using tilde in variable from the user

Question

I saw a lot of posts about it all over the internet but i couldn't make it run as i wanted.

I have an input from the user of a path. If the user will type ~/Desktop/ for example i can't use $ans in grep and other commands because of the tilde.

here is an example of my code. the second line is my main problem right now.

read -p "Enter new path or type 'exit':   " ans
file=`grep -x $ans ~/.Log` 

Appreciate your help.

Answer 1

Several problems here:

1. You aren't quoting your variable. This means that whatever the user passes in will be subjected (after parameter expansion) to word splitting on whitespace and to glob expansion. This will break things. See:

   • Why does my shell script choke on whitespace or other special characters?
   • Security implications of forgetting to quote a variable in bash/POSIX shells

2. The user input could start with a hyphen. If it does, it will be interpreted as an option to grep. To avoid this, use the -e option to tell grep that the argument is specifically a search pattern, no matter what it looks like.

3. You are using regex search. You probably want fixed text search, not regex. So use -F.

4. Backticks are deprecated. Use $(...) instead. See also:

   • Have backticks (i.e. `cmd`) in *sh shells been deprecated?

---

Putting these points together, you will have code that looks more like:

read -p "Enter new path or type 'exit':   " ans
file="$(grep -Fxe "$ans" ~/.Log)"

---

But now we come to the additional problems:

5. The variable file will only contain exactly what the user typed, anyway. Or possibly it will contain nothing. Or possibly it will contain multiple copies of what the user typed, delimited by newlines.

   If we omit the -F flag which I added, then the variable file will only contain full-line matches of the regex the user typed in.

   Neither of these things are necessarily files. The fact that you named the variable file indicates to me that you have some other expectation for what the code does than what it really does in actual fact.

6. There are also many potential problems associated with using read, which may not matter for your script but you should at least be aware of them; see:

   • Why is using a shell loop to process text considered bad practice?

   (Though that post discusses using a shell loop to process text, which you're not doing, it also explains with good examples some of the quirks involved in using read.)

7. Also, whatever it is that you're doing with that file variable later on could be much more simply accomplished with a pipeline.

I can't take this any further, though, because I don't know what the rest of your script looks like.

Answer 2

Quote wrap the search variable

file=`grep -x "$ans" ~/.Log`

Answer 3

See this question and the answers there. It has to do with the order and quoting of shell expansions, specifically tilde expansion. For me the simplest way is my answer there to search and replace ~ with the value of $HOME. So you can add this line before your grep:

ans="${ans//\~/$HOME}"

This is a bashism, but this is tagged with bash.