4.4BSD/usr/src/contrib/gawk-2.15.2/vms/gawk.hlp
! Gawk.Hlp
! Pat Rankin, Jun'90
! revised, Jun'91
! revised, Jul'92
! Online help for GAWK.
!
1 GAWK
GAWK is GNU awk, the Free Software Foundation's implementation of
the awk programming language. awk is an interpretive language which
can handle many data-reformatting jobs with just a few lines of code.
It has powerful string manipulation and pattern matching capabilities
built in. This version should be compatible with POSIX 1003.2 awk.
The VMS version of GAWK supports both the original UN*X-style command
interface and a DCL interface. The only setup requirement for GAWK
is to define it as a 'foreign' command: a DCL symbol with a value
which begins with '$'.
$ GAWK :== $disk:[directory]GAWK
2 GNU_syntax
GAWK's UN*X-style interface uses the 'dash' convention for specifying
options and uses spaces to separate multiple arguments.
There are two main alternatives, depending on how the awk program is
to be passed to GAWK. Both alternatives share most options.
Usage: $ gawk [-W opts] [-F fs] [-v var=val] -f progfile [--] file ...
or $ gawk [-W opts] [-F fs] [-v var=val] [--] "program" file ...
The options are case-sensitive. On VMS, the DCL command interpreter
converts unquoted text into uppercase before passing it to the running
program. However, GAWK is written in 'C' and the C Run-Time Library
(VAXCRTL) converts unquoted text into *lowercase*. Therefore, the
-Fval and -W options must be enclosed in quotes.
Note: under VMS POSIX, the usual shell command line processing occurs.
3 options
-f file use the specified file as the awk program source; if more
than one instance of -f is used, each file will be read
in succession
-Fstring define a value for the FS variable (field separator)
-v var=val assign a value of 'val' to the variable 'var'
-W 'options' additional gawk-specific options; multiple values may
be separated by commas, or by spaces if they're quoted,
or mulitple occurrences of -W may be used.
-W compat use awk "compatibility mode" to disable GAWK extensions
and get the behavior of UN*X awk.
-W copyright [or -W copyleft] display an abbreviated version of
the GNU copyright information
-W lint warn about suspect or non-portable awk program code
-W posix compatibility mode with additional restrictions
-W version display program version number
-- don't check further arguments for leading dash
3 program_text
If the '-f file' option is not used on the command line, then the
first "non-dash" argument is assumed to be a string of text containing
the awk source program. Here is a complete sample program:
$ gawk -- "BEGIN {print ""\nHello, World!\n""}"
This program would print a blank line (based on first "\n"), followed
by a line reading "Hello, World!", followed by another blank line
(since awk's 'print' statement includes the trailing 'newline').
On VMS, to include a quote character inside of a quoted string, two
successive quotes ("") must be used. (Not necessary for VMS POSIX.)
3 data_files
After all dash-options are examined, and after the program text if
there were no occurrences of the -f option, remaining (space separated)
command line arguments are considered to be data files for the awk
program to process. If any of these actually contains an equals sign
(=), then it is interpreted as a variable assignment instead of a data
file. The syntax is 'variable_name=value'. For example, the command
$ gawk -f myprog.awk infile.one flag=2 start=0 infile.two
would read file 'infile.one' for the program in 'myprog.awk', then it
would set 'flag' to 2 and 'start' to 0, and finally it would read file
'infile.two' for the program. Note that in a case like this, the two
assignments actually occur after the first file has been processed,
not at program startup when the command line is first scanned.
3 IO_redirection
The command parsing in the VMS implementation of GAWK does some
emulation of a UN*X-style shell, where certain characters on the
command line have special meaning. In particular, the symbols '<',
'>', '|', '*', and '?' receive special handling before the main part
of the program has a chance to see them. The symbols '<' and '>'
perform some file manipulation from the command line:
<ifile open file 'ifile' (readonly) as 'stdin' [SYS$INPUT]
>nfile create 'nfile' as 'stdout' [SYS$OUTPUT], in stream-lf format
>>ofile append to 'ofile' for 'stdout'; create it if necessary
>&efile point 'stderr' [SYS$ERROR] at 'efile', but don't open it yet
>$vfile create 'vfile' as 'stdout', using RMS attributes appropriate
for a standard text file (variable length records with
implied carriage control)
2>&1 route error messages into the regular output stream
1>&2 send output data to the error destination
<<sentinel error; reading stdin until 'sentinel' not supported
<-, >- error; closure of stdin or stdout from cmd line not supported
>>$vfile incorrect; would be interpreted as file "$vfile" in stream-lf
format rather than as file "vfile" in RMS 'text' format
| error; command line pipes not supported
Note: under VMS POSIX these features are implemented by the shell
rather than inside GAWK, so consult the shell documentation for
specific details.
3 wildcard_expansion
The command parsing in the VMS implementation of GAWK does some
emulation of a UN*X-style shell, where certain characters on the
command line have special meaning. In particular, the symbols '<',
'>', '*', '%', and '?' receive special handling before the main part
of the program has a chance to see them. The symbols '*', '%' and '?'
are used as wildcards in filenames. '*' and '%' have their usual VMS
meanings of multiple character and single character wildcards,
respectively, and '?' is also treated as a single character wildcard.
When a command line argument that should be a filename contains any
of the wildcard characters, a directory lookup is attempted for files
which match the specified pattern. If one or more matching files are
found, those filenames are put into the command line in place of the
original pattern. If no matching files are found, the original
pattern is left in place.
Note: under VMS POSIX wildcard expansion, or "file globbing", is
performed by the shell rather than inside GAWK, so consult the shell
documentation for details. In particular, the last sentence of the
previous paragraph does not apply.
2 DCL_syntax
GAWK's DCL-style interface is more or less a standard DCL command, with
one required parameter. Multiple values--when present--are separated
by commas.
There are two main alternatives, depending on how the awk program is
to be passed to GAWK. Both alternatives share most options.
Usage: GAWK /COMMANDS="awk program text" data_file[,data_file,...]
or GAWK /INPUT=awk_file data_file[,"Var=value",data_file,...]
( or GAWK /INPUT=(awk_file1,awk_file2,...) data_file[,...] )
Not applicable under VMS POSIX.
3 Parameter
data_file[,datafile,...] (data_file data_file ...)
data_file[,"Var=value",...,data_file,...] (data_file Var=value &c)
Data file(s) for the awk program to process. If any of these
actually contains an equals sign (=), then it is interpreted as
a variable assignment instead of a data file. The syntax is
"variable_name=value". Quotes are required for non-file parameters.
For example, the command
$ gawk/input=myprog.awk infile.one,"flag=2","start=0",infile.two
would read file 'infile.one' for the program in 'myprog.awk', then it
would set 'flag' to 2 and 'start' to 0, and finally it would read file
'infile.two' for the program. Note that in a case like this, the two
assignments actually occur after the first file has been processed,
not at program startup when the command line is first scanned.
Wildcard file lookups are attempted on data file specifications. See
subtopic 'GAWK GNU_syntax wildcard_expansion' for details.
At least one data_file parameter value is required. An exception is
made if /usage, /version, or /copyright is specified *and* if GAWK is
defined as a 'foreign' command rather than a 'native' DCL command.
3 Qualifiers
/COMMANDS
/COMMANDS="awk program text" (-- "awk program text")
For short programs, it is possible to include the complete program
on the command line. The quotes are required. Here is a complete
sample program:
$ gawk/commands="BEGIN {print ""\nHello, World!\n""}" NL:
This program would print a blank line (based on first "\n"), followed
by a line reading "Hello, World!", followed by another blank line
(since awk's 'print' statement includes the trailing 'newline').
To include a quote character inside of a quoted string, two
successive quotes ("") must be used.
Either /COMMANDS or /INPUT (but not both) must be supplied.
/INPUT
/INPUT=(awk_file1,awk_file2) (-f awk_file1 -f awk_file2)
Used to specify one or more files containing the source code of
the awk program. If more than one file is used, separate them
with commas and enclose the list in parentheses.
Multiple source files are processed in order as if they had been
concatenated together.
Either /INPUT or /COMMANDS (but not both) must be supplied.
/FIELD_SEPARATOR
/FIELD_SEPARATOR="FS_value" (-F"FS_value")
Assign a value to the built in variable FS (field separator).
/VARIABLES
/VARIABLES=("Var1=val1","Var2=val2",...) (-v Var1=val1 -v Var2=val2)
Assign value(s) to the specified variable(s).
/REG_EXPR
/REG_EXPR={AWK | EGREP | POSIX} (-a vs -e options [obsolete])
This qualifier is obsolete and has no effect.
/STRICT
/[NO]STRICT (-"W compat" option)
Use strict awk compatibility mode (/strict) and suppress GAWK
extensions. The default is /NOSTRICT.
/POSIX
/[NO]POSIX (-"W posix" option)
Use POSIX compatibility mode (/posix) and suppress GAWK extensions.
The default is /NOPOSIX. Slightly more restrictive than /strict.
/LINT
/[NO]LINT (-"W lint" option)
Check the awk program cafefully for potential problems that might
be encountered if it were to be used with other awk implementations,
and print warnings for anything found. The default in /NOLINT.
/VERSION
/VERSION (-"W version" option)
Print GAWK's version number.
/COPYRIGHT
/COPYRIGHT (-"W copyright" or -"W copyleft" option)
Print a brief version of GAWK's copyright notice.
/USAGE
/USAGE (no corresponding GNU_syntax option)
Print a compact summary of the command line options.
After the 'usage' message is printed, GAWK terminates regardless
of any other command line options.
/OUTPUT
/OUTPUT=out_file (>$out_file)
Write program output into 'out_file'. The default is SYS$OUTPUT.
2 awk_language
An awk program consists of one or more pattern-action pairs, sometimes
referred to as "rules". For each record of an input (data) file, the
rules are checked sequentially. Any pattern which matches the input
record triggers that rule's action. Actions are instructions which
resemble statements in the 'C' programming language. Patterns come
in several varieties, including field comparisons, regular expression
matching, and special cases defined by reserved keywords.
All awk keywords and variables are case-sensitive. Text matching is
also sensitive to character case unless the builtin variable IGNORECASE
is set to a non-zero value.
3 rules
The syntax for a pattern-action 'rule' is simply
PATTERN { ACTION }
where the braces ({}) are required punctuation for the action.
Semicolons (;) or 'newlines' (ie, having the text on a separate line)
delimit multiple rules and also multiple actions within a given rule.
Either the pattern or the action may be omitted; an empty pattern
matches every record of the input file; a missing action (not an empty
action inside of braces), is an implicit request to print the current
record; an empty action (ie, {}) is legal but not very useful.
3 patterns
There are several types of patterns available for awk rules.
expression an 'expression' is something to be evaluated (perhaps
a comparison or function call) which will
be considered true if non-zero (for numeric
results) or if non-null (for strings)
/regular_expression/ slashes (/) delimit a regular expression
which is used as a pattern
pattern1, pattern2 a pair of patterns separated by a comma (,),
which causes a range of records to trigger
the associated action; the records which
match the patterns are included in the range
<null> an omitted pattern (in this text, the string '<null>'
is displayed, but in an awk program, it
would really be blank) matches every record
BEGIN keyword for specifying a rule to be executed prior to
reading the 1st record of the 1st input file
END keyword for specifying a rule to be executed after
handling the last input record of last file
4 examples
Some example patterns (mostly with the corresponding actions omitted)
NF > 0 # comparison expression: matches non-null records
$0 # implied comparison: also matches non-null records
$2 > 1000 && sum <= 999999 # slightly more elaborate expression
/x/ # regular expression matching any record with an 'x' in it
/^ / # reg-expr matching records beginning with a space
$1 == "start", $NF == "stop" # range pattern for input in which
some data lines begin with 'start' and/or end with
'stop' in order to collect groups of records
{ sum += $1 } # null pattern: it's action (add field #1 to
variable 'sum') would be executed for every record
BEGIN { sum = 0 } # keyword 'BEGIN': perform this action before
reading the input file (note: initialization to 0 is
unnecessary in awk)
END { print "total =", sum } # keyword 'END': perform this
action after the last input record has been processed
3 actions
An 'action' is something to do when a given record has matched the
corresponding pattern in a rule. In general, actions resemble 'C'
statements and expressions. The action in a rule must be enclosed
in braces ({}).
Each action can contain more than one statement or expression to be
executed, provided that they're separated by semicolons (;) and/or
on separate lines.
An omitted action is equivalent to
{ print $0 }
which prints the current record.
3 operators
Relational operators
== compare for equality
!= compare for inequality
<, <=, >, >= numerical or lexical comparison (less than, less or
equal, greater than, greater or equal, respectively)
~ match against a regular expression
!~ match against a regular expression, but accept failed matches
instead of successful ones
Arithmetic operators
+ addition
- subtraction
* multiplication
/ division
% remainder
^, ** exponentiation ('**' is a synonym for '^', unless POSIX
compatibility is specified, in which case it's invalid)
Boolean operators (aka Logical operators)
a value is considered false if it's 0 or a null string,
it is true otherwise; the result of a boolean operation
(and also of a comparison operation) will be 0 when false
or 1 when true
|| or [expression (a || b) is true if either a is true or b
is true or both a and b are true; it is false otherwise;
b is not evaluated unless a is false (ie, short-circuit)]
&& and [expression (a && b) is true if both a and b are true;
it is false otherwise; b is only evaluated if a is true]
! not [expression (!a) is true if a is false, false otherwise]
in array membership; the keyword 'in' tests whether the value
on the left represents a current subscript in the array
named on the right
Conditional operator
? : the conditional operator takes three operands; the first is
an expression to evaluate, the second is the expression to
use if the first was true, the third is the expression to
use if it was false [simple example (a < b ? b : a) gives
the maximum of a and b]
Assignment operators
= store the value on the right into the variable or array slot
on the left [expression (a = b) stores the value of b in a]
+=, -=, *=, /=, %=, ^=, **= perform the indicated arithmetic
operation using the current value of the variable or array
element of the left side and the expression on the right
side, then store the result in the left side
++ increment by 1 [expression (++a) gets the current value of
a and adds 1 to it, stores that back in a, and returns the
new value; expression (a++) gets the current value of a,
adds 1 to it, stores that back in a, but returns the
original value of a]
-- decrement by 1 (analogous to increment)
String operators
there is no explicit operator for string concatenation;
two values and/or variables side-by-side are implicitly
concatenated into a string (numeric values are first
converted into their string equivalents)
Conversion between numeric and string values
there is no explicit operator for conversion; adding 0
to a string with force it to be converted to a number
(the numeric value will be 0 if the string does not
represent an integer or floating point number); the
reverse, converting a number into a string, is done by
concatenating a null string ("") to it [the expression
(5.75 "") evaluates to "5.75"]
Field 'operator'
$ prefixing a number or variable with a dollar sign ($)
causes the appropriate record field to be returned [($2)
gives the second field of the record, ($NF) gives the
last field (since the builtin variable NF is set to the
number of fields in the current record)]
Array subscript operator
, multi-dimensional arrays are simulated by using comma (,)
separated array indices; the actual index is generated
by replacing commas with the value of builtin SUBSEP,
then concatenating the expression into a string index
[comma is also used to separate arguments in function
calls and user-defined function definitions]
[comma is *also* used to indicate a range pattern in an
awk rule]
Escape 'operator'
\ In quoted character strings, the backslash (\) character
causes the following character to be interpreted in a
special manner [string "one\ntwo" has an embedded newline
character (linefeed on VMS, but treated as if it were both
carriage-return and linefeed); string "\033[" has an ASCII
'escape' character (which has octal value 033) followed by
a 'right-bracket' character]
Backslash is also used in regular expressions
Redirection operators
< Read-from -- valid with 'getline'
> Write-to (create new file) -- valid with 'print' and 'printf'
>> Append-to (create file if it doesn't already exist)
| Pipe-from/to -- valid with 'getline', 'print', and 'printf'
4 precedence
Operator precedence, listed from highest to lowest. Assignment,
conditional, and exponentiation operators group from right to left;
all others group from left to right. Parentheses may be used to
override the normal order.
field ($)
increment (++), decrement (--)
exponentiation (^, **)
unary plus (+), unary minus (-), boolean not (!)
multiplication (*), division (/), remainder (%)
addition (+), subtraction (-)
concatenation (no special symbol; implied by context)
relational (==, !=, <, >=, etc), and redirection (<, >, >>, |)
Relational and redirection operators have the same precedence
and use similar symbols; context distinguishes between them
matching (~, !~)
array membership ('in')
boolean and (&&)
boolean or (||)
conditional (? :)
assignment (=, +=, etc)
4 escaped_characters
Inside of a quoted string or constant regular expression, the
backslash (\) character gives special meaning to the character(s)
after it. Special character letters are case sensitive.
\\ results in one backslash in the string
\a is an 'alert' (<ctrl/G>. the ASCII <bell> character)
\b is a backspace (BS, <ctrl/H>)
\f is a form feed (FF, <ctrl/L>)
\n 'newline' (<ctrl/J> [line feed treated as CR+LF]
\r carriage return (CR, <ctrl/M> [re-positions at the
beginning of the current line]
\t tab (HT, <ctrl/I>)
\v vertical tab (VT, <ctrl/K>)
\### is an arbitrary character, where '###' represents 1 to 3
octal (ie, 0 thru 7) digits
\x## is an alternate arbitrary character, where '##' represents
1 or more hexadecimal (ie, 0 thru 9 and/or A through E
and/or a through e) digits; if more than two digits
follow, the result is undefined; not recognized if POSIX
compatibility mode is specified.
3 statements
A statement refers to a unit of instruction found in the action
part of an awk rule, and also found in the definition of a function.
The distinction between action, statement, and expression usually
won't matter to an awk programmer.
Compound statements consist of multiple statements separated by
semicolons or newlines and enclosed within braces ({}). They are
sometimes referred to as 'blocks'.
4 expressions
An expression such as 'a = 10' or 'n += i++' is a valid statement.
Function invocations such as 'reformat_field($3)' are also valid
statements.
4 if-then-else
A conditional statement in awk uses the same syntax as for the 'C'
programming language: the 'if' keyword, followed by an expression
in parentheses, followed by a statement--or block of statements
enclosed within braces ({})--which will be executed if the expression
is true but skipped if it's false. This can optionally be followed
by the 'else' keyword and another statement--or block of statements--
which will be executed if (and only if) the expression was false.
5 examples
Simple example showing a statement used to control how many numbers
are printed on a given line.
if ( ++i <= 10 ) #check whether this would be the 11th
printf(" %5d", k) #print on current line if not
else {
printf("\n %5d", k) #print on next line if so
i = 1 #and reset the counter
}
Another example ('next' is described under 'action-controls')
if ($1 > $2) { print "rejected"; next } else diff = $2 - $1
4 loops
Three types of loop statements are available in awk. Each uses
the same syntax as 'C'. The simplest of the three is the 'while'
statement. It consists of the 'while' keyword, followed by an
expression enclosed within parentheses, followed by a statement--or
block of statements in braces ({})--which will be executed if the
expression evaluates to true. The expression is evaluated before
attempting to execute the statement; if it's true, the statement is
executed (the entire block of statements if there is a block) and
then the expression is re-evaluated.
The second type of loop is the do-while loop. It consists of the
'do' keyword, followed by a statement (usually a block of statements
enclosed within braces), followed by the 'while' keyword, followed
by a test expression enclosed within parentheses. The statement--or
block--is always executed at least once. Then the test expression
is evaluated, and the statement(s) re-executed if the result was
true (followed by re-evaluation of the test, and so on).
The most complex of the three loops is the 'for' statement, and it
has a second variant that is not found in 'C'. The ordinary for-loop
consists of the 'for' keyword, followed by three semicolon-separated
expressions enclosed within parentheses, followed by a statement or
brace-enclosed block of statements. The first of the three
expressions is an initialization clause; it is done before starting
the loop. The second expression is used as a test, just like the
expression in a while-loop. It is checked before attempting to
execute the statement block, and then re-checked after each execution
(if any) of the block. The third expression is an 'increment' clause;
it is evaluated after an execution of the statement block and before
re-evaluation of the test (2nd) expression. Normally, the increment
clause will change a variable used in the test clause, in such a
fashion that the test clause will eventually evaluate to false and
cause the loop to finish.
Note to 'C' programmers: the comma (,) operator commonly used in
'C' for-loop expressions is not valid in awk.
The awk-specific variant of the for-loop is used for processing
arrays. Its syntax is 'for' keyword, followed by variable_name 'in'
array_name (where 'var in array' is enclosed in parentheses),
followed by a statement (or block). Each valid subscript value for
the array in question is successively placed--in no particular
order--into the specified 'index' variable.
5 while_example
# strip fields from the input record until there's nothing left
while (NF > 0) {
$1 = "" #this will affect the value of $0
$0 = $0 #this causes $0 and NF to be re-evaluated
print
}
5 do_while_example
# This is a variation of the while_example; it gives a slightly
# different display due to the order of operation.
# echo input record until all fields have been stripped
do {
print #output $0
$1 = "" #this will affect the value of $0
$0 = $0 #this causes $0 and NF to be re-evaluated
} while (NF > 0)
5 for_example
# echo command line arguments (won't include option switches)
for ( i = 0; i < ARGC; i++ ) print ARGV[i]
# display contents of builtin environment array
for (itm in ENVIRON)
print itm, ENVIRON[itm]
4 loop-controls
There are two special statements--both from 'C'--for changing the
behavior of loop execution. The 'continue' statement is useful in
a compound (block) statement; when executed, it effectively skips
the rest of the block so that the increment-expression (only for
for-loops) and loop-termination expression can be re-evaluated.
The 'break' statement, when executed, effectively skips the rest
of the block and also treats the test expression as if it were
false (instead of actually re-evaluating it). In this case, the
increment-expression of a for-loop is also skipped.
'break' is only allowed within a loop ('for', 'while', or
'do-while'). If 'continue' is used outside of a loop, it is
treated like 'next' (see action-controls). Inside nested loops,
both 'break' and 'continue' only apply to the innermost loop.
4 action-controls
There are two special statements for controlling statement execution.
The 'next' statement, when executed, causes the rest of the current
action and all further pattern-action rules to be skipped, so that
the next input record will be immediately processed. This is useful
if any early action knows that the current record will fail all the
remaining patterns; skipping those rules will reduce processing time.
An extended form, 'next file', is also available. It causes the
remainder of the current file to be skipped, and then either the
next input file will be processed, if any, or the END action will be
performed. 'next file' is not available in traditional awk.
The 'exit' statement causes GAWK execution to terminate. All open
files are closed, and no further processing is done. The END rule,
if any, is executed. 'exit' takes an optional numeric value as a
argument which is used as an exit status value, so that some sort
of indication of why execution has stopped can be passed on to the
user's environment.
4 other_statements
The delete statement is used to remove an element from an array.
The syntax is 'delete' keyword followed by array name, followed
by index value enclosed in square brackets ([]).
The return statement is used in user-defined functions. The syntax
is the keyword 'return' optionally followed by a string or numeric
expression.
See also subtopic 'functions IO_functions' for a description of
'print', 'printf', and 'getline'.
3 fields
When an input record is read, it is automatically split into fields
based on the current values of FS (builtin variable defining field
separator expression) and RS (builtin variable defining record
separator character). The default value of FS is an expression
which matches one or more spaces and tabs; the default for RS is
newline. If the FIELDWIDTHS variable is set to a space separated
list of numbers (as in ``FIELDWIDTHS = "2 3 2"'') then the input
is treated as if it had fixed-width fields of the indicated sizes
and the FS value will be ignored.
The field prefix operator ($), is used to reference a particular
field. For example, $3 designates the third field of the current
record. The entire record can be referenced via $0 (and it holds
the actual input record, not the values of $1, $2, ... concatenated
together, so multiple spaces--when present--remain intact, unless
a new value gets assigned).
The builtin variable NF holds the number of fields in the current
record. $NF is therefore the value of the last field. Attempts to
access fields beyond NF result in null values (if a record contained
3 fields, the value of $5 would be "").
Assigning a new value to $0 causes all the other field values (and NF)
to be re-evaluated. Changing a specific field will cause $0 to receive
a new value once it's re-evaluated, but until then the other existing
fields remain unchanged.
3 variables
Variables in awk can hold both numeric and string values and do not
have to be pre-declared. In fact, there is no way to explicitly
declare them at all. Variable names consist of a leading letter
(either upper or lower case, which are distinct from each other)
or underscore (_) character followed by any number of letters,
digits, or underscores.
When a variable that didn't previously exist is referenced, it is
created and given a null value. A null value is treated as 0 when
used as a number, and is a string of zero characters in length if
used as a string.
4 builtin_variables
GAWK maintains several 'built-in' variables. All have default values;
some are updated automatically. All the builtins have uppercase-only
names.
These builtin variables control how awk behaves
FS input field separator; default is a single space, which is
treated as if it were a regular expression for matching
one or more spaces and/or tabs; a value of " " also has a
second special-case side-effect of causing leading blanks
to be ignored instead of producing a null first field;
initial value can be specified on the command line with
the -F option (or /field_separator); the value can be a
regular expression
RS input record separator; default value is a newline ("\n");
only a single character is allowed [no regular expressions
or multi-character strings; expected to be remedied in a
future release of gawk]
OFS output field separator; value to place between variables in
a 'print' statement; default is one space; can be arbitrary
string
ORS output record separator; value to implicitly terminate 'print'
statement with; default is newline ("\n"); can be arbitrary
string
OFMT default output format used for printing numbers; default
value is "%.6g"
CONVFMT conversion format used for string-to-number conversions;
default value is also "%.6g", like OFMT
SUBSEP subscript separator for array indices; used when an array
subscript is specified as a comma separated list of values:
the comma is replaced by SUBSEP and the resulting index
is a concatenation of the values and SUBSEP(s); default
value is "\034"; value may be arbitrary string
IGNORECASE regular expression matching flag; if true (non-zero)
matching ignores differences between upper and lower case
letters; affects the '~' and '!~' operators, the 'index',
'match', 'split', 'sub', and 'gsub' functions, and the
field splitting based on FS; default value is false (0);
has no effect if GAWK is in strict compatibility mode (via
the -"W compat" option or /strict)
FIELDWIDTHS space or tab separated list of width sizes; takes
precedence over FS when set, but is cleared if FS has a
value assigned to it; [note: the current implementation
of fixed-field input is considered experimental and is
expected to evolve over time]
These builtin variables provide useful information
NF number of fields in the current record
NR record number (accumulated over all files when more than one
input file is processed by the same program)
FNR current record number of the current input file; reset to 0
each time an input file is completed
RSTART starting position of substring matched by last invocation
of the 'match' function; set to 0 if a match fails and at
the start of each input record
RLENGTH length of substring matched by the last invocation of the
'match' function; set to -1 if a match fails
FILENAME name of the input file currently being processed; the
special name "-" is used to represent the standard input
ENVIRON array of miscellaneous user environment values; the VMS
implementation of GAWK provides values for ["USER"] (the
username), ["PATH"] (current default directory), ["HOME"]
(the user's login directory), and "[TERM]" (terminal type
if available) [all info provided by VAXCRTL's environ]
ARGC number of elements in the ARGV array, counting [0] which is
the program name (ie, "gawk")
ARGV array of command-line arguments (in [0] to [ARGC-1]); the
program name (ie, "gawk") in held in ARGV[0]; command line
parameters (data files and "var=value" expressions, but not
program options or the awk program text string if present)
are stored in ARGV[1] through ARGV[ARGC-1]; the awk program
can change values of ARGC and ARGV[] during execution in
order to alter which files are processed or which between-
file assignments are made
4 arrays
awk supports associative arrays to collect data into tables. Array
elements can be either numeric or string, as can the indices used to
access them. Each array must have a unique name, but a given array
can hold both string and numeric elements at the same time. Arrays
are one-dimensional only, but multi-dimensional arrays can be
simulated using comma (,) separated indices, whereby a single index
value gets created by replacing commas with SUBSEP and concatenating
the resulting expression into a single string.
Referencing an array element is done with the expression
Array[Index]
where 'Array' represents the array's name and 'Index' represents a
value or expression used for a subscript. If the requested array
element did not exist, it will be created and assigned an initial
null value. To check whether an element exists without creating it,
use the 'in' boolean operator.
Index in Array
would check 'Array' for element 'Index' and return 1 if it existed
or 0 otherwise. To remove an element from an array, use the 'delete'
statement
delete Array[Index]
Note: there is no way to delete an ordinary variable or an entire
array; 'delete' only works on a specific array element.
To process all elements of an array (in succession) when their
subscripts might be unknown, use the 'in' variant of the for-loop
for (Index in Array) { ... }
3 functions
awk supports both built-in and user-defined functions. A function
may be considered a 'black-box' which accepts zero or more input
parameters, performs some calculations or other manipulations based
on them, and returns a single result.
The syntax for calling a function consists of the function name
immediately followed by an open parenthesis (left parenthesis '('),
followed by an argument list, followed by a closing parenthesis
(right parenthesis ')'). The argument list is a sequence of values
(numbers, strings, variables, array references, or expressions
involving the above and/or nested function calls), separated by
commas and optional white space.
The parentheses are required punctuation, except for the 'print' and
'printf' builtin IO functions, where they're optional, and for the
builtin IO function 'getline', where they're not allowed. Some
functions support optional [trailing] arguments which can be simply
omitted (along with the corresponding comma if applicable).
4 numeric_functions
Builtin numeric functions
int(n) returns the value of 'n' with any fraction truncated
[truncation of negative values is towards 0]
sqrt(n) the square root of n
exp(n) the exponential of n ('e' raised to the 'n'th power)
log(n) natural logarithm of n
sin(n) sine of n (in radians)
cos(n) cosine of n (radians)
atan2(m,n) arctangent of m/n (radians)
rand() random number in the range 0 to 1 (exclusive)
srand(s) sets the random number 'seed' to s, so that a sequence
of 'random' numbers can be repeated; returns the
previous seed value; srand() [argument omitted] sets
the seed to an 'unpredictable' value (based on date
and time, for instance, so should be unrepeatable)
4 string_functions
Builtin string functions
index(s,t) search string s for substring t; result is 1-based
offset of t within s, or 0 if not found
length(s) returns the length of string s; either 'length()'
with its argument omitted or 'length' without any
parenthesized argument list will return length of $0
match(s,r) search string s for regular expression r; the offset
of the longest, left-most substring which matches
is returned, or 0 if no match was found; the builtin
variables RSTART and RLENGTH are also set [RSTART to
the return value and RLENGTH to the size of the
matching substring, or to -1 if no match was found]
split(s,a,f) break string s into components based on field
separator f and store them in array a (into elements
[1], [2], and so on); the last argument is optional,
if omitted, the value of FS is used; the return value
is the number of components found
sprintf(f,e,...) format expression(s) e using format string f and
return the result as a string; formatting is similar
to the printf function
sub(r,t,s) search string target s for regular expression r, and
if a match is found, replace the matching text with
substring t, then store the result back in s; if s
is omitted, use $0 for the string; the result is
either 1 if a match+substitution was made, or 0
otherwise; if substring t contains the character
'&', the text which matched the regular expression
is used instead of '&' [to suppress this feature
of '&', 'quote' it with a backslash (\); since this
will be inside a quoted string which will receive
'backslash' processing before being passed to sub(),
*two* consecutive backslashes will be needed "\\&"]
gsub(r,t,s) similar to sub(), but gsub() replaces all nonoverlapping
substrings instead of just the first, and the return
value is the number of substitutions made
substr(s,p,l) extract a substring l characters long starting at
offset p in string s; l is optional, if omitted then
the remainder of the string (p thru end) is returned
tolower(s) return a copy of string s in which every uppercase
letter has been converted into lowercase
toupper(s) analogous to tolower(); convert lowercase to uppercase
4 time_functions
Builtin time functions
systime() return the current time of day as the number of seconds
since some reference point; on VMS the reference point
is January 1, 1970, at 12 AM local time (not UTC)
strftime(f,t) format time value t using format f; if t is omitted,
the default is systime()
5 time_formats
Formatting directives similar to the 'printf' & 'sprintf' functions
(each is introduced in the format string by preceding it with a
percent sign (%)); the directive is substituted by the corresponding
value
a abbreviated weekday name (Sun,Mon,Tue,Wed,Thu,Fri,Sat)
A full weekday name
b abbreviated month name (Jan,Feb,...)
B full month name
c date and time (Unix-style "aaa bbb dd HH:MM:SS YYYY" format)
C century prefix (19 or 20) [not century number, ie 20th]
d day of month as two digit decimal number (01-31)
D date in mm/dd/yy format
e day of month with leading space instead of leading 0 ( 1-31)
E ignored; following format character used
H hour (24 hour clock) as two digit number (00-23)
h abbreviated month name (Jan,Feb,...) [same as %b]
I hour (12 hour clock) as two digit number (01-12)
j day of year as three digit number (001-366)
m month as two digit number (01-12)
M minute as two digit number (00-59)
n 'newline' (ie, treat %n as \n)
O ignored; following format character used
p AM/PM designation for 12 hour clock
r time in AM/PM format ("II:MM:SS p")
R time without seconds ("HH:MM")
S second as two digit number (00-59)
t tab (ie, treat %t as \t)
T time ("HH:MM:SS")
U week of year (00-53) [first Sunday is first day of week 1]
V date (VMS-style "dd-bbb-YYYY" with 'bbb' forced to uppercase)
w weekday as decimal digit (0 [Sunday] through 6 [Saturday])
W week of year (00-53) [first _Monday_ is first day of week 1]
x date ("aaa bbb dd YYYY")
X time ("HH:MM:SS")
y year without century (00-99)
Y year with century (19yy-20yy)
Z time zone name (always "local" for VMS)
% literal percent sign (%)
4 IO_functions
Builtin I/O functions
print x,... print the values of one or more expressions; if none
are listed, $0 is used; parentheses are optional;
when multiple values are printed, the current value
of builtin OFS (default is 1 space) is used to
separate them; the print line is implicitly
terminated with the current value of ORS (default
is newline); print does not have a return value
printf(f,x,...) print the values of one or more expressions, using
the specified format string; null strings are used
to supply missing values (if any); no between field
or trailing newline characters are printed, they
should be specified within the format string; the
argument-enclosing parentheses are optional;
printf does not have a return value
getline v read a record into variable v; if v is omitted, $0 is
used (and NF, NR, and FNR are updated); if v is
specified, then field-splitting won't be performed;
note: parentheses around the argument are *not*
allowed; return value is 1 for successful read, 0
if end of file is encountered, or -1 if some sort
of error occurred; [see 'redirection' for several
variants]
close(s) close a file or pipe specified by the string s; the
string used should have the same value as the one
used in a getline or print/printf redirection
system(s) pass string s to executed by the operating system;
the command string is executed in a subprocess
5 redirection
Both getline and print/printf support variant forms which use
redirection and pipes.
To read from a file (instead of from the primary input file), use
getline var < "file"
or getline < "file" (read into $0)
where the string "file" represents either an actual file name (in
quotes) or a variable which contains a file name string value or an
expression which evaluates to a string filename.
To create a pipe executing some command and read the result into
a variable (or into $0), use
"command" | getline var
or "command" | getline (read into $0)
where "command" is a literal string containing an operating system
command or a variable with a string value representing such a
command.
To output into a file other that the primary output, use
print x,... > "file" (or >> "file")
or printf(f,x,...) > "file" (or >> "file")
similar to the 'getline' example above. '>>' causes output to be
appended to an existing file if it exists, or create the file if
it doesn't already exist. '>' always creates a new file. The
alternate redirection method of '>$' (for RMS text file attributes)
is *only* available on the command line, not with 'print' or
'printf' in the current release.
To output an error message, use 'print' or 'printf' and redirect
the output to file "/dev/stderr" (or equivalently to "SYS$ERROR:"
on VMS). 'stderr' will normally be the user's terminal, even if
ordinary output is being redirected into a file.
To feed awk output into another command, use
print x,... | "command" (similarly for 'printf')
similar to the second 'getline' example. In this case, output
from awk will be passed as input to the specified operating system
command. The command must be capable of reading input from 'stdin'
("SYS$INPUT:" on VMS) in order to receive data in this manner.
The 'close' function operates on the "file" or "command" argument
specified here (either a literal string or a variable or expression
resulting in a string value). It completely closes the file or
pipe so that further references to the same file or command string
would re-open that file or command at the beginning. Closing a
pipe or redirection also releases some file-oriented resources.
Note: the VMS implementation of GAWK uses temporary files to
simulate pipes, so a command must finish before 'getline' can get
any input from it, and 'close' must be called for an output pipe
before any data can be passed to the specified command.
5 formats
Formatting characters used by the 'printf' and 'sprintf' functions
(each is introduced in the format string by preceding it with a
percent sign (%))
% include a literal percent sign (%) in the result
c format the next argument as a single ASCII character
(prints first character of string argument, or corresponding
ASCII character if numeric argument, e.g. 65 is 'A')
s format the next argument as a string (numeric arguments are
converted into strings on demand)
d decimal number (ie, integer value in base 10)
i integer (equivalent to decimal)
o octal number (integer in base 8)
x hexadecimal number (integer in base 16) [lowercase]
X hexadecimal number [digits 'A' thru 'E' in uppercase]
f floating point number (digits, decimal point, fraction digits)
e exponential (scientific notation) number (digit, decimal
point, fraction digits, letter 'e', sign '+' or '-',
exponent digits)
g 'fractional' number in either 'e' or 'f' format, whichever
produces shorter result
Three optional modifiers can be placed between the initiating
percent sign and the format character (doesn't apply to %%).
- left justify (only matters when width specifier is present)
NN width ['NN' represents 1 or more decimal digits]; actually
minimum width to use, longer items will not be truncated; a
leading 0 will cause right-justified numbers to be padded on
the left with zeroes instead of spaces when they're aligned
.MM precision [decimal point followed by 1 or more digits]; used
as maximum width for strings (causing truncation if they're
actually longer) or as number of fraction digits for 'f' or
'e' numeric formats, or number of significant digits for 'g'
numeric format
4 user_defined_functions
User-defined functions may be created as needed to simplify awk
programs or to collect commonly used code into one place. The
general syntax of a user-defined function is the 'function' keyword
followed by unique function name, followed by a comma-separated
parameter list enclosed in parentheses, followed by statement(s)
enclosed within braces ({}). A 'return' statement is customary
but is not required.
function FuncName(arg1,arg2) {
# arbitrary statements
return (arg1 + arg2) / 2
}
If a function does not use 'return' to specify an output value, the
result received by the caller will be unpredictable.
Functions may be placed in an awk program before, between, or after
the pattern-action rules. The abbreviation 'func' may be used in
place of 'function', unless POSIX compatibility mode is in effect.
3 regular_expressions
A regular expression is a shorthand way of specifying a 'wildcard'
type of string comparison. Regular expression matching is very
fundamental to awk's operation.
Meta symbols
^ matches beginning of line or beginning of string; note that
embedded newlines ('\n') create multi-line strings, so
beginning of line is not necessarily beginning of string
$ matches end of line or end of string
. any single character (except newline)
[ ] set of characters; [ABC] matches either 'A' or 'B' or 'C'; a
dash (other than first or last of the set) denotes a range
of characters: [A-Z] matches any upper case letter; if the
first character of the set is '^', then the sense of match
is reversed: [^0-9] matches any non-digit; several
characters need to be quoted with backslash (\) if they
occur in a set: '\', ']', '-', and '^'
| alternation (similar to boolean 'or'); match either of two
patterns [for example "^start|stop$" matches leading 'start'
or trailing 'stop']
( ) grouping, alter normal precedence [for example, "^(start|stop)$"
matches lines reading either 'start' or 'stop']
* repeated matching; when placed after a pattern, indicates that
the pattern should match any number of times [for example,
"[a-z][0-9]*" matches a lower case letter followed by zero or
more digits]
+ repeated matching; when placed after a pattern, indicates that
the pattern should match one or more times ["[0-9]+" matches
any non-empty sequence of digits]
? optional matching; indicates that the pattern can match zero or
one times ["[a-z][0-9]?" matches lower case letter alone or
followed by a single digit]
\ quote; prevent the character which follows from having special
meaning
A regular expression which matches a string or line will match against
the first (left-most) substring which meets the pattern and include
the longest sequence of characters which still meets that pattern.
3 comments
Comments in awk programs are introduced with '#'. Anything after
'#' on a line is ignored by GAWK. It's a good idea to include an
explanation of what an awk program is doing and also who wrote it
and when.
3 further_information
For complete documentation on GAWK, see "The_GAWK_Manual" from FSF.
Source text for it is present in the file GAWK.TEXINFO. A postscript
version is available via anonymous FTP from host prep.ai.mit.edu in
directory pub/gnu/.
For additional documentation on awk--above and beyond that provided in
The_GAWK_Manual--see "The_AWK_Programming_Language" by Aho, Weinberger,
and Kernighan (2nd edition, 1988), published by Addison-Wesley. It is
both a reference on the awk language and a tutorial on awk's use, with
many sample programs.
3 authors
The awk programming language was originally created by Alfred V. Aho,
Peter J. Weinberger, and Brian W. Kernighan in 1977. The language
was revised and enhanced in a new version which was released in 1985.
GAWK, the GNU implementation of awk, was written in 1986 by Paul Rubin
and Jay Fenlason, with advice from Richard Stallman, and with
contributions from John Woods. In 1988 and 1989, David Trueman and
Arnold Robbins revised GAWK for compatibility with the newer awk.
GAWK version 2.11.1 was ported to VMS by Pat Rankin in November, 1989,
with further revisions in the Spring of 1990. The VMS port was
incorporated into the official GNU distribution of version 2.13 in
Spring 1991. (Version 2.12 was never publically released.)
2 release_notes
GAWK 2.14 tested under VMS V5.5, July, 1992; compatible with VMS
versions V4.6 and later. Current source code compatible with DEC's
VAXC v3.x and v2.4 or v2.3; also compiles successfully with GNUC
(GNU's gcc). VMS POSIX uses c89 and requires VAXC V3.x.
3 AWK_LIBRARY
GAWK uses a built in search path when looking for a program file
specified by the -f option (or the /input qualifier) when that file
name does not include a device and/or directory. GAWK will first
look in the current default directory, then if the file wasn't found
it will look in the directory specified by the translation of logical
name "AWK_LIBRARY".
Not applicable under VMS POSIX.
3 known_problems
There are several known problems with GAWK running on VMS. Some can
be ignored, others require work-arounds. Note: GAWK in the VMS POSIX
environment does not have these problems.
4 command_line_parsing
The command
gawk "program text"
will pass the first phase of DCL parsing (the single required
parameter is present), then it will give an error that a required
element (either /input=awk_file or /commands="program text") is
missing. If what was intended (as is most likely) is to pass the
program text to the UN*X-style command interface, the following
variation is required
gawk -- "program text"
The presence of "--", which is normally optional, will inhibit the
attempt to use DCL parsing (as will any '-' option or redirection).
4 file_formats
If a file having the RMS attribute "Fortran carriage control" is
read as input, it will generate an empty first record if the first
actual record begins with a space (leading space becomes a newline).
Also, the last record of the file will give a "record not terminated"
warning. Both of these minor problems are due to the way that the
C Run-Time Library (VAXCRTL) converts record attributes.
Another poor feature without a work-around is that there's no way to
specify "append if possible, create with RMS text attributes if not"
with the current command line I/O redirection. '>>$' isn't supported.
4 RS_peculiarities
Changing the record separator to something other than newline ('\n')
will produce anomalous results for ordinary files. For example,
using RS = "\f" and FS = "\n" with the following input
|rec 1, line 1
|rec 1, line 2
|^L (form feed)
|rec 2, line 1
|rec 2, line 2
|^L (form feed)
|rec 3, line 1
|rec 3, line 2
|(end of file)
will produce two fields for record 1, but three fields each for
records 2 and 3. This is because the form-feed record delimiter is
on its own line, so awk sees a newline after it. Since newline is
now a field separator, records 2 and 3 will have null first fields.
The following awk code will work-around this problem by inserting
a null first field in the first record, so that all records can be
handled the same by subsequent processing.
# fix up for first record (RS != "\n")
FNR == 1 { if ( $0 == "" ) #leading separator
next #skip its null record
else #otherwise,
$0 = FS $0 #realign fields
}
There is a second problem with this same example. It will always
trigger a "record not terminated" warning when it reaches the end of
file. In the sample shown, there is no final separator; however, if
a trailing form-feed were present, it would produce a spurious final
record with two null fields. This occurs because the I/O system
sees an implicit newline at the end of the last record, so awk sees
a pair of null fields separated by that newline. The following code
fragment will fix that provided there are no null records (in this
case, that would be two consecutive lines containing just form-feeds).
# fix up for last record (RS != "\n")
$0 == FS { next } #drop spurious final record
Note that the "record not terminated" warning will persist.
4 cmd_inconsistency
The DCL qualifier /OUTPUT is internally equivalent to '>$' output
redirection, but the qualifier /INPUT corresponds to the -f option
rather than to '<' input redirection.
4 exit
The exit statement can optionally pass a final status value to the
operating system. GAWK expects a UN*X-style value instead of a
VMS status value, so 0 indicates success and non-zero indicates
failure. The final exit status will be 1 (VMS success) if 0 is
used, or even (VMS non-success) if non-zero is used.
3 changes
Changes between version 2.14 and 2.13.2:
General
'next file' construct added
'continue' outside of any loop is treated as 'next'
Assorted bug fixes and efficiency improvements
_The_GAWK_Manual_ updated
Test suite expanded
VMS-specific
VMS POSIX support added
Disk I/O throughput enhanced
Pipe emulation improved and incorrect interaction with user-mode
redefinition of SYS$OUTPUT eliminated
3 prior_changes
Changes between version 2.13 and 2.11.1: (2.12 was not released)
General
CONVFMT and FIELDWIDTHS builtin control variables added
systime() and strftime() date/time functions added
'lint' and 'posix' run-time options added
'-W' command line option syntax supercedes '-c', '-C', and '-V'
'-a' and '-e' regular expression options made obsolete
Various bug fixes and efficiency improvements
More platforms supported ('officially' including VMS)
VMS-specific
%g printf format fixed
Handling of '\' on command line modified; no longer necessary to
double it up
Problem redirecting stderr (>&efile) at same time as stdin (<ifile)
or stdout (>ofile) has been fixed
``2>&1'' and ``1>&2'' redirection constructs added
Interaction between command line I/O redirection and gawk pipes
fixed; also, name used for pseudo-pipe temporary file expanded
3 license
GAWK is covered by the "GNU General Public License", the gist of which
is that if you supply this software to a third party, you are expressly
forbidden to prevent them from supplying it to a fourth party, and if
you supply binaries you must make the source code available to them
at no additional cost. Any revisions or modified versions are also
covered by the same license. There is no warranty, express or implied,
for this software. It is provided "as is."
[Disclaimer: This is just an informal summary with no legal basis;
refer to the actual GNU General Public License for specific details.]
!2 examples
!