for num in $(cat 1.txt|awk '{print $1}'); do grep "^$num" [21].txt | tr -d '\n' | sed 's/\([0-9].txt:\)//g' | sed 's/\([0-9]\)[a-zA-Z]/\1/g'; echo ''; done | sed 's/^[ \t]*//;s/[ \t]*$//;/^$/d' | awk 'int($3) {print $0}'

for num in $(cat 1.txt|awk '{print $1}'); do 
    # grep - Pull lines from relevant files and output them
    # Will be in the format '<filename>:<line>
    # Trim the newline so it's all one one-line for each entry.
    # Use sed to remove the filenames from the line
    # We'll have a <number>a <number2> -  sed removes the letter after the number
    grep "^$num" [21].txt | \
    tr -d '\n' | \
    sed 's/\([0-9].txt:\)//g' | \
    sed 's/\([0-9]\)[a-zA-Z]/\1/g'
    #We removed CRs, add one back to delimit EOL
    echo ' '
done | \
    sed 's/^[ \t]*//;s/[ \t]*$//;/^$/d' | \
    awk 'int($3) {print $0}'
    # Use sed to trim all leading/trailing whitespace and delete empty lines
    # Use awk to only print lines that have a 3rd parameter, IE it was found in both files.

$ for num in $(cat 1.txt|awk '{print $1}'); do grep "^$num" [21].txt | tr -d '\n' | sed 's/\([0-9].txt:\)//g' | sed 's/\([0-9]\)[a-zA-Z]/\1/g'; echo ' '; done | sed 's/^[ \t]*//;s/[ \t]*$//;/^$/d' | awk 'int($3) {print $0}'
a 10 21
b 11 22
c 12 23
g 16 24
h 17 25
i 18 26