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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.
10.1 Built-in Variables that Control awk
Built-in variables that you change to control awk
.
10.2 Built-in Variables that Convey Information Built-in variables where awk
gives you information.10.3 Using ARGC
andARGV
Ways to use ARGC
andARGV
.
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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
sprintf
function
(see section Built-in Functions for String Manipulation).
Its default value is "%.6g"
.
CONVFMT
was introduced by the POSIX standard.
FIELDWIDTHS *
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, tabs, and/or newlines is a single separator.(9) It also causes
spaces, tabs, and newlines 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 *
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.
The value of IGNORECASE
does not affect array subscripting.
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
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
print
statement. Its
default value is " "
, a string consisting of a single space.
ORS
print
statement. Its default value is "\n"
.
(See section 6.3 Output Separators.)
RS
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).
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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
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 *
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
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 *
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
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.(10) (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
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.
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ARGC
and ARGV
In 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. This includes variable assignments done with the `-v'
option (see section Command Line Options).
Normal variable assignments on the command line are
treated as arguments, and do show up in the ARGV
array.
$ cat showargs.awk -| BEGIN { -| printf "A=%d, B=%d\n", A, B -| for (i = 0; i < ARGC; i++) -| printf "\tARGV[%d] = %s\n", i, ARGV[i] -| } -| END { printf "A=%d, B=%d\n", A, B } $ awk -v A=1 -f showargs.awk B=2 /dev/null -| A=1, B=0 -| ARGV[0] = awk -| ARGV[1] = B=2 -| ARGV[2] = /dev/null -| A=1, B=2 |
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
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] } } |
To actually get the options into the awk
program, you have to
end the awk
options with `--', and then supply your options,
like so:
awk -f myprog -- -v -d file1 file2 ... |
This is not necessary in gawk
: Unless `--posix' has been
specified, gawk
silently puts any unrecognized options into
ARGV
for the awk
program to deal with.
As soon as it
sees an unknown option, gawk
stops looking for other options it might
otherwise recognize. The above example with gawk
would be:
gawk -f myprog -d -v file1 file2 ... |
Since `-d' is not a valid gawk
option, the following `-v'
is passed on to the awk
program.
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