old file = composing_commands.sh.1
new file = composing_commands.sh (I added one one to it)
 
so users have old file and I want to provide for them to update.
 
tiias5:shellscripts/>diff composing_commands.sh composing_commands.sh.1
36d35
< if com1 && com2 && com3; then echo "commands 1 and 2"; fi
tiias5:shellscripts/>diff -e composing_commands.sh composing_commands.sh.1
36d
tiias5:shellscripts/>diff composing_commands.sh.1 composing_commands.sh
35a36
> if com1 && com2 && com3; then echo "commands 1 and 2"; fi
tiias5:shellscripts/>diff -c composing_commands.sh.1 composing_commands.sh
*** composing_commands.sh.1     Thu Mar  3 16:56:26 2005
--- composing_commands.sh       Thu Mar  3 15:37:12 2005
***************
*** 33,35 ****
--- 33,36 ----
  #using short circuit command we do conditional execution of if block
  #which we could have wrapped in function and then use above style
 
+ if com1 && com2 && com3; then echo "commands 1 and 2"; fi
tiias5:shellscripts/>diff -c composing_commands.sh.1 composing_commands.sh > mypatch.txt
tiias5:shellscripts/>patch composing_commands.sh.1 mypatch.txt
patching file composing_commands.sh.1
tiias5:shellscripts/>diff composing_commands.sh composing_commands.sh.1
tiias5:shellscripts/>


diff -e oldfile newfile
will output all changes needed to update oldfile to newfile.  This output we call patch. 
 
To apply this patch first we need to modify the patch.  Add
w
Q
to end of patch file.  patch output did not have those to commands to persists changes.  Without it when patch is applied it will print ? which is ed way for prompting user should you quit with unsaved changes.
 
ex. we have old file composing_commands.sh.1 and new file composing_commands.sh. 
Here old = composing_commands.sh.1 while 
     new  = composing_commands.sh  
 
pisdb40:shellscripts/>diff composing_commands.sh.1 composing_commands.sh
36c36,37
< if com1 && com2 && com4; then echo "commands 1 and 2"; fi
---
> if com1 && com2 && com3; then echo "commands 1 and 2"; fi
> echo "hello world"
pisdb40:shellscripts/>diff -e composing_commands.sh.1 composing_commands.sh
36c
if com1 && com2 && com3; then echo "commands 1 and 2"; fi
echo "hello world"
.
pisdb40:shellscripts/>sh.1 composing_commands.sh > mypatch.txt               <
pisdb40:shellscripts/>cat mypatch.txt
36c
if com1 && com2 && com3; then echo "commands 1 and 2"; fi
echo "hello world"
.
pisdb40:shellscripts/>ed composing_commands.sh.1 < mypatch.txt
590
?
pisdb40:shellscripts/>vi mypatch.txt
pisdb40:shellscripts/>ed composing_commands.sh.1 < mypatch.txt
590
609
pisdb40:shellscripts/>diff composing_commands.sh composing_commands.sh.1
pisdb40:shellscripts/>