! -d lmb ]
    then
        mkdir lmb
    fi
    cd lmb
</verbatim>

'That means that the =test= command can be replaced by a pair of square brackets (=[]=), separated by blank spaces between the arguments, which would considerably increase the legibility of the command, since if would present a syntax similar to the one of other languages and that is why the =test= command will be used henceforth.'

---++ Please help me to finish or to correct this translation (from Portuguese)

Send me a e-mail <a href="mailto:julio.neves@gmail.com?Subject=Julio I wanna help the Pub Talk translation">julio.neves@gmail.com</a>
-- Main.JulioNeves - 30 Sep 2006
Topic revision: r4 - 16 Feb 2007 - 13:16:21 - JulioNeves
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