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# 1  
Old 06-07-2013
sed substitution

Hi everyone,

I need very simple sed command to change a parameter in a text file.

I have a line in this text which is like

Code:
set xx 0.5

A program reads this file and does some algebraic calculations. So to make a parameter scan I need to change the value of xx. I thought I can do this with a simple sed command in a shell loop. so the shell simply will look like

Code:
#!/bin/sh
xx=0.0
for ((i=0; i<=10; i++))
do xx = xx + i*0.5

% here comes the sed command which substitutes the existing value of xx with the one set above, and afterwards runs the program which reads that text file

done

thanks for all the help
# 2  
Old 06-07-2013
Not sure I understand. The sed command will not run a program to read a text file. And if I interpret your request correctly, you want the text file to be modified to look like set 0.0 0.5 in the first loop, in which i is 0 and thus xx = 0.0. Why do you need the file at all, when you can set xx within the shell itself?
# 3  
Old 06-07-2013
ok, first of all i don't need sed to run the program, running the program comes after I change the parameter, forget about running the program. Second, I want the text file to be modified to look like
after the 1st iteration
Code:
set xx 0.0

after 2nd iteration
Code:
set xx 0.5

after 3rd iteration
Code:
set xx 1.0

etc.
# 4  
Old 06-07-2013
Try
Code:
sed -i "s/^set xx .*/set xx $xx/" file

Pls note that the -i option is controversially discussed in this forum.
This User Gave Thanks to RudiC For This Post:
# 5  
Old 06-07-2013
Code:
sed -i 's/^\(set xx \)[0-9.][0-9.]*/\1'$xx'/' file

Maybe you can pass an input stream to your program?
It is more efficient to have
Code:
sed 's/^\(set xx \)[0-9.][0-9.]*/\1'$xx'/' file | your_program

# 6  
Old 06-07-2013
thanks RudiC, this works very well. If you don't mind I would like to ask another simple question.

now I need to set a condition, after I run the program it produces an output file. Somewhere in this output
there is a line that looks like
Code:
#  Integrated weight  :  .60573E+01

in my shell script I need to read this line and if that number is in certain interval continue to do further calculations, if not skip to next iteration. So how can I write this if .. else statement?
# 7  
Old 06-07-2013
This is one way:
Code:
n1=$(printf "%f" ".60573E+01")
n2=6
if (( $(bc -l <<< "$n2 > $n1") )); then
  echo n2 gt n1
else
  echo n2 ngt n1
fi

For example:
Code:
$ n2=6

$ if (( $(bc -l <<< "$n2 > $n1") )); then   echo n2 gt n1; else   echo n2 ngt n1; fi
n2 ngt n1

$ n2=7

$ if (( $(bc -l <<< "$n2 > $n1") )); then   echo n2 gt n1; else   echo n2 ngt n1; fi
n2 gt n1


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