How do I tell if a regular file does not exist in Bash?

The test command ([ here) has a "not" logical operator which is the exclamation point (similar to many other languages). Try this:

if [ ! -f /tmp/foo.txt ]; then    echo "File not found!"fi

Bash File Testing

-b filename - Block special file
-c filename - Special character file
-d directoryname - Check for directory Existence
-e filename - Check for file existence, regardless of type (node, directory, socket, etc.)
-f filename - Check for regular file existence not a directory
-G filename - Check if file exists and is owned by effective group ID
-G filename set-group-id - True if file exists and is set-group-id
-k filename - Sticky bit
-L filename - Symbolic link
-O filename - True if file exists and is owned by the effective user id
-r filename - Check if file is a readable
-S filename - Check if file is socket
-s filename - Check if file is nonzero size
-u filename - Check if file set-user-id bit is set
-w filename - Check if file is writable
-x filename - Check if file is executable

How to use:

#!/bin/bashfile=./fileif [ -e "$file" ]; then    echo "File exists"else     echo "File does not exist"fi 

A test expression can be negated by using the ! operator

#!/bin/bashfile=./fileif [ ! -e "$file" ]; then    echo "File does not exist"else     echo "File exists"fi 

You can negate an expression with "!":

#!/bin/bashFILE=$1if [ ! -f "$FILE" ]then    echo "File $FILE does not exist"fi

The relevant man page is man test or, equivalently, man [ -- or help test or help [ for the built-in bash command.