Shell Programming and Scripting

#!/bin/bash
if [[ -f $1 ]]
then
        file=$1
        file2="${file%R1.fastq}R2.fastq"
        echo fetching data from R2 file ...
        sleep 3
        cat $file $file2 > infile
else
        echo "Input_file passed as an argument $1 is NOT found."
        exit;
fi

        file=$1
        file2=eval echo "$file" | sed -r 's/(.*)R1(.*)/\1R2\2/'

file="${1%1.*}"
file2="${1/R1/R2}"
cat $file*
cat "$1" "${1/R1/R2}"
cat "$1" "$file2"
file2=$(sed 's/R1/R2/' <<<"$1")

file2=$(...)

cat ${file%R1*}*