UNIX for Beginners Questions & Answers

Hi Team,

I know that "&" holds the result of current pattern match.
But what does "&&" means and its use please?

Thanks & Regards,
Batta Archana

In an awk script or a shell script it is used as a logical and operator:
awk :

Code:

if ($1=="A" && $2=="B") {print $3}

In shell command lines (or scripts) it is also used to create a multiple statement.
It tests the return code of the previous statement, and if it returns OK which in shell is 0, then it does the next step
Example: if directory a exists, list it

Code:

[ -d a ] && ls a

Hi,

I am not sure about awk but in Shell scripting the logical and operator should be "-a" but not "&&". Isn't it??

I tried below script using && which didn't work. But it worked only after replacing && with "-a".

Code:

if [ $a && $b ]
then
   echo "I am in IF block"
fi

Below syntax worked fine

Code:

if [ $a -a $b ]
then
   echo "I am in IF block"
fi

Thanks & Regards
Batta Archana

Moderator's Comments:

edit by bakunin: please use CODE-tags for code and sample output. Thank you.

No, it isn't. Note that there is no such thing as a "meaning of <whatever> in UNIX" and this is where your confusion perhaps comes from.

When you write the following in a shell script:

Code:

if [ .... ] ; then
    something
else
    other
fi

what happens is the following:

The keyword if is interpreted by the shell. "if" will execute the command immediately following, evaluate its return code and execute everything between the keyword then and the keyword else if this return code is 0 (zero), otherwise the part between else and fi.

You can try this out by doing the following: "root" is a user known to exist and will therefore have a line in the file /etc/passwd, "blabla" is a use i suppose will not exist (if it does, use another name known to not exist).

Code:

if grep -q "root" /etc/passwd ; then
     echo "this user exists"
else
     echo "this user does not exist"
fi

Replace the "root" by "blabla" and run again.

You probably ask yourself what the "[" now is. In fact it is a command: the command /bin/test in disguise. Test it (this is from a Fedora installation, your result might look slightly different):

Code:

# ls -l /bin/[
-rwxr-xr-x. 1 root root 41496 Sep 16 20:06 /bin/[

To make shell scripts look more like ordinary programming languages the developers of the UNIX shell devised this (alias-name) and, because this would have resulted in command lines like this:

Code:

if [ <condition> ; then

where the bracket is opened but not closed, they further decreed that, if /bin/test is called as [, the ] is to be given as the last argument.

You can test that easily by trying [ as a single command, without any if:

Code:

[ "x" == "y" ] ; echo $?
[ "x" == "x" ] ; echo $?

"echo $?" displays the return code of /bin/test, alias [: a 1 for the first line (which means logically FALSE) and a 0 for the second (which means logically TRUE).

Now, back to your original question: the command test does indeed not know about "&&" - for a list of parameters and logical evaluations issue man test and read there. But the shell (actually: some shells - the Bourne shell and all of its descendants, ksh, bash, .... do. There are other shells and they may or may not do the same) has a device which is, written generally:

Code:

command1 && command2

and it does the following: command1 is executed and if its return code is 0 then command2 is executed too, otherwise not. There is the opposite of "&&" too: "||". This:

Code:

command1 || command2

means that command1 is executed and if it returns a non-zero return code command2 is executed, otherwise not. An example would be:

Code:

mkdir "/some/dir" || echo "command mkdir /some/thing failed"

I hope this helps.

bakunin

The -a operand is deprecated in the POSIX standard. The recommended way is to use the && operator like so (leaving the variable expansions unquoted, like in your example):

Code:

if [ $a ] && [ $b ]
then
   echo "I am in IF block"
fi

Thank you guys. Now, i could understand the difference between && and -a.

That's true only in regexes of certain text processing commands like sed or awk. In the shell (that your are dealing with in the rest of the post), depending of contexts, it means
- send command / pipe to background
- duplicate file descriptors for redirection
- bitwise AND
- certain behaviour of case statements
and some other very special meanings/behaviours.