Understanding of statement in Linux shell script

RHCE_ran · 11-15-2014, 05:00 AM

There is a command in one of the script being used in our system (values changed) which is like:

export EMAIL="ABC <abc@example.com>" && /usr/bin/mutt -s "Test subject” -a <attachment> abc@example.com <”Test content”

In the above statement, I am not able to understand below points:

1) export EMAIL
2) How does concatenation like EMAIL="ABC <abc@example.com>" && /usr/bin/mutt work. I understand what

mutt -s "Test subject” -a <attachment> abc@example.com <”Test content”

means, but am not able to understand how the concatenation works.

3) Is a mail going to be sent with the above command?

I hope my question is clear of not understanding above 3 points.

Request a reply to my post.

Regards

Teufel · 11-15-2014, 05:17 AM

Regarding export EMAIL
this statement exports EMAIL variable from the current process to some "global" level, so this variable may be used by child processes.

Code:

$ a=somevalue
$ echo $a
$ somevalue
$ bash
$ echo $a
$

the last echo $a output will be empty because $a not initialized for the child bash process.
However, if we will set it (export) in global scope, we can use it in child process:

Code:

$ a=somevalue
$ echo $a
$ somevalue
$ export a
$ bash
$ echo $a
$ somevalue

As for concatenation: it isn't ordinary concatenation. "&&" means that the following command will be executed only if previous command exited successfully. Otherwise following command will not be launched

RHCE_ran · 11-15-2014, 05:46 AM

Thanks for your answer but the points 2 & 3 are not answered in the context of my question. I have a few add on queries now:

1) Is the export going to be for EMAIL only or the complete concatenation?
2) Could this entire statement not be written as something like,

/usr/bin/mutt -s "Test subject” -a <attachment> abc@example.com <”Test content”

in the script which could also result in a mail being sent?

Regards

Teufel · 11-15-2014, 06:19 AM

1. export assigns "ABC <abc@example.com>" string to EMAIL variable. This variable will be available for /usr/bin/mutt since it is a child process of your terminal process. I'm not aware of will mutt really use this variable or not, but it will be available for mutt.
2. Of course you can put this string:
/usr/bin/mutt -s "Test subject” -a <attachment> abc@example.com <”Test content”
in the script and run it (don't forget to make script executable).
Will it send email? Do not know. Why wouldn't you test it yourself? I have no mutt installed, so I can't check it. Just replace abc@example.com with real email address and try to send.

TB0ne · 11-15-2014, 09:12 AM

Quote:

Originally Posted by RHCE_ran

There is a command in one of the script being used in our system (values changed) which is like:
export EMAIL="ABC <abc@example.com>" && /usr/bin/mutt -s "Test subject” -a <attachment> abc@example.com <”Test content”

In the above statement, I am not able to understand below points:

1) export EMAIL
2) How does concatenation like EMAIL="ABC <abc@example.com>" && /usr/bin/mutt work. I understand what mutt -s "Test subject” -a <attachment> abc@example.com <”Test content” means, but am not able to understand how the concatenation works.
3) Is a mail going to be sent with the above command?

I hope my question is clear of not understanding above 3 points.

...and putting "how to use command aliases in linux shell script" into Google pulls up close to ONE MILLION HITS. Did you try that?
http://www.computerworld.com/article...-commands.html
http://tldp.org/LDP/abs/html/aliases.html
http://linuxcommand.org/wss0020.php

Out of that many hits, were you not able to find answers? Again, it's odd that someone with an 'RHCE certification', would not know answers to very basic things like this. While they may not have been covered extensively on the exam, any administrator with even a small amount of experience should be familiar with these things.

nbritton · 11-23-2014, 09:17 PM

Code:

$ true && printf "foo"; echo
foo
$ false && printf "foo"; echo

$ true || printf "foo"; echo

$ false || printf "foo"; echo
foo

From the mutt manual page:

Quote:

from

Type: e-mail address
Default: ""

When set, this variable contains a default from address. It can be overridden using my_hdr (including from send-hooks) and `` $reverse_name''.

Defaults to the EMAIL environment variable's content.

RHCE_ran · 11-24-2014, 03:38 AM

Thanks for your answer but I did not understand the || in the lines,

true || printf "foo"; echo
false || printf "foo"; echo

Request a reply.

Regards

TB0ne · 11-24-2014, 09:26 AM

Quote:

Originally Posted by RHCE_ran

Thanks for your answer but I did not understand the || in the lines,

true || printf "foo"; echo
false || printf "foo"; echo

Request a reply.

Again, did you try to look this up? From the bash scripting tutorial you've been pointed to in the past:
http://tldp.org/LDP/abs/html/ops.html#ANDOR

nbritton · 11-24-2014, 10:09 AM

Quote:

Originally Posted by RHCE_ran

Thanks for your answer but I did not understand the || in the lines,

true || printf "foo"; echo
false || printf "foo"; echo

Request a reply.

Regards

http://wiki.bash-hackers.org/syntax/basicgrammar#lists

RHCE_ran · 11-26-2014, 02:29 AM

The link, http://tldp.org/LDP/abs/html/ops.html#ANDOR says nothing helpful, just that:

The && and || operators also find use in an arithmetic context without explaining how || works.

I been trying to get a good explanation but have not got it.

Regards

RHCE_ran · 11-26-2014, 02:33 AM

Thanks, I understood ||. It is opposite of &&.

Regards

Soderlund · 11-26-2014, 09:02 AM

Quote:

Originally Posted by RHCE_ran

Thanks, I understood ||. It is opposite of &&.

Not really. They are logic operators. && means "and", and || means "or".

If (foo && bar), and foo is false, then bar is not checked because the condition is false no matter what bar is.

If (foo || bar), and foo is true, then bar is not checked because the condition is true no matter what bar is.

Therefore a statement like this means to run "upgrade" ONLY if "update" succeeds:

Code:

apt-get update && apt-get upgrade

The following means to print "FAILURE!" ONLY if "update" fails:

Code:

apt-get update || echo "FAILURE!"

Which is demonstrated by the code that nbritton posted.

TB0ne · 11-26-2014, 10:08 AM

Quote:

Originally Posted by RHCE_ran

The link, http://tldp.org/LDP/abs/html/ops.html#ANDOR says nothing helpful, just that:

The && and || operators also find use in an arithmetic context without explaining how || works. I been trying to get a good explanation but have not got it.

We can only give you an explanation...we can't make you understand it. That link tells you EXACTLY what those operators do, and how they function, with examples. If you clicked on that link, and read ANYTHING around where it took you, you'd see the "||" is an OR operator, and the "&&" is AND. It even has examples on how they're used. Soderlund gave you an explanation, but you really need to read documentation, and start thinking about and applying the knowledge you're given.

szboardstretcher · 11-26-2014, 10:13 AM

&& works like this

Code:

cat whatever && echo "it worked"

explained:

Code:

cat whatever (and if that succeeds, then run this ->) echo "it worked"

|| works like the opposite

Code:

cat whatever || echo "it didn't work"

explained:

Code:

cat whatever (and if that does NOT succeed, then run this ->) echo "it didn't work"

You can combine them as well...

Code:

cat whatever && echo "it worked" || echo "it didn't work"

RHCE_ran · 11-27-2014, 06:28 AM

I do not think it is correct, please check the posting of szboardstretcher for the correct explanation.

Regards

---------- Post added 11-27-14 at 07:29 AM ----------

&& and || is not AND or OR, please check the posting of szboardstretcher for the correct explanation.

Regards