Write to a text file in #!/bin/sh

[gsopitan]

I am creating a command that does the following;

commandname increment file num

file contains an integer
num is an integer

I want my command to add the value of arg2 to the content arg1

I was thinking;

#!/bin/sh

for $3 in ![0-9] # I am not even sure this would work
do
echo 1>&2 Error: You must enter an integer value
exit 5
done

# This is where I am completely lost. I am not sure how to write the value of "num" + `cat file` in "file"

Any help would be greatly appreciated.

Thanks

 

[SamBones]

This sounds like a class assignment, so please don't ask us to code it for you. Your code example says "I am not even sure this would work". Well what have you actually tried on a system? And what were the results or error messages?