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(gawk.info)More Complex

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A More Complex Example
======================

   Now that we've mastered some simple tasks, let's look at what
typical `awk' programs do.  This example shows how `awk' can be used to
summarize, select, and rearrange the output of another utility.  It uses
features that haven't been covered yet, so don't worry if you don't
understand all the details:

     ls -l | awk '$6 == "Nov" { sum += $5 }
                  END { print sum }'

   This command prints the total number of bytes in all the files in the
current directory that were last modified in November (of any year).
(1) The `ls -l' part of this example is a system command that gives you
a listing of the files in a directory, including each file's size and
the date the file was last modified. Its output looks like this:

     -rw-r--r--  1 arnold   user   1933 Nov  7 13:05 Makefile
     -rw-r--r--  1 arnold   user  10809 Nov  7 13:03 awk.h
     -rw-r--r--  1 arnold   user    983 Apr 13 12:14 awk.tab.h
     -rw-r--r--  1 arnold   user  31869 Jun 15 12:20 awk.y
     -rw-r--r--  1 arnold   user  22414 Nov  7 13:03 awk1.c
     -rw-r--r--  1 arnold   user  37455 Nov  7 13:03 awk2.c
     -rw-r--r--  1 arnold   user  27511 Dec  9 13:07 awk3.c
     -rw-r--r--  1 arnold   user   7989 Nov  7 13:03 awk4.c

The first field contains read-write permissions, the second field
contains the number of links to the file, and the third field
identifies the owner of the file. The fourth field identifies the group
of the file.  The fifth field contains the size of the file in bytes.
The sixth, seventh and eighth fields contain the month, day, and time,
respectively, that the file was last modified.  Finally, the ninth field
contains the name of the file.(2)

   The `$6 == "Nov"' in our `awk' program is an expression that tests
whether the sixth field of the output from `ls -l' matches the string
`Nov'.  Each time a line has the string `Nov' for its sixth field, the
action `sum += $5' is performed.  This adds the fifth field (the file's
size) to the variable `sum'.  As a result, when `awk' has finished
reading all the input lines, `sum' is the total of the sizes of the
files whose lines matched the pattern.  (This works because `awk'
variables are automatically initialized to zero.)

   After the last line of output from `ls' has been processed, the
`END' rule executes and prints the value of `sum'.  In this example,
the value of `sum' is 140963.

   These more advanced `awk' techniques are covered in later sections
(Note: Actions.).  Before you can move on to more
advanced `awk' programming, you have to know how `awk' interprets your
input and displays your output.  By manipulating fields and using
`print' statements, you can produce some very useful and impressive
looking reports.

   ---------- Footnotes ----------

   (1) In the C shell (`csh'), you need to type a semicolon and then a
backslash at the end of the first line; see Note: `awk' Statements
Versus Lines, for an explanation as to why.  In a
POSIX-compliant shell, such as the Bourne shell or `bash', you can type
the example as shown.  If the command `echo $path' produces an empty
output line, you are most likely using a POSIX-compliant shell.
Otherwise, you are probably using the C shell or a shell derived from
it.

   (2) On some very old systems, you may need to use `ls -lg' to get
this output.