4.6 Redirecting Output of print and printf

So far, the output from print and printf has gone to the standard output, usually the terminal. Both print and printf can also send their output to other places. This is called redirection.

A redirection appears after the print or printf statement. Redirections in awk are written just like redirections in shell commands, except that they are written inside the awk program.

There are four forms of output redirection: output to a file, output appended to a file, output through a pipe to another command, and output to a coprocess. They are all shown for the print statement, but they work identically for printf:

print items > output-file

This type of redirection prints the items into the output file named output-file. The file name output-file can be any expression. Its value is changed to a string and then used as a file name (see Expressions).

When this type of redirection is used, the output-file is erased before the first output is written to it. Subsequent writes to the same output-file do not erase output-file, but append to it. (This is different from how you use redirections in shell scripts.) If output-file does not exist, it is created. For example, here is how an awk program can write a list of BBS names to one file named name-list, and a list of phone numbers to another file named phone-list:

          $ awk '{ print $2 > "phone-list"
          >        print $1 > "name-list" }' BBS-list
          $ cat phone-list
          -| 555-5553
          -| 555-3412
          ...
          $ cat name-list
          -| aardvark
          -| alpo-net
          ...
     

Each output file contains one name or number per line.

print items >> output-file

This type of redirection prints the items into the pre-existing output file named output-file. The difference between this and the single-`>' redirection is that the old contents (if any) of output-file are not erased. Instead, the awk output is appended to the file. If output-file does not exist, then it is created.

print items | command

It is also possible to send output to another program through a pipe instead of into a file. This type of redirection opens a pipe to command, and writes the values of items through this pipe to another process created to execute command.

The redirection argument command is actually an awk expression. Its value is converted to a string whose contents give the shell command to be run. For example, the following produces two files, one unsorted list of BBS names, and one list sorted in reverse alphabetical order:

          awk '{ print $1 > "names.unsorted"
                 command = "sort -r > names.sorted"
                 print $1 | command }' BBS-list
     

The unsorted list is written with an ordinary redirection, while the sorted list is written by piping through the sort utility.

The next example uses redirection to mail a message to the mailing list `bug-system'. This might be useful when trouble is encountered in an awk script run periodically for system maintenance:

          report = "mail bug-system"
          print "Awk script failed:", $0 | report
          m = ("at record number " FNR " of " FILENAME)
          print m | report
          close(report)
     

The message is built using string concatenation and saved in the variable m. It's then sent down the pipeline to the mail program. (The parentheses group the items to concatenate—see Concatenation.)

The close function is called here because it's a good idea to close the pipe as soon as all the intended output has been sent to it. See Close Files And Pipes, for more information.

This example also illustrates the use of a variable to represent a file or command—it is not necessary to always use a string constant. Using a variable is generally a good idea, because awk requires that the string value be spelled identically every time.

print items |& command

This type of redirection prints the items to the input of command. The difference between this and the single-`|' redirection is that the output from command can be read with getline. Thus command is a coprocess, which works together with, but subsidiary to, the awk program.

This feature is a gawk extension, and is not available in POSIX awk. See Two-way I/O, for a more complete discussion.

Redirecting output using `>', `>>', `|', or `|&' asks the system to open a file, pipe, or coprocess only if the particular file or command you specify has not already been written to by your program or if it has been closed since it was last written to.

It is a common error to use `>' redirection for the first print to a file, and then to use `>>' for subsequent output:

     # clear the file
     print "Don't panic" > "guide.txt"
     ...
     # append
     print "Avoid improbability generators" >> "guide.txt"

This is indeed how redirections must be used from the shell. But in awk, it isn't necessary. In this kind of case, a program should use `>' for all the print statements, since the output file is only opened once.

As mentioned earlier (see Getline Notes), many Many awk implementations limit the number of pipelines that an awk program may have open to just one! In gawk, there is no such limit. gawk allows a program to open as many pipelines as the underlying operating system permits.

Advanced Notes: Piping into sh

A particularly powerful way to use redirection is to build command lines and pipe them into the shell, sh. For example, suppose you have a list of files brought over from a system where all the file names are stored in uppercase, and you wish to rename them to have names in all lowercase. The following program is both simple and efficient:

     { printf("mv %s %s\n", $0, tolower($0)) | "sh" }
     
     END { close("sh") }

The tolower function returns its argument string with all uppercase characters converted to lowercase (see String Functions). The program builds up a list of command lines, using the mv utility to rename the files. It then sends the list to the shell for execution.