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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 them, and never affect anything except when your program examines them.

A few variables 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 where their areas of activity are described.

Built-in Variables That Control awk

This is a list of the variables which you can change to control how awk does certain things.

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.

The default value is " ", a string consisting of a single space. As a special exception, this value actually means that any sequence of spaces and tabs is a single separator. It also causes spaces and tabs at the beginning or end of a line to be ignored.

You can set the value of FS on the command line using the `-F' option:

awk -F, 'program' input-files

IGNORECASE
If IGNORECASE is nonzero, then all regular expression matching is done in a case-independent fashion. In particular, regexp matching with `~' and `!~', and the gsub index, match, split and sub functions all ignore case when doing their particular regexp operations. Note: since field splitting with the value of the FS variable is also a regular expression operation, that too is done with case ignored. See section Case-sensitivity in Matching.

If gawk is in compatibility mode (see section Invocation of awk), then IGNORECASE has no special meaning, and regexp operations are always case-sensitive.

OFMT
This string is used by awk to control 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. Its default value is "%.6g".

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 a string containing a single newline character, which could be written as "\n". (See section Output Separators).

RS
This is awk's 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. (See section How Input is Split into Records.)

SUBSEP
SUBSEP is a subscript separator. It has the default value of "\034", and is used to separate the parts of the name of a multi-dimensional array. Thus, if you access foo[12,3], it really accesses foo["12\0343"]. (See section Multi-dimensional Arrays).

Built-in Variables That Convey Information to You

This is a list of the variables that are set automatically by awk on certain occasions so as to provide information for your program.

ARGC
ARGV
The command-line arguments available to awk are stored in an array called ARGV. ARGC is the number of command-line arguments present. ARGV is indexed from zero to ARGC - 1. See section Invocation of awk. For example:

awk '{ print ARGV[$1] }' 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 3, one more than the index of the last element in ARGV since the elements are numbered from zero.

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.

ENVIRON
This is an 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 `/u/close'. 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 array ENVIRON is empty.

FILENAME
This is the name of the file that awk is currently reading. If awk is reading from the standard input (in other words, there are no files listed on the command line), FILENAME is set to "-". FILENAME is changed each time a new file is read (see section Reading Input Files).

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 0 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 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 0 if no match was found.

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