7.5.2 Built-in Variables That Convey Information

The following is an alphabetical list of variables that awk sets automatically on certain occasions in order to provide information to your program.

The variables that are specific to gawk are marked with a pound sign (‘#’). These variables are gawk extensions. In other awk implementations or if gawk is in compatibility mode (see section Command-Line Options), they are not special:

ARGC, ARGV

The command-line arguments available to awk programs are stored in an array called ARGV. ARGC is the number of command-line arguments present. See section Other Command-Line Arguments. Unlike most awk arrays, ARGV is indexed from 0 to ARGC - 1. In the following example:

$ awk 'BEGIN {
>         for (i = 0; i < ARGC; i++)
>             print ARGV[i]
>      }' inventory-shipped mail-list
-| awk
-| inventory-shipped
-| mail-list

ARGV[0] contains ‘awk’, ARGV[1] contains ‘inventory-shipped’, and ARGV[2] contains ‘mail-list’. The value of ARGC is three, one more than the index of the last element in ARGV, because the elements are numbered from zero.

The names ARGC and ARGV, as well as the convention of indexing the array from 0 to ARGC - 1, are derived from the C language’s method of accessing command-line arguments.

The value of ARGV[0] can vary from system to system. Also, you should note that the program text is not included in ARGV, nor are any of awk’s command-line options. See section Using ARGC and ARGV for information about how awk uses these variables. (d.c.)

ARGIND #

The index in ARGV of the current file being processed. Every time gawk opens a new data file for processing, it sets ARGIND to the index in ARGV of the file name. When gawk is processing the input files, ‘FILENAME == ARGV[ARGIND]’ is always true.

This variable is useful in file processing; it allows you to tell how far along you are in the list of data files as well as to distinguish between successive instances of the same file name on the command line.

While you can change the value of ARGIND within your awk program, gawk automatically sets it to a new value when it opens the next file.

ENVIRON

An associative array containing the values of the environment. The array indices are the environment variable names; the elements are the values of the particular environment variables. For example, ENVIRON["HOME"] might be /home/arnold.

For POSIX awk, changing this array does not affect the environment passed on to any programs that awk may spawn via redirection or the system() function.

However, beginning with version 4.2, if not in POSIX compatibility mode, gawk does update its own environment when ENVIRON is changed, thus changing the environment seen by programs that it creates. You should therefore be especially careful if you modify ENVIRON["PATH"], which is the search path for finding executable programs.

This can also affect the running gawk program, since some of the built-in functions may pay attention to certain environment variables. The most notable instance of this is mktime() (see section Time Functions), which pays attention the value of the TZ environment variable on many systems.

Some operating systems may not have environment variables. On such systems, the ENVIRON array is empty (except for ENVIRON["AWKPATH"] and ENVIRON["AWKLIBPATH"]; see section The AWKPATH Environment Variable and see section The AWKLIBPATH Environment Variable).

ERRNO #

If a system error occurs during a redirection for getline, during a read for getline, or during a close() operation, then ERRNO contains a string describing the error.

In addition, gawk clears ERRNO before opening each command-line input file. This enables checking if the file is readable inside a BEGINFILE pattern (see section The BEGINFILE and ENDFILE Special Patterns).

Otherwise, ERRNO works similarly to the C variable errno. Except for the case just mentioned, gawk never clears it (sets it to zero or ""). Thus, you should only expect its value to be meaningful when an I/O operation returns a failure value, such as getline returning -1. You are, of course, free to clear it yourself before doing an I/O operation.

If the value of ERRNO corresponds to a system error in the C errno variable, then PROCINFO["errno"] will be set to the value of errno. For non-system errors, PROCINFO["errno"] will be zero.

FILENAME

The name of the current input file. When no data files are listed on the command line, awk reads from the standard input and FILENAME is set to "-". FILENAME changes each time a new file is read (see section Reading Input Files). Inside a BEGIN rule, the value of FILENAME is "", because there are no input files being processed yet.³⁸ (d.c.) Note, though, that using getline (see section Explicit Input with getline) inside a BEGIN rule can give FILENAME a value.

FNR

The current record number in the current file. awk increments FNR each time it reads a new record (see section How Input Is Split into Records). awk resets FNR to zero each time it starts a new input file.

NF

The number of fields in the current input record. NF is set each time a new record is read, when a new field is created, or when $0 changes (see section Examining Fields).

Unlike most of the variables described in this subsection, assigning a value to NF has the potential to affect awk’s internal workings. In particular, assignments to NF can be used to create fields in or remove fields from the current record. See section Changing the Contents of a Field.

FUNCTAB #

An array whose indices and corresponding values are the names of all the built-in, user-defined, and extension functions in the program.

NOTE: Attempting to use the delete statement with the FUNCTAB array causes a fatal error. Any attempt to assign to an element of FUNCTAB also causes a fatal error.

NR

The number of input records awk has processed since the beginning of the program’s execution (see section How Input Is Split into Records). awk increments NR each time it reads a new record.

PROCINFO #

The elements of this array provide access to information about the running awk program. The following elements (listed alphabetically) are guaranteed to be available:

PROCINFO["argv"]

The PROCINFO["argv"] array contains all of the command-line arguments (after glob expansion and redirection processing on platforms where that must be done manually by the program) with subscripts ranging from 0 through argc - 1. For example, PROCINFO["argv"][0] will contain the name by which gawk was invoked. Here is an example of how this feature may be used:

gawk '
BEGIN {
        for (i = 0; i < length(PROCINFO["argv"]); i++)
                print i, PROCINFO["argv"][i]
}'

Please note that this differs from the standard ARGV array which does not include command-line arguments that have already been processed by gawk (see section Using ARGC and ARGV).

PROCINFO["egid"]

The value of the getegid() system call.

PROCINFO["errno"]

The value of the C errno variable when ERRNO is set to the associated error message.

PROCINFO["euid"]

The value of the geteuid() system call.

PROCINFO["FS"]

This is "FS" if field splitting with FS is in effect, "FIELDWIDTHS" if field splitting with FIELDWIDTHS is in effect, "FPAT" if field matching with FPAT is in effect, or "API" if field splitting is controlled by an API input parser.

PROCINFO["gid"]

The value of the getgid() system call.

PROCINFO["identifiers"]

A subarray, indexed by the names of all identifiers used in the text of the awk program. An identifier is simply the name of a variable (be it scalar or array), built-in function, user-defined function, or extension function. For each identifier, the value of the element is one of the following:

"array"

The identifier is an array.

"builtin"

The identifier is a built-in function.

"extension"

The identifier is an extension function loaded via @load or -l.

"scalar"

The identifier is a scalar.

"untyped"

The identifier is untyped (could be used as a scalar or an array; gawk doesn’t know yet).

"user"

The identifier is a user-defined function.

The values indicate what gawk knows about the identifiers after it has finished parsing the program; they are not updated while the program runs.

PROCINFO["platform"]

This element gives a string indicating the platform for which gawk was compiled. The value will be one of the following:

"djgpp"

"mingw"

Microsoft Windows, using either DJGPP or MinGW, respectively.

"os2"

OS/2.

"os390"

OS/390.

"posix"

GNU/Linux, Cygwin, Mac OS X, and legacy Unix systems.

"vms"

OpenVMS or Vax/VMS.

PROCINFO["pgrpid"]

The process group ID of the current process.

PROCINFO["pid"]

The process ID of the current process.

PROCINFO["ppid"]

The parent process ID of the current process.

PROCINFO["strftime"]

The default time format string for strftime(). Assigning a new value to this element changes the default. See section Time Functions.

PROCINFO["uid"]

The value of the getuid() system call.

PROCINFO["version"]

The version of gawk.

The following additional elements in the array are available to provide information about the MPFR and GMP libraries if your version of gawk supports arbitrary-precision arithmetic (see section Arithmetic and Arbitrary-Precision Arithmetic with gawk):

PROCINFO["gmp_version"]

The version of the GNU MP library.

PROCINFO["mpfr_version"]

The version of the GNU MPFR library.

PROCINFO["prec_max"]

The maximum precision supported by MPFR.

PROCINFO["prec_min"]

The minimum precision required by MPFR.

The following additional elements in the array are available to provide information about the version of the extension API, if your version of gawk supports dynamic loading of extension functions (see section Writing Extensions for gawk):

PROCINFO["api_major"]

The major version of the extension API.

PROCINFO["api_minor"]

The minor version of the extension API.

On some systems, there may be elements in the array, "group1" through "groupN" for some N. N is the number of supplementary groups that the process has. Use the in operator to test for these elements (see section Referring to an Array Element).

The following elements allow you to change gawk’s behavior:

PROCINFO["NONFATAL"]

If this element exists, then I/O errors for all redirections become nonfatal. See section Enabling Nonfatal Output.

PROCINFO["name", "NONFATAL"]

Make I/O errors for name be nonfatal. See section Enabling Nonfatal Output.

PROCINFO["command", "pty"]

For two-way communication to command, use a pseudo-tty instead of setting up a two-way pipe. See section Two-Way Communications with Another Process for more information.

PROCINFO["input_name", "READ_TIMEOUT"]

Set a timeout for reading from input redirection input_name. See section Reading Input with a Timeout for more information.

PROCINFO["input_name", "RETRY"]

If an I/O error that may be retried occurs when reading data from input_name, and this array entry exists, then getline returns -2 instead of following the default behavior of returning -1 and configuring input_name to return no further data. An I/O error that may be retried is one where errno has the value EAGAIN, EWOULDBLOCK, EINTR, or ETIMEDOUT. This may be useful in conjunction with PROCINFO["input_name", "READ_TIMEOUT"] or situations where a file descriptor has been configured to behave in a non-blocking fashion. See section Retrying Reads After Certain Input Errors for more information.

PROCINFO["sorted_in"]

If this element exists in PROCINFO, its value controls the order in which array indices will be processed by ‘for (indx in array)’ loops. This is an advanced feature, so we defer the full description until later; see Using Predefined Array Scanning Orders with gawk.

RLENGTH

The length of the substring matched by the match() function (see section String-Manipulation Functions). RLENGTH is set by invoking the match() function. Its value is the length of the matched string, or -1 if no match is found.

RSTART

The start index in characters of the substring that is matched by the match() function (see section String-Manipulation Functions). RSTART is set by invoking the match() function. Its value is the position of the string where the matched substring starts, or zero if no match was found.

RT #

The input text that matched the text denoted by RS, the record separator. It is set every time a record is read.

SYMTAB #

An array whose indices are the names of all defined global variables and arrays in the program. SYMTAB makes gawk’s symbol table visible to the awk programmer. It is built as gawk parses the program and is complete before the program starts to run.

The array may be used for indirect access to read or write the value of a variable:

foo = 5
SYMTAB["foo"] = 4
print foo    # prints 4

The isarray() function (see section Getting Type Information) may be used to test if an element in SYMTAB is an array. Also, you may not use the delete statement with the SYMTAB array.

Prior to version 5.0 of gawk, you could use an index for SYMTAB that was not a predefined identifier:

SYMTAB["xxx"] = 5
print SYMTAB["xxx"]

This no longer works, instead producing a fatal error, as it led to rampant confusion.

The SYMTAB array is more interesting than it looks. Andrew Schorr points out that it effectively gives awk data pointers. Consider his example:

# Indirect multiply of any variable by amount, return result

function multiply(variable, amount)
{
    return SYMTAB[variable] *= amount
}

You would use it like this:

BEGIN {
    answer = 10.5
    multiply("answer", 4)
    print "The answer is", answer
}

When run, this produces:

$ gawk -f answer.awk
-| The answer is 42

NOTE: In order to avoid severe time-travel paradoxes,³⁹ neither FUNCTAB nor SYMTAB is available as an element within the SYMTAB array.

$ echo '1
> 2
> 3
> 4' | awk 'NR == 2 { NR = 17 }
> { print NR }'
-| 1
-| 17
-| 18
-| 19