1. # prepend and append some text on each line of a text file:
     awk '{print "prefix "$1" postfix"}' input.txt > /tmp/output.txt 
    
  2. # replace the value of the second column in file1.dat by zero if it is strictly less than 0.1 (cfr., the gnuplot manual):
     awk '{print $1,($2<0.1) ? 0.0 : $2}' file1.da 
    
  3. # multiply the numeric value of the second column in file1.dat by five if the value of the first column is in the range of [1,3] (cfr., the gnuplot manual):
     awk '{print $1,($1<=3 && $1>=1)? $2*5 : $2}' file1.dat