How to get values back from an external function that requires interactivity? - bash

Question

One of the routines I frequently use is a check for valid arguments passed when invoking scripts. Ideally, I'd like to make these, and other, similar, routines external functions that I could call from any script, for handling these more trivial processes. But, I'm having trouble retrieving the values I need from said function(s), without making the process more complicated.
I have tried using command substitution (e.g., echoing the output of the external function into a variable name local to the calling script), which seems to at least work with simpler functions. However, working with this file checking function, requires the read command in a loop, and, thus, user interactivity, which causes the script to hang when trying to resolve the variable that function call is stored in:
#!/bin/bash
# This is a simple function I want to call from other scripts.
exist(){
# If the first parameter passed is not a directory, then the input is
#+ invalid.
if [ ! -d "$1" ]; then
# Rename $1, so we can manipulate its value.
userDir="$1"
# Ask the user for new input while his input is invalid.
while [ ! -d "$userDir" ]; do
echo "\"$userDir\" does not exist."
echo "Enter the path to the directory: "
read userDir
# Convert any tildes in the variable b/c the shell didn't get to
#+ perform expansion.
userDir=`echo "$userDir" | sed "s|~|$HOME|"`
done
fi
}
exist "$1"
How can I retrieve the value of userDir in the calling script without adding (much) complexity?

Answer 1

You can have the exist function interact with the user over stderr and still capture the variable with command substitution. Let's take a simplified example:
exist() { read -u2 -p "Enter dir: " dir; echo "$dir"; }
The option -u2 tells read to use file descriptor 2 (stderr) for interacting with the user. This will continue to work even if stdout has been redirected via command substitution. The option -p "Enter dir: " allows read to set the prompt and capture the user input in one command.
As an example of how it works:
$ d=$(exist)
Enter dir: SomeDirectory
$ echo "$d"
SomeDirectory
Complete example
exist() {
local dir="$1"
while [ ! -d "$dir" ]; do
echo "'$dir' is not a directory." >&2
read -u2 -p "Enter the path to the directory: " dir
dir="${dir/\~/$HOME}"
done
echo "$dir"
}
As an example of this in use:
$ d=$(exist /asdf)
'/asdf' is not a directory.
Enter the path to the directory: /tmp
$ echo "new directory=$d"
new directory=/tmp
Notes:
There is no need for an if statement and a while loop. The while is sufficient on its own.
Single quotes can be put in double-quoted strings without escapes. So, if we write the error message as "'$dir' is not a directory.", escapes are not needed.
All shell variables should be double-quoted unless one wants them to be subject to word splitting and pathname expansion.

Answer 2

Right off the bat I'd say you can 'echo' to the user on stderr and echo your intended answer on stdout.
I had to rearrange a bit to get it working, but this is tested:
exist(){
# If the first parameter passed is not a directory, then the input is
#+ invalid.
userDir="$1"
if [ ! -d "$userDir" ]; then
# Ask the user for new input while his input is invalid.
while [ ! -d "$userDir" ]; do
>&2 echo "\"$userDir\" does not exist."
>&2 echo "Enter the path to the directory: "
read userDir
done
else
>&2 echo "'$1' is indeed a directory"
fi
echo "$userDir"
}
When I tested, I saved that to a file called exist.inc.func
Then I wrote another script that uses it like this:
#!/bin/sh
source ./exist.inc.func
#Should work with no input:
varInCallingProg=$(exist /root)
echo "Got back $varInCallingProg"
#Should work after you correct it interactively:
varInCallingProg2=$(exist /probablyNotAdirOnYourSystem )
echo "Got back $varInCallingProg2"