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Sh vs bash

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Top Forums Shell Programming and Scripting Sh vs bash
# 1  
Old 08-06-2014
Sh vs bash
I don't know if this is an anomaly or by design.
Code:
bash-3.2# cat t                             
#!/bin/sh                                   
wc -l wpd*.prg >wc.out                      
totallines=0                                
totalprg=0                                  
while read a b                              
do                                          
        totalprg=`expr $totalprg + 1`       
        totallines=`expr $totallines + $a`  
        echo $totalprg $totallines          
done <wc.out                                
echo Total Programs $totalprg               
echo Total Lines $totallines                
bash-3.2# ./t                               
1 3146                                      
2 6299                                      
3 9453                                      
4 18906                                     
Total Programs 0                            
Total Lines 0                               
bash-3.2#

Code:
  bash-3.2# cat t                           
#!/bin/bash                               
wc -l wpd*.prg >wc.out                    
totallines=0                              
totalprg=0                                
while read a b                            
do                                        
        totalprg=`expr $totalprg + 1`     
        totallines=`expr $totallines + $a`
        echo $totalprg $totallines        
done <wc.out                              
echo Total Programs $totalprg             
echo Total Lines $totallines              
bash-3.2# ./t                             
1 3146                                    
2 6299                                    
3 9453                                    
4 18906                                   
Total Programs 4                          
Total Lines 18906                         
bash-3.2#

and finally
Code:
bash-3.2# cat t                            
#!/bin/sh                                  
#wc -l wpd*.prg >wc.out                    
totallines=0                               
totalprg=0                                 
while read a b                             
do                                         
        totalprg=`expr $totalprg + 1`      
        totallines=`expr $totallines + $a` 
        echo $totalprg $totallines         
done                                       
echo Total Programs $totalprg              
echo Total Lines $totallines               
bash-3.2# ./t <wc.out                      
1 3146                                     
2 6299                                     
3 9453                                     
4 18906                                    
Total Programs 4                           
Total Lines 18906                          
bash-3.2#

These 2 Users Gave Thanks to jgt For This Post:
CarloM Scrutinizer
# 2  
Old 08-06-2014
There are various threads going around that draw attention to how Bash spawns a sub-shell to handle while-loops, etc, which is why the value outside a while-loop is "not preserved".
# 3  
Old 08-06-2014
That does not apply when 'while read' is only reading from a file, Scott.

What is 'sh' on your system? So far BASH, ASH, and DASH all work here.
This User Gave Thanks to Corona688 For This Post:
Scott
# 4  
Old 08-06-2014
What version/type of shell are you using in your 1st example (/bin/sh) ?

I get result 2 with everything I have here ash, dash, ksh, bash and bash --posix
# 5  
Old 08-06-2014
Strangely enough I found this some time ago in the early stages of the AudioScope.sh development:-
Code:
Last login: Wed Aug  6 22:34:25 on ttys000
AMIGA:barrywalker~> for n in {1..10}; do printf "$n\n"; n=$((n+2)); done
1
2
3
4
5
6
7
8
9
10
AMIGA:barrywalker~> _

I still don't understand what is happening, as I would have thought that the "n" inside the loop would either create an error or be the equivalent of a "step of 2".
Would this be a bug or am I missing something...
# 6  
Old 08-06-2014
I ran the original scripts on SCO 6.0.0 mp4
On SUSE 11.2 I get the expected results using sh.
# 7  
Old 08-06-2014
@wisecracker remember that a for loop is just fetching the variable as a string from the parameter list one at a time it isn't incrementing the variable at all.

So you can see that changing n within the for loop will have no effect on the next parameter fetched, think about:

Code:
for n in A B C D
do
   ...
   n=Z
done

@jgt, was /bin/sh a linked to another shell?
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