I am having problem writing the following script, this is more like a boolean case where i need to check for existance of a string in a file and if it exist i have to do certain things.

If ( such string exist in a file)

sed 's/car/bus/g' file

else

do something else.

May try this way...

grep car file > /dev/null
if [ $? -eq 0 ]; then
sed 's/car/bus/g' file
else
do something else
fi

Why do you need the > /dev/null? (Im new to this myself)

This works too:

$ cat grep-from.ksh
#!/bin/ksh
grep car file4                                                                                                                 
if [ $? == 0 ]; then                                                                                                                
sed 's/car/bus/g' file                                                                                                             
else                                                                                                                                
echo "That didn't go well"                                                                                                          
fi

$ if grep "car" gr.txt > /dev/null
> then
> sed -i 's/car/BUS/g' gr.txt
> else
> echo "No car found in gr.txt"
> fi

$ cat gr.txt
my BUS
this is new
BUS is great