Calling an executable program using awk

Question

I have a program in C that I want to call by using awk in shell scripting. How can I do something like this?

Answer 1

From the AWK man page:

 system(cmd)               executes cmd and returns its exit status 

The GNU AWK manual also has a section that, in part, describes the system function and provides an example:

system("date | mail -s 'awk run done' root") 

Answer 2

A much more robust way would be to use the getline() function of GNU awk to use a variable from a pipe. In form cmd | getline result, cmd is run, then its output is piped to getline. It returns 1 if got output, 0 if EOF, -1 on failure.

First construct the command to run in a variable in the BEGIN clause if the command is not dependant on the contents of the file, e.g. a simple date or an ls.

A simple example of the above would be

awk 'BEGIN {     cmd = "ls -lrth"     while ( ( cmd | getline result ) > 0 ) {         print result     }     close(cmd); }' 

When the command to run is part of the columnar content of a file, you generate the cmd string in the main {..} as below. E.g. consider a file whose $2 contains the name of the file and you want it to be replaced with the md5sum hash content of the file. You can do

awk '{ cmd = "md5sum "$2        while ( ( cmd | getline md5result ) > 0 ) {             $2 = md5result        }        close(cmd);  }1' 

Another frequent usage involving external commands in awk is during date processing when your awk does not support time functions out of the box with mktime(), strftime() functions.

Consider a case when you have Unix EPOCH timestamp stored in a column and you want to convert that to a human readable date format. Assuming GNU date is available

awk '{ cmd = "date -d @" $1 " +\"%d-%m-%Y %H:%M:%S\""         while ( ( cmd | getline fmtDate) > 0 ) {             $1 = fmtDate        }        close(cmd); }1'  

for an input string as

1572608319 foo bar zoo 

the above command produces an output as

01-11-2019 07:38:39 foo bar zoo 

The command can be tailored to modify the date fields on any of the columns in a given line. Note that -d is a GNU specific extension, the *BSD variants support -f ( though not exactly similar to -d).

More information about getline can be referred to from this AllAboutGetline article at awk.freeshell.org page.