Generate a file name for a new file, based on the specified prefix and suffix. Uniqueness is guaranteed by a sequence number. E.g. "newfile test- .txt" will return a file name in the form test-NNN.txt where NNN is the first number (001, 002, etc.) that results in a file name that doesn't exist yet.

newfile.sh

# Return a file name consisting of the specified prefix, number, and suffix
# 3 arguments:
# $1 - prefix
# $2 - an integer, which will be padded with zeroes to length 3
# $3 - suffix
# e.g. "mkfile test- 2 .txt" will return "test-002.txt"
mkfile ()
{
    echo "${1}$(printf '%03d' ${2})${3}"
}

# Generate a file name for a new file, based on the specified prefix and
# suffix. Uniqueness is guaranteed by a sequence number. 
# E.g. "newfile test- .txt" will return a file name in the form
# test-NNN.txt where NNN is the first number (001, 002, etc.) that results in
# a file name that doesn't exist yet.
#
# 2 arguments:
# $1 - prefix
# $2 - suffix
# E.g. calling "newfile test- .txt" will return, consecutively:
# test-001.txt, test-002.txt, test-003.txt, etc.
newfile ()
{
    i=1

    while [ -f "$(mkfile $1 $i $2)" ]; do
        ((i++))
    done
    mkfile $1 $i $2
}