Shell Programming and Scripting

mv myFile.txt myFile.txt.bak
sed s/foo/bar/g myFile.txt.bak > myFile.txt

Turn the two-line version, above, of the substitution commands into a shell script, subst1 taking three parameters:
the string to be replaced
the string with which to replace it
the name of the file in which to make the substitution
For example,


~/UnixCourse/scriptAsst/subst1 foo bar myFile.txt
should replace all occurrences of "foo" in the file myFile.txt by "bar", leaving the original file as myFile.txt.bak.

Similarly,


~/UnixCourse/scriptAsst/subst1 abc "" aardvark.dat
should remove (replace by the empty string) all occurrences of "abc" in the file aardvark.dat by, leaving the original file as aardvark.dat.bak.

One problem with this approach is that the file's creation and modification dates are altered, even if no change was made to the file contents (because the string being relaced did not occur within the file).

Create a new script, subst2, taking the same three parameters, that performs the substitution and backup file creation when the target string occurs somewhere in the file, but leaves the file completely unchanged (and does not create the .bak file) if the string being replaced is nowhere in the file. (Hint: there are two ways to do this. One is to check first for the presence of the target string and, if it is found, to do the two-step substitution as above. On average, though, it may be faster to go ahead and do the substitution, and then to check and see if the resulting files are identical.)

Generalize your subst2 script, producing a new script subst that will apply a substitution to any number of files given on the command line. For example

~/UnixCourse/scriptAsst/subst foo bar myFile1.txt myFile2.txt myFile3.txt
should apply the same substitution throughout each of the three files named there.

Below is the script I started writing, but I'm not exactly sure how to get the script to make the substitution of the test file.


#!/bin/csh
set string1=$find
set string=$replace
set fileSub=$file


sed "s/$find/$replace/g" $3

and when I give the command ~/UnixCourse/scriptAsst/subst1 and or test1.txt it gives me an error message saying that find is undefined. I am lost with this and any help will be very much appreciated.

---------- Post updated at 11:39 AM ---------- Previous update was at 11:30 AM ----------

This is part of an assignment that I am trying to figure out and I got stuck at this particular part.

Wrong subforum , you should post on Homework & Coursework Questions.
Thread reported.