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 Forum index » Off-Topic Area » Programming
Bash script variables assignment syntax question
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emily22

Joined: 06 May 2012
Posts: 3
PostPosted: Sun 06 May 2012, 05:06    Post subject:  Bash script variables assignment syntax question  
hey there to all!

I'm writing a bash script and I wonder what is the correct syntax for arithmetic expression when it comes to integers which are stored in variables.

For example

aa=23
bbb=35
cccc=479

and I want to assign:

dd=$(($aa+$bbb))

or

aa=$(($aa+$bbb))

or

ee=$(($cccc-$aa))

Is this correct or let is working better?
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musher0


Joined: 04 Jan 2009
Posts: 2219
Location: Gatineau (Qc), Canada
PostPosted: Sun 06 May 2012, 18:57    Post subject:  
Hello!

I know it is never wrong to put a 'let' statement before this type of variable, although it takes an extra 4 bytes.

Best of luck.
_________________
Wink "To err is human; to really foul things up, you need a computer!" / "L'erreur est humaine; pour vraiment f... la m..., il faut un ordinateur." (Carleton University, banderole à la Rentrée 1979 / banner, start of 1979 school year) Wink
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sunburnt


Joined: 08 Jun 2005
Posts: 4006
Location: Arizona, U.S.A.
PostPosted: Sun 06 May 2012, 23:50    Post subject:  
Hi emily22; "Let" is kinda diminished, or depreciated as it were ( not used ).

I can`t remember the forum name of the competent member that showed me this:
Code:
aa=23 ; bbb=35

((dd=$aa+$bbb))

echo $dd
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jpeps

Joined: 31 May 2008
Posts: 2422
PostPosted: Mon 07 May 2012, 00:56    Post subject:  
sunburnt wrote:
Hi emily22; "Let" is kinda diminished, or depreciated as it were ( not used ).

I can`t remember the forum name of the competent member that showed me this:
Code:
aa=23 ; bbb=35

((dd=$aa+$bbb))

echo $dd


works, but what's wrong with dd=$(($aa + $bbb)) ?
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amigo

Joined: 02 Apr 2007
Posts: 1759
PostPosted: Mon 07 May 2012, 03:30    Post subject:  
'$((dd=$aa+$bbb))' would be the most portable among different shells. 'let' is bash-specific. While '((dd=$aa+$bbb))' works with bash, it may not with other shells. Whitespace around the operators has o effect.
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musher0


Joined: 04 Jan 2009
Posts: 2219
Location: Gatineau (Qc), Canada
PostPosted: Tue 15 May 2012, 01:22    Post subject:  
Hello.

Even if the "let" statements are officially deprecated, the following script does not work if the "let" statements are removed.

Code:

#!/bin/sh
# ~/$MAPPS/bin/urxvt-fordo-cascade-4.sh
####
# Ce script-ci doit demeurer en l'état.
# (c) Christian L'Écuyer, Gatineau (Qc), Canada, janvier 2012. Gratuiciel, mais tous droits réservés.
# Courriel : http://scr.im/reyucelc
#
# Si ça ne marche pas, assurez-vous qu'un autre "rxvt" ne tourne pas quelque part en mémoire.
####
if [ "`pidof rxvt`" != "" ];then
   for i in `seq 4`;
   do
      wmctrl -c Console
   done
   exit
   else   
      export gm="g 80x16"
      let x=45
      let y=80
      for i in `seq 4`;
           do
                   rxvt -bd grey10 -b 4 -fg white -cr green -sr +st +tcw -bc -tint black -sh 57 -T "Console $i" -$gm-$x-$y &
         let x=$x+120
         let y=$y+170
         sleep 0.17s
           done
fi


Is it me, bash (version 4.1.0-1_i686, 2009), my computer, my Puppy version (lupu 5.25 retro4), my attitude towards life, a spell from my ex? Laughing Someone has an explanation? Thanks in advance.

Also, before you ask, I have found a reason to open 4 console windows at the same time! Very Happy

BFN.
_________________
Wink "To err is human; to really foul things up, you need a computer!" / "L'erreur est humaine; pour vraiment f... la m..., il faut un ordinateur." (Carleton University, banderole à la Rentrée 1979 / banner, start of 1979 school year) Wink
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sunburnt


Joined: 08 Jun 2005
Posts: 4006
Location: Arizona, U.S.A.
PostPosted: Tue 15 May 2012, 19:26    Post subject:  
musher0;
Code:
x=45 ; y=80
........
((x=$x+120)) ; ((y=$y+170))
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Peterm321

Joined: 29 Jan 2009
Posts: 196
PostPosted: Tue 15 May 2012, 20:10    Post subject:  
emily22 wrote:
For example

aa=23
bbb=35
cccc=479

and I want to assign:

dd=$(($aa+$bbb))


Code:

#!/bin/bash
aa=23
bbb=35
cccc=479
let dd=aa+bbb
echo -e "\ndd= $dd\n"
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technosaurus


Joined: 18 May 2008
Posts: 3843
PostPosted: Tue 15 May 2012, 23:28    Post subject:  
musher0 wrote:
Hello.

Even if the "let" statements are officially deprecated, the following script does not work if the "let" statements are removed.

...

Is it me, bash (version 4.1.0-1_i686, 2009), my computer, my Puppy version (lupu 5.25 retro4), my attitude towards life, a spell from my ex? Laughing Someone has an explanation? Thanks in advance.

Also, before you ask, I have found a reason to open 4 console windows at the same time! Very Happy

BFN.
see amigo's post for portable, alternative syntax
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Shep

Joined: 08 Nov 2008
Posts: 840
Location: GIRT-BY-SEA
PostPosted: Sun 20 May 2012, 10:54    Post subject:  
sunburnt wrote:
I can`t remember the forum name of the competent member that showed me this:
Code:
aa=23 ; bbb=35

((dd=$aa+$bbb))

echo $dd

Inside $((....)) bash allows you to omit the $ preface to all the variables:

a=24 b=56 c=$((a+b)); echo $c
80
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musher0


Joined: 04 Jan 2009
Posts: 2219
Location: Gatineau (Qc), Canada
PostPosted: Sun 20 May 2012, 15:03    Post subject:  
Thanks to all for their answers.
_________________
Wink "To err is human; to really foul things up, you need a computer!" / "L'erreur est humaine; pour vraiment f... la m..., il faut un ordinateur." (Carleton University, banderole à la Rentrée 1979 / banner, start of 1979 school year) Wink
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sunburnt


Joined: 08 Jun 2005
Posts: 4006
Location: Arizona, U.S.A.
PostPosted: Sun 20 May 2012, 18:27    Post subject:  
Shep; You`re right of course, I was winging it from memory.
So...
Quote:
((dd=aa+bb))
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jpeps

Joined: 31 May 2008
Posts: 2422
PostPosted: Sun 20 May 2012, 19:23    Post subject:  
sunburnt wrote:
Shep; You`re right of course, I was winging it from memory.
So...
Quote:
((dd=aa+bb))


interesting; I'd never tried that before. Just goes to prove that $$ is dispensable.
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