GAWK: Effective AWK Programming

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182 GAWK: Effective AWK ProgrammingThe ‘-f’ option may be used more than once on the command line. If it is, awk readsits program source from all of the named files, as if they had been concatenated togetherinto one big file. This is useful for creating libraries of awk functions. These functions canbe written once and then retrieved from a standard place, instead of having to be includedinto each individual program. (As mentioned in Section 8.2.1 [Function Definition Syntax],page 153, function names must be unique.)Library functions can still be used, even if the program is entered at the terminal, byspecifying ‘-f /dev/tty’. After typing your program, type Ctrl-d (the end-of-file character)to terminate it. (You may also use ‘-f -’ to read program source from the standardinput but then you will not be able to also use the standard input as a source of data.)Because it is clumsy using the standard awk mechanisms to mix source file and commandlineawk programs, gawk provides the ‘--source’ option. This does not require you to preemptthe standard input for your source code; it allows you to easily mix command-lineand library source code (see Section 11.4 [The AWKPATH Environment Variable], page 183).If no ‘-f’ or ‘--source’ option is specified, then gawk uses the first non-option commandlineargument as the text of the program source code.If the environment variable POSIXLY_CORRECT exists, then gawk behaves in strict POSIXmode, exactly as if you had supplied the ‘--posix’ command-line option. Many GNUprograms look for this environment variable to turn on strict POSIX mode. If ‘--lint’is supplied on the command line and gawk turns on POSIX mode because of POSIXLY_CORRECT, then it issues a warning message indicating that POSIX mode is in effect. Youwould typically set this variable in your shell’s startup file. For a Bourne-compatible shell(such as bash), you would add these lines to the ‘.profile’ file in your home directory:POSIXLY_CORRECT=trueexport POSIXLY_CORRECTFor a csh-compatible shell, 1 you would add this line to the ‘.login’ file in your homedirectory:setenv POSIXLY_CORRECT trueHaving POSIXLY_CORRECT set is not recommended for daily use, but it is good for testingthe portability of your programs to other environments.11.3 Other Command-Line ArgumentsAny additional arguments on the command line are normally treated as input files to beprocessed in the order specified. However, an argument that has the form var=value,assigns the value value to the variable var—it does not specify a file at all. (This wasdiscussed earlier in Section 5.3.2 [Assigning Variables on the Command Line], page 78.)All these arguments are made available to your awk program in the ARGV array (seeSection 6.5 [Built-in Variables], page 110). Command-line options and the program text (ifpresent) are omitted from ARGV. All other arguments, including variable assignments, areincluded. As each element of ARGV is processed, gawk sets the variable ARGIND to the indexin ARGV of the current element.The distinction between file name arguments and variable-assignment arguments is madewhen awk is about to open the next input file. At that point in execution, it checks the file1 Not recommended.
Chapter 11: Running awk and gawk 183name to see whether it is really a variable assignment; if so, awk sets the variable insteadof reading a file.Therefore, the variables actually receive the given values after all previously specifiedfiles have been read. In particular, the values of variables assigned in this fashion arenot available inside a BEGIN rule (see Section 6.1.4 [The BEGIN and END Special Patterns],page 99), because such rules are run before awk begins scanning the argument list.The variable values given on the command line are processed for escape sequences (seeSection 2.2 [Escape Sequences], page 25).In some earlier implementations of awk, when a variable assignment occurred beforeany file names, the assignment would happen before the BEGIN rule was executed. awk’sbehavior was thus inconsistent; some command-line assignments were available inside theBEGIN rule, while others were not. Unfortunately, some applications came to depend uponthis “feature.” When awk was changed to be more consistent, the ‘-v’ option was added toaccommodate applications that depended upon the old behavior.The variable assignment feature is most useful for assigning to variables such as RS, OFS,and ORS, which control input and output formats before scanning the data files. It is alsouseful for controlling state if multiple passes are needed over a data file. For example:awk ’pass == 1 { pass 1 stuff }pass == 2 { pass 2 stuff }’ pass=1 mydata pass=2 mydataGiven the variable assignment feature, the ‘-F’ option for setting the value of FS is notstrictly necessary. It remains for historical compatibility.11.4 The AWKPATH Environment VariableIn most awk implementations, you must supply a precise path name for each program file,unless the file is in the current directory. But in gawk, if the file name supplied to the‘-f’ option does not contain a ‘/’, then gawk searches a list of directories (called the searchpath), one by one, looking for a file with the specified name.The search path is a string consisting of directory names separated by colons. gawkgets its search path from the AWKPATH environment variable. If that variable does notexist, gawk uses a default path, ‘.:/usr/local/share/awk’. 2 (Programs written for useby system administrators should use an AWKPATH variable that does not include the currentdirectory, ‘.’.)The search path feature is particularly useful for building libraries of useful awk functions.The library files can be placed in a standard directory in the default path and then specifiedon the command line with a short file name. Otherwise, the full file name would have tobe typed for each file.By using both the ‘--source’ and ‘-f’ options, your command-line awk programs can usefacilities in awk library files (see Chapter 12 [A Library of awk Functions], page 186). Pathsearching is not done if gawk is in compatibility mode. This is true for both ‘--traditional’and ‘--posix’. See Section 11.2 [Command-Line Options], page 177.2 Your version of gawk may use a different directory; it will depend upon how gawk was built and installed.The actual directory is the value of ‘$(datadir)’ generated when gawk was configured. You probablydon’t need to worry about this, though.
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