Sh vs bash


 
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Top Forums Shell Programming and Scripting Sh vs bash
# 15  
Old 08-07-2014
Quote:
Originally Posted by achenle
In my experience, on Linux sh is just a soft link to bash.
It is not just a softlink. When sh is a symlink to bash on certain Linux distributions and sh is invoked, bash is run with the --posix option.
# 16  
Old 08-07-2014
This is an interesting example. It shows that pure Bourne shells execute while loops in a subshell environment when the input to that loop is redirected from a file, but does not create a subshell when the input is the script's standard input. If I knew that Bourne shell "feature", I had forgotten it.
# 17  
Old 08-07-2014
An ugly work-around is this descriptor-magic:
Code:
#!/bin/sh
wc -l wpd*.prg >wc.out
totallines=0
totalprg=0
exec 3<&0 # save stdin
exec 0<wc.out # redirect stdin
while read a b
do
        totalprg=`expr $totalprg + 1`
        totallines=`expr $totallines + $a`
        echo $totalprg $totallines
done
exec 0<&3 # restore stdin
echo Total Programs $totalprg
echo Total Lines $totallines

# 18  
Old 08-08-2014
I wonder how old read's -u flag is. It might have been invented as a workaround to this problem, it'd simplify your solution a bit.

Code:
#!/bin/sh
wc -l wpd*.prg >wc.out
totallines=0
totalprg=0

exec 5<wc.out
while read -u 5 a b
do
        totalprg=`expr $totalprg + 1`
        totallines=`expr $totallines + $a`
        echo $totalprg $totallines
done
exec 5<&-
echo Total Programs $totalprg
echo Total Lines $totallines

# 19  
Old 08-08-2014
Quote:
Originally Posted by Corona688
I wonder how old read's -u flag is. It might have been invented as a workaround to this problem, it'd simplify your solution a bit.

Code:
#!/bin/sh
wc -l wpd*.prg >wc.out
totallines=0
totalprg=0

exec 5<wc.out
while read -u 5 a b
do
        totalprg=`expr $totalprg + 1`
        totallines=`expr $totallines + $a`
        echo $totalprg $totallines
done
exec 5<&-
echo Total Programs $totalprg
echo Total Lines $totallines

Pure Bourne shells didn't have a -u option for read. I'm not sure where or when it first appeared, but it was in ksh88.
# 20  
Old 08-08-2014
Quote:
Originally Posted by Don Cragun
I'm not sure where or when it first appeared, but it was in ksh88.
Right. I always thought this to be one of the definitive advantages of ksh (at least for scripting) over all the other shells, because with "print -u" and "read -u" you can easily put all the I/O-descriptors to work. In Bourne-shell as well as in bash this was always a hassle. Picture the equivalent like:

Code:
exec 3>file.1
exec 4>file.2
exec 5>file.3

print -u3 - "this goes to file.1"
print -u4 - "this goes to file.2"
print -u5 - "this goes to file.3"
print -u3 - "this goes to file.1 again"

exec 3>&-
exec 4>&-
exec 5>&-

in bash. It is not impossible, but the code would be a lot less clean. In addition you'd need different redirections (one overwriting, one appending) for the two lines going to file.1.

bakunin
# 21  
Old 08-12-2014
I guess I'm not seeing the problem:

Code:
exec 3>file.1
exec 4>file.2
exec 5>file.3

printf "this goes to file.1\n" >&3
printf "this goes to file.2\n" >&4
printf "this goes to file.3\n" >&5
printf "this goes to file.1 again\n" >&3

exec 3>&-
exec 4>&-
exec 5>&-

It's true that you can't put 3, 4, 5 in variables like you could with -u parameters, but you can append just fine.
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