shell script help
 

I am creating a new script, sub2, taking three parameters, that treats the string to be replaced as plain text instead of as a regular expression.

The script works for everything except spaces.

#!/bin/csh
set firstString="$1"
set secondString="$2"
set file="$3"

#adds on escape characters to be used later in sed
set arg=`echo "$firstString" | sed 's:[]\[\^\$\.\*\/]:\\&:g'`


sed "s/$arg/$secondString/g" "$file" > "$file.updated"
mv "$file.updated" "$file"

from my terminal here are a few examples:

sub2 o a test.txt

test.txt will change from

"hello world" to "hella warld"

sub2 . ! test.txt

"hello..." to "hello!!!"

my problem is I do not understand much about shell scripting or sed in general.

when i type :

sub " " x test.txt

I expect all the spaces to be replaced with "x".

But I get an error:

sed: -e expression #1, char 0: no previous regular expression

Any help would be greatly appreciated.

Hi there,

Maybe you should use sed with a -e option:

Hope it helps, tell if not.

EDIT:
1.- can you post the value of $arg?
2.- Shouldn't you escape secondString too? At least for slashes and variables...

sed -e "s/$arg/$secondString/g" "$file" > "$file.updated"

with

echo "$firstString"
echo "$secondString"
echo "$file"
echo "$arg"

this was the output:


sub2 " " a test1.txt (command I ran from the terminal) spaces indicate the values firstString and secondString

a
test1.txt

sed: -e expression #1, char 0: no previous regular expression

So still not there yet. If I eventually figure it out ill post on here what I was missing.

Hi again,
What about using bash instead of csh?

I am using ssh to connect to the linux box. So I don't think I have that option. It should work on either but I'm not sure. Why I am posting on the newbie forum.

Hi again,
Sorry, I don't know nearly anything about csh. I know it's doing some substitutions differently from what bash does (it strips initial/final spaces, for example). To use bash in your script instead of csh you only need to change csh to bash in the hash bang (#!/bin/bash, script's first line). Then, you can stop using 'set' and you can use $(command) instead of `command`. The example I posted before works as expected if bash is installed in the system (nearly all modern linux have it installed).

I tried what you suggested and it isn't working on cases that previously were. Would you please post what you got working so I know that I changed everything properly.

I switched it over to sh correctly. Now the expression arg="`echo "$firstString" | sed 's:[]\[\^\$\.\*\/]:\\&:g'`" isnt returning the proper variable.

What is a good expression to change special characters to special characters with escape characters.

For example * to \* etc

Maybe [:punct:]?