Bourne Shell Script - sh

Question

I'm attempting to write a script in the Bourne shell that will do the following:
Read in a filename
If the file does not exist in the target directory, it will display a message to the user stating such
If the file exists in the target directory, it will be moved to a /trash folder
If the file exists in the target directory, but a file of the same name is in the /trash folder, it will still move the file to the /trash directory, but will attach a _bak extention to the file.
My use of the Bourne shell is minimal, so here's what I have so far. Any pointers or tips would be greatly appreciated, thanks!
#!/bin/sh
#Scriptname: Trash Utility
source_dir=~/p6_tmp
target_dir=~/trash
echo "Please enter the filename you wish to trash:"
read filename
if [ -f $source_dir $filename]
then mv "$filename" "$target_dir"
else
echo "$filename does not exist"
fi

Answer 1

You cannot use ~ to refer to $HOME in a sh script. Switch to $HOME (or change the shebang to a shell which supports this, such as #!/bin/bash).
To refer to a file in a directory, join them with a slash:
if [ -f "$source_dir/$filename" ]
Notice also the required space before the terminating ] token.
To actually move the file you tested for, use the same expression for the source argument to mv:
mv "$source_dir/$filename" "$target_dir"
As a general design, a script which takes a command-line parameter is much easier to integrate into future scripts than one wich does interactive prompting. Most modern shells offer file name completion and history mechanisms, so a noninteractive script also tends to be more usable (you practically never need to transcribe a file name manually).

Answer 2

A Bash Solution:
#!/bin/bash
source_dir="~/p6_tmp"
target_dir="~/trash"
echo "Please enter the filename you wish to trash:"
read filename
if [ -f ${source_dir}/${filename} ]
then
if [ -f ${target_dir}/${filename} ]
then
mv "${source_dir}/${filename}" "${target_dir}/${filename}_bak"
else
mv "${source_dir}/${filename}" "$target_dir"
fi
else
echo "The file ${source_dir}/${filename} does not exist"
fi

Answer 3

Here's the completed script. Thanks again to all who helped!
#!/bin/sh
#Scriptname: Trash Utility
#Description: This script will allow the user to enter a filename they wish to send to the trash folder.
source_dir=~/p6_tmp
target_dir=~/trash
echo "Please enter the file you wish to trash:"
read filename
if [ -f "$source_dir/$filename" ]
then
if [ -f "$target_dir/$filename" ]
then mv "$source_dir/$filename" "$target_dir/$(basename "$filename")_bak"
date "+%Y-%m-%d %T - Trash renamed ~/$(basename "$source_dir")/$filename to ~/$(basename "/$target_dir")/$(basename "$filename")_bak" >> .trashlog
else mv "$source_dir/$filename" "$target_dir"
date "+%Y-%m-%d %T - Trash moved ~/$(basename "/$source_dir")/$filename to ~/$(basename "/$target_dir")/$filename" >> .trashlog
fi
else
date "+%Y-%m-%d %T - Trash of ~/$(basename "/$source_dir")/$filename does not exist" >> .trashlog
fi