13.3.9 An Easy Way to Use Library Functions

In Including Other Files Into Your Program, we saw how gawk provides a built-in file-inclusion capability. However, this is a gawk extension. This section provides the motivation for making file inclusion available for standard awk, and shows how to do it using a combination of shell and awk programming.

Using library functions in awk can be very beneficial. It encourages code reuse and the writing of general functions. Programs are smaller and therefore clearer. However, using library functions is only easy when writing awk programs; it is painful when running them, requiring multiple ‘-f’ options. If gawk is unavailable, then so too is the AWKPATH environment variable and the ability to put awk functions into a library directory (see section Command-Line Options). It would be nice to be able to write programs in the following manner:

# library functions
@include getopt.awk
@include join.awk
…

# main program
BEGIN {
    while ((c = getopt(ARGC, ARGV, "a:b:cde")) != -1)
        …
    …
}

The following program, ‘igawk.sh’, provides this service. It simulates gawk’s searching of the AWKPATH variable and also allows nested includes; i.e., a file that is included with ‘@include’ can contain further ‘@include’ statements. igawk makes an effort to only include files once, so that nested includes don’t accidentally include a library function twice.

igawk should behave just like gawk externally. This means it should accept all of gawk’s command-line arguments, including the ability to have multiple source files specified via ‘-f’, and the ability to mix command-line and library source files.

The program is written using the POSIX Shell (sh) command language.(79) It works as follows:

1. Loop through the arguments, saving anything that doesn’t represent awk source code for later, when the expanded program is run.
2. For any arguments that do represent awk text, put the arguments into a shell variable that will be expanded. There are two cases:
   1. Literal text, provided with ‘--source’ or ‘--source=’. This text is just appended directly.
   2. Source file names, provided with ‘-f’. We use a neat trick and append ‘@include filename’ to the shell variable’s contents. Since the file-inclusion program works the way gawk does, this gets the text of the file included into the program at the correct point.
3. Run an awk program (naturally) over the shell variable’s contents to expand ‘@include’ statements. The expanded program is placed in a second shell variable.
4. Run the expanded program with gawk and any other original command-line arguments that the user supplied (such as the data file names).

This program uses shell variables extensively: for storing command-line arguments, the text of the awk program that will expand the user’s program, for the user’s original program, and for the expanded program. Doing so removes some potential problems that might arise were we to use temporary files instead, at the cost of making the script somewhat more complicated.

The initial part of the program turns on shell tracing if the first argument is ‘debug’.

The next part loops through all the command-line arguments. There are several cases of interest:

--

This ends the arguments to igawk. Anything else should be passed on to the user’s awk program without being evaluated.

-W

This indicates that the next option is specific to gawk. To make argument processing easier, the ‘-W’ is appended to the front of the remaining arguments and the loop continues. (This is an sh programming trick. Don’t worry about it if you are not familiar with sh.)

-v, -F

These are saved and passed on to gawk.

-f, --file, --file=, -Wfile=

The file name is appended to the shell variable program with an ‘@include’ statement. The expr utility is used to remove the leading option part of the argument (e.g., ‘--file=’). (Typical sh usage would be to use the echo and sed utilities to do this work. Unfortunately, some versions of echo evaluate escape sequences in their arguments, possibly mangling the program text. Using expr avoids this problem.)

--source, --source=, -Wsource=

The source text is appended to program.

--version, -Wversion

igawk prints its version number, runs ‘gawk --version’ to get the gawk version information, and then exits.

If none of the ‘-f’, ‘--file’, ‘-Wfile’, ‘--source’, or ‘-Wsource’ arguments are supplied, then the first nonoption argument should be the awk program. If there are no command-line arguments left, igawk prints an error message and exits. Otherwise, the first argument is appended to program. In any case, after the arguments have been processed, program contains the complete text of the original awk program.

The program is as follows:

#! /bin/sh
# igawk --- like gawk but do @include processing

if [ "$1" = debug ]
then
    set -x
    shift
fi

# A literal newline, so that program text is formatted correctly
n='
'

# Initialize variables to empty
program=
opts=

while [ $# -ne 0 ] # loop over arguments
do
    case $1 in
    --)     shift
            break ;;

    -W)     shift
            # The ${x?'message here'} construct prints a
            # diagnostic if $x is the null string
            set -- -W"${@?'missing operand'}"
            continue ;;

    -[vF])  opts="$opts $1 '${2?'missing operand'}'"
            shift ;;

    -[vF]*) opts="$opts '$1'" ;;

    -f)     program="$program$n@include ${2?'missing operand'}"
            shift ;;

    -f*)    f=$(expr "$1" : '-f\(.*\)')
            program="$program$n@include $f" ;;

    -[W-]file=*)
            f=$(expr "$1" : '-.file=\(.*\)')
            program="$program$n@include $f" ;;

    -[W-]file)
            program="$program$n@include ${2?'missing operand'}"
            shift ;;

    -[W-]source=*)
            t=$(expr "$1" : '-.source=\(.*\)')
            program="$program$n$t" ;;

    -[W-]source)
            program="$program$n${2?'missing operand'}"
            shift ;;

    -[W-]version)
            echo igawk: version 3.0 1>&2
            gawk --version
            exit 0 ;;

    -[W-]*) opts="$opts '$1'" ;;

    *)      break ;;
    esac
    shift
done

if [ -z "$program" ]
then
     program=${1?'missing program'}
     shift
fi

# At this point, `program' has the program.

The awk program to process ‘@include’ directives is stored in the shell variable expand_prog. Doing this keeps the shell script readable. The awk program reads through the user’s program, one line at a time, using getline (see section Explicit Input with getline). The input file names and ‘@include’ statements are managed using a stack. As each ‘@include’ is encountered, the current file name is “pushed” onto the stack and the file named in the ‘@include’ directive becomes the current file name. As each file is finished, the stack is “popped,” and the previous input file becomes the current input file again. The process is started by making the original file the first one on the stack.

The pathto() function does the work of finding the full path to a file. It simulates gawk’s behavior when searching the AWKPATH environment variable (see section The AWKPATH Environment Variable). If a file name has a ‘/’ in it, no path search is done. Similarly, if the file name is "-", then that string is used as-is. Otherwise, the file name is concatenated with the name of each directory in the path, and an attempt is made to open the generated file name. The only way to test if a file can be read in awk is to go ahead and try to read it with getline; this is what pathto() does.(80) If the file can be read, it is closed and the file name is returned:

expand_prog='

function pathto(file,    i, t, junk)
{
    if (index(file, "/") != 0)
        return file

    if (file == "-")
        return file

    for (i = 1; i <= ndirs; i++) {
        t = (pathlist[i] "/" file)

        if ((getline junk < t) > 0) {
            # found it
            close(t)
            return t
        }

    }
    return ""
}

The main program is contained inside one BEGIN rule. The first thing it does is set up the pathlist array that pathto() uses. After splitting the path on ‘:’, null elements are replaced with ".", which represents the current directory:

BEGIN {
    path = ENVIRON["AWKPATH"]
    ndirs = split(path, pathlist, ":")
    for (i = 1; i <= ndirs; i++) {
        if (pathlist[i] == "")
            pathlist[i] = "."
    }

The stack is initialized with ARGV[1], which will be ‘/dev/stdin’. The main loop comes next. Input lines are read in succession. Lines that do not start with ‘@include’ are printed verbatim. If the line does start with ‘@include’, the file name is in $2. pathto() is called to generate the full path. If it cannot, then the program prints an error message and continues.

The next thing to check is if the file is included already. The processed array is indexed by the full file name of each included file and it tracks this information for us. If the file is seen again, a warning message is printed. Otherwise, the new file name is pushed onto the stack and processing continues.

Finally, when getline encounters the end of the input file, the file is closed and the stack is popped. When stackptr is less than zero, the program is done:

    stackptr = 0
    input[stackptr] = ARGV[1] # ARGV[1] is first file

    for (; stackptr >= 0; stackptr--) {
        while ((getline < input[stackptr]) > 0) {
            if (tolower($1) != "@include") {
                print
                continue
            }
            fpath = pathto($2)

            if (fpath == "") {
                printf("igawk:%s:%d: cannot find %s\n",
                    input[stackptr], FNR, $2) > "/dev/stderr"
                continue
            }

            if (! (fpath in processed)) {
                processed[fpath] = input[stackptr]
                input[++stackptr] = fpath  # push onto stack
            } else
                print $2, "included in", input[stackptr],
                    "already included in",
                    processed[fpath] > "/dev/stderr"
        }
        close(input[stackptr])
    }
}'  # close quote ends `expand_prog' variable

processed_program=$(gawk -- "$expand_prog" /dev/stdin << EOF
$program
EOF
)

The shell construct ‘command << marker’ is called a here document. Everything in the shell script up to the marker is fed to command as input. The shell processes the contents of the here document for variable and command substitution (and possibly other things as well, depending upon the shell).

The shell construct ‘$(…)’ is called command substitution. The output of the command inside the parentheses is substituted into the command line. Because the result is used in a variable assignment, it is saved as a single string, even if the results contain whitespace.

The expanded program is saved in the variable processed_program. It’s done in these steps:

1. Run gawk with the ‘@include’-processing program (the value of the expand_prog shell variable) on standard input.
2. Standard input is the contents of the user’s program, from the shell variable program. Its contents are fed to gawk via a here document.
3. The results of this processing are saved in the shell variable processed_program by using command substitution.

The last step is to call gawk with the expanded program, along with the original options and command-line arguments that the user supplied.

eval gawk $opts -- '"$processed_program"' '"$@"'

The eval command is a shell construct that reruns the shell’s parsing process. This keeps things properly quoted.

This version of igawk represents my fifth version of this program. There are four key simplifications that make the program work better:

• Using ‘@include’ even for the files named with ‘-f’ makes building the initial collected awk program much simpler; all the ‘@include’ processing can be done once.
• Not trying to save the line read with getline in the pathto() function when testing for the file’s accessibility for use with the main program simplifies things considerably.
• Using a getline loop in the BEGIN rule does it all in one place. It is not necessary to call out to a separate loop for processing nested ‘@include’ statements.
• Instead of saving the expanded program in a temporary file, putting it in a shell variable avoids some potential security problems. This has the disadvantage that the script relies upon more features of the sh language, making it harder to follow for those who aren’t familiar with sh.

Also, this program illustrates that it is often worthwhile to combine sh and awk programming together. You can usually accomplish quite a lot, without having to resort to low-level programming in C or C++, and it is frequently easier to do certain kinds of string and argument manipulation using the shell than it is in awk.

Finally, igawk shows that it is not always necessary to add new features to a program; they can often be layered on top.

As an additional example of this, consider the idea of having two files in a directory in the search path:

‘default.awk’

This file contains a set of default library functions, such as getopt() and assert().

‘site.awk’

This file contains library functions that are specific to a site or installation; i.e., locally developed functions. Having a separate file allows ‘default.awk’ to change with new gawk releases, without requiring the system administrator to update it each time by adding the local functions.

One user suggested that gawk be modified to automatically read these files upon startup. Instead, it would be very simple to modify igawk to do this. Since igawk can process nested ‘@include’ directives, ‘default.awk’ could simply contain ‘@include’ statements for the desired library functions.