Shell Programming and Scripting

i have a file call text and i what the user to enter any word he/she wants to replace with any word

i current have

cat text

echo "Please enter the word to be amended:"
read old
echo "Please enter a new word:"
read new

sed "s/$old/$new/g" $1 > text

echo "press ENTER to continue"
read key

what is not right

What is $1?
I guess you are using text as input and output , you can replace (if perl installed):

Code:

sed "s/$old/$new/g" $1 > text

with

Code:

perl -pi -e "s/$old/$new/g" text

the script looks fine.

What's happening that's wrong? consider using set -x to see what you are doing where. Of course, [ "$1" == "text" ] && you get a problem.

What if $old or $new contains a "/" char? You have to escape appropriately your variables to use them in sed regular expressions.

might help to understand what it is you really mean by "what is not right"? A better question would be .... After I run the script my text file is empty or it gives me such and such results and I am looking for .... or it does nothing .... help us understand what your real problem is

I see several different potential problems .....

how do you call this script? (it is expecting a file name ... but the "cat text" always displays the file "text" )

the $1 does not necessarly have to be the file text

if you were to call this script without a file name (or with the file name of text ... you would indeed end up with an empty "text" file.

here is what I would do

change all references of "text" to "$1"

change the sed command to

cat $1 | sed s/'$old'/'$new'/g > $1

The way you wrote the sed empties the text file first then tries to put the now empty file back into itself. The cat/sed combination will empty the text file after it has done the cat.

The following leaves an empty file.

Code:

cat $1 | sed s/'$old'/'$new'/g > $1
Need to direct to a tmp file and then move it back (is there a better way?)
 cat $1 | sed s/'$old'/'$new'/g > $1.$$tmp
 mv $1.$$tmp $1

Moderator's Comments:

Please use CODE tags when displaying sample input, output, and code segments.

I tried my script under ksh, bash, and sh ... all worked with the cat $1 | sed .... > $1 approach ....

Which shell were you using that resulted in an empty file?

Your approach will work, just curious as to the portability of the code if yours fails (on the questioned sed) and mine works.

Thanks