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AWK Language Programming - Built-in Variables

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Original source (kirste.userpage.fu-berlin.de)
Tags: awk documentation stream-manipulation stream-editing gawk builtins
Clipped on: 2018-01-22

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Built-in Variables

Most awk variables are available for you to use for your own purposes; they never change except when your program assigns values to them, and never affect anything except when your program examines them. However, a few variables in awk have special built-in meanings. Some of them awk examines automatically, so that they enable you to tell awk how to do certain things. Others are set automatically by awk, so that they carry information from the internal workings of awk to your program.

This chapter documents all the built-in variables of gawk. Most of them are also documented in the chapters describing their areas of activity.

Built-in Variables that Control awk

This is an alphabetical list of the variables which you can change to control how awk does certain things. Those variables that are specific to gawk are marked with an asterisk, `*'.

CONVFMT
This string controls conversion of numbers to strings (see section Conversion of Strings and Numbers). It works by being passed, in effect, as the first argument to the sprintf function (see section Built-in Functions for String Manipulation). Its default value is "%.6g". CONVFMT was introduced by the POSIX standard.
FIELDWIDTHS *
This is a space separated list of columns that tells gawk how to split input with fixed, columnar boundaries. It is an experimental feature. Assigning to FIELDWIDTHS overrides the use of FS for field splitting. See section Reading Fixed-width Data, for more information. If gawk is in compatibility mode (see section Command Line Options), then FIELDWIDTHS has no special meaning, and field splitting operations are done based exclusively on the value of FS.
FS
FS is the input field separator (see section Specifying How Fields are Separated). The value is a single-character string or a multi-character regular expression that matches the separations between fields in an input record. If the value is the null string (""), then each character in the record becomes a separate field. The default value is " ", a string consisting of a single space. As a special exception, this value means that any sequence of spaces and tabs is a single separator. It also causes spaces and tabs at the beginning and end of a record to be ignored. You can set the value of FS on the command line using the `-F' option:
awk -F, 'program' input-files
If gawk is using FIELDWIDTHS for field-splitting, assigning a value to FS will cause gawk to return to the normal, FS-based, field splitting. An easy way to do this is to simply say `FS = FS', perhaps with an explanatory comment.
IGNORECASE *
If IGNORECASE is non-zero or non-null, then all string comparisons, and all regular expression matching are case-independent. Thus, regexp matching with `~' and `!~', and the gensub, gsub, index, match, split and sub functions, record termination with RS, and field splitting with FS all ignore case when doing their particular regexp operations. See section Case-sensitivity in Matching. If gawk is in compatibility mode (see section Command Line Options), then IGNORECASE has no special meaning, and string and regexp operations are always case-sensitive.
OFMT
This string controls conversion of numbers to strings (see section Conversion of Strings and Numbers) for printing with the print statement. It works by being passed, in effect, as the first argument to the sprintf function (see section Built-in Functions for String Manipulation). Its default value is "%.6g". Earlier versions of awk also used OFMT to specify the format for converting numbers to strings in general expressions; this is now done by CONVFMT.
OFS
This is the output field separator (see section Output Separators). It is output between the fields output by a print statement. Its default value is " ", a string consisting of a single space.
ORS
This is the output record separator. It is output at the end of every print statement. Its default value is "\n". (See section Output Separators.)
RS
This is awk's input record separator. Its default value is a string containing a single newline character, which means that an input record consists of a single line of text. It can also be the null string, in which case records are separated by runs of blank lines, or a regexp, in which case records are separated by matches of the regexp in the input text. (See section How Input is Split into Records.)
SUBSEP
SUBSEP is the subscript separator. It has the default value of "\034", and is used to separate the parts of the indices of a multi-dimensional array. Thus, the expression foo["A", "B"] really accesses foo["A\034B"] (see section Multi-dimensional Arrays).

Built-in Variables that Convey Information

This is an alphabetical list of the variables that are set automatically by awk on certain occasions in order to provide information to your program. Those variables that are specific to gawk are marked with an asterisk, `*'.

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 zero to ARGC - 1. For example:
$ awk 'BEGIN {
>        for (i = 0; i < ARGC; i++) 
>            print ARGV[i] 
>      }' inventory-shipped BBS-list
-| awk
-| inventory-shipped
-| BBS-list
In this example, ARGV[0] contains "awk", ARGV[1] contains "inventory-shipped", and ARGV[2] contains "BBS-list". The value of ARGC is three, one more than the index of the last element in ARGV, since the elements are numbered from zero. The names ARGC and ARGV, as well as the convention of indexing the array from zero to ARGC - 1, are derived from the C language's method of accessing command line arguments. See section Using ARGC and ARGV, for information about how awk uses these variables.
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, it is always true that `FILENAME == ARGV[ARGIND]'. This variable is useful in file processing; it allows you to tell how far along you are in the list of data files, and to distinguish between successive instances of the same filename on the command line. While you can change the value of ARGIND within your awk program, gawk will automatically set it to a new value when the next file is opened. This variable is a gawk extension. In other awk implementations, or if gawk is in compatibility mode (see section Command Line Options), it is not special.
ENVIRON
An associative array that contains the values of the environment. The array indices are the environment variable names; the values are the values of the particular environment variables. For example, ENVIRON["HOME"] might be `/home/arnold'. Changing this array does not affect the environment passed on to any programs that awk may spawn via redirection or the system function. (In a future version of gawk, it may do so.) Some operating systems may not have environment variables. On such systems, the ENVIRON array is empty (except for ENVIRON["AWKPATH"]).
ERRNO *
If a system error occurs either doing a redirection for getline, during a read for getline, or during a close operation, then ERRNO will contain a string describing the error. This variable is a gawk extension. In other awk implementations, or if gawk is in compatibility mode (see section Command Line Options), it is not special.
FILENAME
This is the name of the file that awk is currently reading. When no data files are listed on the command line, awk reads from the standard input, and FILENAME is set to "-". FILENAME is changed each time a new file is read (see section Reading Input Files). Inside a BEGIN rule, the value of FILENAME is "", since there are no input files being processed yet.(7) (d.c.)
FNR
FNR is the current record number in the current file. FNR is incremented each time a new record is read (see section Explicit Input with getline). It is reinitialized to zero each time a new input file is started.
NF
NF is 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).
NR
This is the number of input records awk has processed since the beginning of the program's execution (see section How Input is Split into Records). NR is set each time a new record is read.
RLENGTH
RLENGTH is the length of the substring matched by the match function (see section Built-in Functions for String Manipulation). RLENGTH is set by invoking the match function. Its value is the length of the matched string, or -1 if no match was found.
RSTART
RSTART is the start-index in characters of the substring matched by the match function (see section Built-in Functions for String Manipulation). 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 *
RT is set each time a record is read. It contains the input text that matched the text denoted by RS, the record separator. This variable is a gawk extension. In other awk implementations, or if gawk is in compatibility mode (see section Command Line Options), it is not special.

A side note about NR and FNR. awk simply increments both of these variables each time it reads a record, instead of setting them to the absolute value of the number of records read. This means that your program can change these variables, and their new values will be incremented for each record (d.c.). For example:

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

Before FNR was added to the awk language (see section Major Changes between V7 and SVR3.1), many awk programs used this feature to track the number of records in a file by resetting NR to zero when FILENAME changed.

Using ARGC and ARGV

In section Built-in Variables that Convey Information, you saw this program describing the information contained in ARGC and ARGV:

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

In this example, ARGV[0] contains "awk", ARGV[1] contains "inventory-shipped", and ARGV[2] contains "BBS-list".

Notice that the awk program is not entered in ARGV. The other special command line options, with their arguments, are also not entered. But variable assignments on the command line are treated as arguments, and do show up in the ARGV array.

Your program can alter ARGC and the elements of ARGV. Each time awk reaches the end of an input file, it uses the next element of ARGV as the name of the next input file. By storing a different string there, your program can change which files are read. You can use "-" to represent the standard input. By storing additional elements and incrementing ARGC you can cause additional files to be read.

If you decrease the value of ARGC, that eliminates input files from the end of the list. By recording the old value of ARGC elsewhere, your program can treat the eliminated arguments as something other than file names.

To eliminate a file from the middle of the list, store the null string ("") into ARGV in place of the file's name. As a special feature, awk ignores file names that have been replaced with the null string. You may also use the delete statement to remove elements from ARGV (see section The delete Statement).

All of these actions are typically done from the BEGIN rule, before actual processing of the input begins. See section Splitting a Large File Into Pieces, and see section Duplicating Output Into Multiple Files, for an example of each way of removing elements from ARGV.

The following fragment processes ARGV in order to examine, and then remove, command line options.

BEGIN {
    for (i = 1; i < ARGC; i++) {
        if (ARGV[i] == "-v")
            verbose = 1
        else if (ARGV[i] == "-d")
            debug = 1
        else if (ARGV[i] ~ /^-?/) {
            e = sprintf("%s: unrecognized option -- %c",
                    ARGV[0], substr(ARGV[i], 1, ,1))
            print e > "/dev/stderr"
        } else
            break
        delete ARGV[i]
    }
}


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