Gawk

https://www.gnu.org/software/gawk/manual/html_node/
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Gawk is the GNU implementation of the AWK programming language, used for text processing, data...
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### Initialization of a scalar cookie

Source: https://www.gnu.org/software/gawk/manual/html_node/Symbol-table-by-cookie.html

This example shows how to install a variable and retrieve its scalar cookie during extension initialization for later use.

```c
static awk_scalar_t magic_var_cookie;    /* cookie for MAGIC_VAR */

static void
my_extension_init()
{
    awk_value_t value;
```

```c

    /* install initial value */
    sym_update("MAGIC_VAR", make_number(42.0, & value));

    /* get the cookie */
    sym_lookup("MAGIC_VAR", AWK_SCALAR, & value);

    /* save the cookie */
    magic_var_cookie = value.scalar_cookie;
    ...
}
```

--------------------------------

### POSIX date utility example

Source: https://www.gnu.org/software/gawk/manual/html_node/Time-Functions.html

An example of using the date utility with a format string starting with a plus sign.

```shell
$ `date '+Today is %A, %B %d, %Y.'`
-| Today is Monday, September 22, 2014.
```

--------------------------------

### Switch statement example with getopt

Source: https://www.gnu.org/software/gawk/manual/html_node/Switch-Statement.html

An example demonstrating the use of a switch statement to handle command-line options parsed by getopt.

```awk
while ((c = getopt(ARGC, ARGV, "aksx")) != -1) {
    switch (c) {
    case "a":
        # report size of all files
        all_files = TRUE;
        break
    case "k":
        BLOCK_SIZE = 1024       # 1K block size
        break
    case "s":
        # do sums only
        sum_only = TRUE
        break
    case "x":
        # don't cross filesystems
        fts_flags = or(fts_flags, FTS_XDEV)
        break
    case "?":
    default:
        usage()
        break
    }
}
```

--------------------------------

### Build and install gawk

Source: https://www.gnu.org/software/gawk/manual/html_node/gawkextlib.html

Commands to configure, build, check, and install gawk into /tmp/newgawk.

```bash
cd .../path/to/gawk/code
./configure --prefix=/tmp/newgawk     _Install in /tmp/newgawk for now_
make && make check                    _Build and check that all is OK_
make install                          _Install gawk_
```

--------------------------------

### Shell command example with output and error

Source: https://www.gnu.org/software/gawk/manual/html_node/Conventions.html

Shows how command-line examples are presented: input is preceded by the shell prompt '$', standard output by '-|', and standard error by 'error→'. The example demonstrates printing to stdout and redirecting to stderr.

```shell
$ `echo hi on stdout`
-| hi on stdout
$ `echo hello on stderr 1>&2`
error→ hello on stderr

```

--------------------------------

### Example awk program file (advice)

Source: https://www.gnu.org/software/gawk/manual/html_node/Long.html

This is the content of the file 'advice' used in the example. It prints the message 'Don't Panic!' when run.

```awk
BEGIN { print "Don't Panic!" }

```

--------------------------------

### Output of myprint() example

Source: https://www.gnu.org/software/gawk/manual/html_node/Function-Example.html

The output produced by the myprint() example when given the sample input. It shows the formatted positive third fields.

```text
   5.6
  21.2
```

--------------------------------

### Sample data file

Source: https://www.gnu.org/software/gawk/manual/html_node/Indirect-Calls.html

Example input file format for the sorting program.

```text
Biology_101 sum average sort rsort data: 87.0 92.4 78.5 94.9
Chemistry_305 sum average sort rsort data: 75.2 98.3 94.7 88.2
English_401 sum average sort rsort data: 100.0 95.6 87.1 93.4
```

--------------------------------

### Install package from source

Source: https://www.gnu.org/software/gawk/manual/html_node/Derived-Files.html

Commands to download, extract, build, and install a GNU package from source. The note mentions that the --no-check-certificate option may be needed for wget due to the https:// URL.

```bash
wget https://ftp.gnu.org/gnu/package/package-x.y.z.tar.gz
tar -xpzvf package-x.y.z.tar.gz
cd package-x.y.z
./configure && make && make check
make install    # as root
```

--------------------------------

### Example array contents for asort/asorti

Source: https://www.gnu.org/software/gawk/manual/html_node/String-Functions.html

Initial contents of array 'a' used in the asort and asorti examples. Shows three key-value pairs with string values.

```awk
a["last"] = "de"
a["first"] = "sac"
a["middle"] = "cul"

```

--------------------------------

### Sample Test Program Runs

Source: https://www.gnu.org/software/gawk/manual/html_node/Getopt-Function.html

Example command-line executions and their corresponding outputs for the getopt test program.

```bash
$ `awk -f getopt.awk -v _getopt_test=1 -- -a -cbARG bax -x`
-| c = <a>, Optarg = <>
-| c = <c>, Optarg = <>
-| c = <b>, Optarg = <ARG>
-| non-option arguments:
-|         ARGV[3] = <bax>
-|         ARGV[4] = <-x>

$ `awk -f getopt.awk -v _getopt_test=1 -- -a -x -- xyz abc`
-| c = <a>, Optarg = <>
error→ x -- invalid option
-| c = <?>, Optarg = <>
-| non-option arguments:
-|         ARGV[4] = <xyz>
-|         ARGV[5] = <abc>

$ `awk -f getopt.awk -v _getopt_test=1 -- -a \
> `--longa -b xx --longb=foo=bar --otherd --otherc arg1 arg2`
-| c = <a>, Optarg = <>
-| c = <longa>, Optarg = <>
-| c = <b>, Optarg = <xx>
-| c = <longb>, Optarg = <foo=bar>
-| c = <otherd>, Optarg = <>
-| c = <otherc>, Optarg = <>
-| non-option arguments:
-|        ARGV[8] = <arg1>
-|        ARGV[9] = <arg2>
```

--------------------------------

### Example usage of ord() and chr()

Source: https://www.gnu.org/software/gawk/manual/html_node/Extension-Sample-Ord.html

Demonstrates loading the extension and using ord() to get the numeric value of a character and chr() to get the character from a numeric value.

```gawk
@load "ordchr"
...
printf("The numeric value of 'A' is %d\n", ord("A"))
printf("The string value of 65 is %s\n", chr(65))
```

--------------------------------

### Initialize getopt and Test Harness

Source: https://www.gnu.org/software/gawk/manual/html_node/Getopt-Function.html

Sets up the initial state for getopt and provides a test block to demonstrate option parsing.

```awk
BEGIN {
    Opterr = 1    # default is to diagnose
    Optind = 1    # skip ARGV[0]

    # test program
    if (_getopt_test) {
        _myshortopts = "ab:cd"
        _mylongopts = "longa,longb:,otherc,otherd"

        while ((_go_c = getopt(ARGC, ARGV, _myshortopts, _mylongopts)) != -1)
            printf("c = <%s>, Optarg = <%s>\n", _go_c, Optarg)
        printf("non-option arguments:\n")
        for (; Optind < ARGC; Optind++)
            printf("\tARGV[%d] = <%s>\n", Optind, ARGV[Optind])
    }
}
```

--------------------------------

### split() example: split string into array and separators

Source: https://www.gnu.org/software/gawk/manual/html_node/String-Functions.html

Splits the string "cul-de-sac" using '-' as separator. The array 'a' gets the pieces and 'seps' gets the separator strings. Returns 3.

```awk
split("cul-de-sac", a, "-", seps)
```

```awk
a[1] = "cul"
a[2] = "de"
a[3] = "sac"
```

```awk
seps[1] = "-"
seps[2] = "-"
```

--------------------------------

### View all configure options

Source: https://www.gnu.org/software/gawk/manual/html_node/Additional-Configuration-Options.html

Use this command to display the full list of available options provided by the configure script.

```bash
./configure --help
```

--------------------------------

### Shell file redirection example

Source: https://www.gnu.org/software/gawk/manual/html_node/Redirection.html

Demonstrates the standard shell approach of using > to create a file and >> for subsequent additions.

```shell
echo Name: Arnold Robbins > data
echo Street Address: 1234 A Pretty Street, NE >> data
echo City and State: MyTown, MyState 12345-6789 >> data
```

--------------------------------

### awk comment example

Source: https://www.gnu.org/software/gawk/manual/html_node/Comments.html

Shows how to add comments to an awk program. Comments start with '#' and continue to the end of the line. The '#' does not have to be the first character on the line.

```awk
# This program prints a nice, friendly message.  It helps
# keep novice users from being afraid of the computer.
BEGIN    { print "Don't Panic!" }

```

--------------------------------

### C extension: create and populate array with subarray

Source: https://www.gnu.org/software/gawk/manual/html_node/Creating-Arrays.html

Complete C extension example that creates a new array, installs it in the symbol table, adds two regular elements, creates a subarray, and populates it. The leading #include directives and boilerplate declarations are omitted for brevity.

```c
/* create_new_array --- create a named array */

static void
create_new_array()
{
    awk_array_t a_cookie;
    awk_array_t subarray;
    awk_value_t index, value;

    a_cookie = create_array();
    value.val_type = AWK_ARRAY;
    value.array_cookie = a_cookie;

    if (! sym_update("new_array", & value))
        printf("create_new_array: sym_update(\"new_array\") failed!\n");
    a_cookie = value.array_cookie;
```

```c
    (void) make_const_string("hello", 5, & index);
    (void) make_const_string("world", 5, & value);
    if (! set_array_element(a_cookie, & index, & value)) {
        printf("fill_in_array: set_array_element failed\n");
        return;
    }

    (void) make_const_string("answer", 6, & index);
    (void) make_number(42.0, & value);
    if (! set_array_element(a_cookie, & index, & value)) {
        printf("fill_in_array: set_array_element failed\n");
        return;
    }
```

```c
    (void) make_const_string("subarray", 8, & index);
    subarray = create_array();
    value.val_type = AWK_ARRAY;
    value.array_cookie = subarray;
    if (! set_array_element(a_cookie, & index, & value)) {
        printf("fill_in_array: set_array_element failed\n");
        return;
    }
    subarray = value.array_cookie;
```

```c
    (void) make_const_string("foo", 3, & index);
    (void) make_const_string("bar", 3, & value);
    if (! set_array_element(subarray, & index, & value)) {
        printf("fill_in_array: set_array_element failed\n");
        return;
    }
}
```

--------------------------------

### Build Gawk from source

Source: https://www.gnu.org/software/gawk/manual/html_node/Installation-summary.html

The canonical build recipe for Gawk on POSIX systems using wget, tar, configure, and make.

```bash
wget https://ftp.gnu.org/gnu/gawk/gawk-5.4.0.tar.gz
tar -xvpzf gawk-5.4.0.tar.gz
cd gawk-5.4.0
./configure && make && make check
```

--------------------------------

### substr(string, start [, length])

Source: https://www.gnu.org/software/gawk/manual/html_node/String-Functions.html

Returns a substring of string starting at character position start, optionally of a specified length. If length is omitted, returns the suffix from start to the end.

```APIDOC
## substr(string, start [, length])

### Description
Returns a length-character-long substring of string, starting at character number start (the first character is number one). If length is not present, returns the whole suffix of string beginning at start. If length is greater than the number of characters remaining, the whole suffix is returned.

### Syntax
substr(string, start [, length])

### Parameters
- **string** (string) - Required - The string from which to extract the substring.
- **start** (number) - Required - The starting character position (1-based). If start is less than one, it is treated as one. If start is greater than the number of characters in the string, the null string is returned.
- **length** (number) - Optional - The number of characters to return. If length is present but less than or equal to zero, the null string is returned.

### Return Value
The extracted substring.

### Examples
```
substr("washington", 5, 3)  # returns "ing"
substr("washington", 5)     # returns "ington"
```

### Notes
- The string returned by substr() cannot be assigned. It is a mistake to attempt to change a portion of a string, e.g., substr(string, 3, 3) = "CDE" is invalid.
- It is also a mistake to use substr() as the third argument of sub() or gsub().
- To replace bits and pieces of a string, combine substr() with string concatenation, e.g., string = substr(string, 1, 2) "CDE" substr(string, 6).
```

--------------------------------

### awk: Toggle flag to print lines between START and END markers

Source: https://www.gnu.org/software/gawk/manual/html_node/Boolean-Ops.html

Prints lines between special bracketing lines. The variable 'interested' starts as zero (false) and is toggled with '!' when START or END is seen. Note: this program has a bug; it prints lines starting with 'END'.

```awk
$1 == "START"   { interested = ! interested; next }
interested      { print }
$1 == "END"     { interested = ! interested; next }
```

--------------------------------

### Initialize getopt() function

Source: https://www.gnu.org/software/gawk/manual/html_node/Getopt-Function.html

The function definition and initial check for empty options and longoptions parameters.

```awk
function getopt(argc, argv, options, longopts,    thisopt, i, j)
{
    if (length(options) == 0 && length(longopts) == 0)
        return -1                # no options given
```

--------------------------------

### Build gawk with make

Source: https://www.gnu.org/software/gawk/manual/html_node/Quick-Installation.html

After running configure, type make to build an executable version of gawk. Run 'make check' to verify it works properly.

```shell
make

```

--------------------------------

### Interval Expression Examples

Source: https://www.gnu.org/software/gawk/manual/html_node/Regexp-Operator-Details.html

Examples of interval expressions matching specific repetition counts.

```regexp
wh{3}y
```

```regexp
wh{3,5}y
```

```regexp
wh{2,}y
```

--------------------------------

### pwcat output example

Source: https://www.gnu.org/software/gawk/manual/html_node/Passwd-Functions.html

Sample output from running pwcat, showing the user database in colon-separated format. The output includes root, nobody, daemon, sys, bin, and other users.

```text
$ `pwcat`
-| root:x:0:1:Operator:/:/bin/sh
-| nobody:*:65534:65534::/:
-| daemon:*:1:1::/:
-| sys:*:2:2::/:/bin/csh
-| bin:*:3:3::/bin:
-| arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/sh
-| miriam:yxaay:112:10:Miriam Robbins:/home/miriam:/bin/sh
-| andy:abcca2:113:10:Andy Jacobs:/home/andy:/bin/sh
...
```

--------------------------------

### grcat command output example

Source: https://www.gnu.org/software/gawk/manual/html_node/Group-Functions.html

Example output produced by running the grcat program.

```text
$ `grcat`
-| wheel:*:0:arnold
-| nogroup:*:65534:
-| daemon:*:1:
-| kmem:*:2:
-| staff:*:10:arnold,miriam,andy
-| other:*:20:
...
```

--------------------------------

### Increment and Decrement Operator Examples

Source: https://www.gnu.org/software/gawk/manual/html_node/Increment-Ops.html

Examples demonstrating the use of increment and decrement operators in AWK.

```awk
b = 6
print b += b++
```

```awk
b = 6
b += ++b + b++
print b
```

--------------------------------

### Input list for getline example

Source: https://www.gnu.org/software/gawk/manual/html_node/Getline_002fVariable.html

The input data used to demonstrate the line-swapping behavior.

```text
wan
tew
free
phore
```

--------------------------------

### Output of length() example

Source: https://www.gnu.org/software/gawk/manual/html_node/Arrays-of-Arrays.html

The output produced by the preceding length() example for the main array a.

```awk
2, 3, 1

```

--------------------------------

### Sample input for myprint() example

Source: https://www.gnu.org/software/gawk/manual/html_node/Function-Example.html

Sample input data used to demonstrate the myprint() function. It contains three lines of numbers.

```text
 1.2   3.4    5.6   7.8
 9.10 11.12 -13.14 15.16
17.18 19.20  21.22 23.24
```

--------------------------------

### Negative remainder example

Source: https://www.gnu.org/software/gawk/manual/html_node/Arithmetic-Ops.html

An example showing that the remainder operator produces a negative result when the dividend is negative.

```awk
-17 % 8 = -1
```

--------------------------------

### Create guide.awk for internationalization

Source: https://www.gnu.org/software/gawk/manual/html_node/I18N-Example.html

The original awk source file using TEXTDOMAIN and the underscore function to mark strings for translation.

```awk
BEGIN {
    TEXTDOMAIN = "guide"
    bindtextdomain(".")  # for testing
    print _"Don't Panic"
    print _"The Answer Is", 42
    print "Pardon me, Zaphod who?"
}
```

--------------------------------

### Execution of Array Traversal Demonstration

Source: https://www.gnu.org/software/gawk/manual/html_node/Controlling-Array-Traversal.html

The command to run the demonstration program and the resulting output showing different sorting behaviors.

```bash
$ `gawk -f compdemo.awk`
-| Sort function: cmp_num_idx      _Sort by numeric index_
-|     data[two] = 20
-|     data[one] = 10              _Both strings are numerically zero_
-|     data[10] = one
-|     data[20] = two
-|     data[100] = 100
-| 
-| Sort function: cmp_str_val      _Sort by element values as strings_
-|     data[one] = 10
-|     data[100] = 100             _String 100 is less than string 20_
-|     data[two] = 20
-|     data[10] = one
-|     data[20] = two
-| 
-| Sort function: cmp_num_str_val  _Sort all numeric values before all strings_
-|     data[one] = 10
-|     data[two] = 20
-|     data[100] = 100
-|     data[10] = one
-|     data[20] = two
```

--------------------------------

### Self-contained awk script example

Source: https://www.gnu.org/software/gawk/manual/html_node/Executable-Scripts.html

An example of an awk script file named 'advice' using the #! mechanism to specify the interpreter.

```awk
#! /bin/awk -f

BEGIN { print "Don't Panic!" }
```

--------------------------------

### Memory ownership example with char pointers

Source: https://www.gnu.org/software/gawk/manual/html_node/API-Ownership-of-MPFR-and-GMP-Values.html

An example demonstrating how ownership of memory can be transferred between pointers for standard character arrays.

```c
char *p = gawk_malloc(42);      p _``owns'' the memory_
char *q = p;
p = NULL;                       _now_ q _``owns'' it_
```

--------------------------------

### Example usage of fork()

Source: https://www.gnu.org/software/gawk/manual/html_node/Extension-Sample-Fork.html

Demonstrates creating a new process and printing different messages based on whether the code is running in the child or parent process.

```awk
@load "fork"
...
if ((pid = fork()) == 0)
    print "hello from the child"
else
    print "hello from the parent"
```

--------------------------------

### Add gawk help to the system library

Source: https://www.gnu.org/software/gawk/manual/html_node/OpenVMS-Installation-Details.html

Load the gawk help file into the OpenVMS help library to enable command-line help access.

```DCL
$ `LIBRARY/HELP sys$help:helplib [.vms]gawk.hlp`
```

--------------------------------

### Example command for flac-edit

Source: https://www.gnu.org/software/gawk/manual/html_node/Shell-Quoting.html

An example command that invokes flac-edit with a song title containing special characters, demonstrating the need for shell quoting.

```sh
$ `flac-edit -song="Whoope! That's Great" file.flac`

```
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