awk: external command

1. Run the external command with "system"
text2:

 3 Chicago  
 5 Los Angeles  
 1 Boston  
 4 Atlantic  

script_1:

 #! /bin/bash  
   
 awk '{  
   print $0 > "text";  
 }  
 END {  
   close(text);   
   #clear buffer, making sure buffer content(4 lines of   
   #records from text2) are saved in "text" file  
   
   system("sort < text");  
   #Run the external command, return code of system is  
   #the return code of external command. And standard   
   #output and standard error of external command are   
   #as awk. In this case, awk standard output is terminal  
   #So it will output content to terminal  
 }' text2  
   
 #output:  
 #1 Boston  
 #3 Chicago  
 #4 Atlantic  
 #5 Los Angeles  
   
 #Following awk program is same as above, unless  
 #that, we define standard output of awk to text3  
 #Then system's external command will output to  
 #text3  
 awk '{  
   print $0 > "text";  
 }  
 END {  
   close(text);   
   system("sort < text");  
 }' text2 >text3  
   
 cat text3;  
 #output:  
 #1 Boston  
 #3 Chicago  
 #4 Atlantic  
 #5 Los Angeles  

2. Run the external command with pipeline
script_1:

 #! /bin/bash  
   
 awk 'BEGIN {  
   shell="/bin/bash"  
   command="echo Hello world!";  
   print command | shell;  
   
   print "======="  
   
   command="v=\"Amazing world!\"";  
   print command | shell;  
   
   print "======="  
   
   command="echo $v";  
   print command | shell;  
   close(shell);  
 }'  
   
 #output:  
 #=======  
 #=======  
 #Hello world!  
 #Amazing world!  
   
 # For all commands we passed to shell, it never get   
 # executed until call "close". That's why 2 lines of  
 # separators get output firstly, then shell's output  
 # get printed out lastly.  
 # Before "close" command is called, all commands we  
 # passed to shell will be buffered, and these commands  
 # will get executed like a script when close is called.  
   
 awk 'BEGIN {  
   command="v=\"Hello world!\"";  
   shell="/bin/bash";  
   print command | shell;  
   close(shell);  
     
   command="echo $v";  
   print command | shell;  
   close(shell);  
 }'  
 #output: empty line  
   
 #Since we closed the shell after feeding command to it.  
 #Then next time, when print feed command to the shell,  
 #awk starts a new shell process, who doesn't recognize  
 #the variable "v" at all! Apparently it will output an  
 #empty line, since v is empty there.