11.1. Managing Output Streams

While printing to the standard output is the default behavior, real-world scripts often need to save results to files for persistence or pass data to other command-line tools for further processing. I/O redirection is AWK’s mechanism for managing these output streams directly from within a script.

11.2. Output to Files

Overwrite Operator ()

This operator directs the output of a print or printf statement to a file. If the file does not exist, it is created. If it does exist, its contents are overwritten.

• Syntax: print DATA > output-file

# This command will overwrite /tmp/message.txt

[jerry]$ awk ‘BEGIN { print “Hello, World !!!” > “/tmp/message.txt” }’

Append Operator ()

This operator also directs output to a file, but it appends the new data to the end of the file, preserving any existing content.

• Syntax: print DATA >> output-file

# This command will add a new line to /tmp/message.txt

[jerry]$ awk ‘BEGIN { print “Hello, World !!!” >> “/tmp/message.txt” }’

11.3. Piping to External Commands

One-Way Pipe ()

The pipe operator sends the output of a print or printf statement as the standard input to another shell command.

• Syntax: print items | command

This example pipes a string to the tr command to convert it to uppercase.

[jerry]$ awk ‘BEGIN { print “hello, world !!!” | “tr [a-z] [A-Z]” }’

Output:

HELLO, WORLD !!!

Two-Way Communication ()

AWK can also establish a two-way pipe with an external command, creating a co-process. This allows a script to both send data to and receive data from the command.

Consider the following script, command.awk:

BEGIN {

cmd = “tr [a-z] [A-Z]”

print “hello, world !!!” |& cmd

close(cmd, “to”)

cmd |& getline out

print out;

close(cmd);

}

Execution Analysis:

1. cmd = “tr …”: A variable stores the command string.
2. print … |& cmd: The string “hello, world !!!” is sent to the tr command’s standard input.
3. close(cmd, “to”): The “to” part of the co-process stream is closed, signaling to tr that there is no more input.
4. cmd |& getline out: AWK reads one line of output from the tr command and stores it in the out variable.
5. print out: The contents of out are printed to standard output.
6. close(cmd): The co-process is fully closed.

Output:

HELLO, WORLD !!!

Whether output is sent to a file or the screen, it is often necessary to format it precisely, which is the role of the printf function.