Running shell commands from awk

There are a few ways to run shell commands from inside an awk command. One is system(...) but it is better for handing values off to a program to do something, not the best way to get some data back from the shell command.

The best way to get data back from a command is to define the command as a variable then execute it piping the output to a new variable using the built in getline function.

The input of this example is a log file were the first field is epoch time and we need to see the time in human readable time format.

tail /var/log/my.log | awk ' {
   DC="date -d@"$1; 
   DC | getline T; 
   printf "%s\t", T; 
   for(i=2;i<NF;i++) {
     printf $i"\t"
   }; 
   printf "\n";
}'

In this example DC becomes the date command that is given the epoch value in variable $1.  T is the variable for the time as a string that we display using printf. Then a loop prints the rest of the data from each line of the log from the second filed to the NF, end number of fields.