Initial version of AWK

awk.md

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+---
+title: awk
+category: CLI
+layout: 2017/sheet
+intro: |
+  Here's some hints on using AWK.
+---
+AWK is a data-driven scripting language for processing textual data. 
+- [Learn AWK in 20 minutes](https://ferd.ca/awk-in-20-minutes.html) _(ferd.ca/awk-in-20-minutes.html)_
+- [Learn AWK in y minutes](https://learnxinyminutes.com/docs/bash/) _(learnxinyminutes.com)_
+- [GNU AWK Manual](https://www.gnu.org/software/gawk/manual/html_node) _(www.gnu.org/software/gawk)_
+
+The name AWK is derived from the initials of the language's three developers: A. Aho, B. W. Kernighan and P. Weinberger.
+
+
+Usage options
+-------------
+
+```bash
+awk -F, '{print $2}' example.csv  # Use other field seperator
+awk -v foo=2 'BEGIN{print foo^2}' # Import variable(s)
+awk -f program.awk input1 input2  # Use file for commands
+```
+
+Program structure
+-----------------
+An AWK program may consist a BEGIN part, one or multiple patterns and an END part. Each of them is followed by an __action__ defined in __{ }__. If no action is defined, the input line, known as variable __$0__, will be printed.
+
+```bash
+awk 'BEGIN {print 2}'           # Action before processing any file
+awk '/abc/ {i++}'    file1.txt  # Do for every matched pattern...
+awk '/a/{a++}/b/b++' file1.txt  # Multiple patterns allowed
+awk 'END{print i}'   file1.txt  # Finally execute END part
+```
+
+Actions
+-------
+The statements in an action can include:
+- `expressions` with constants, variables, assignments, function call etc.
+- __print__ expression-list
+- __printf(__ format,expression-list __)__
+- __if (__ expression __)__ statement_
+- __if (__ expression __)__ statement __else__ statement
+- ___while__ __(__ expression __)__ statement_
+- __do__ statement __while__ expression
+- __for__ __(__ expression1, expression2, expression3 __)__ statement
+- __for__ __(__ variable __in__ array __)__ statement
+- __delete__ arrayname\[expression\]
+- Flow control: __break__, __continue__, __next__, __exit__, __exit expression__
+- Grouping __{statement; statement; ...}__
+
+
+
+
+Patterns
+--------
+
+Column 1 Heading          | Column 2 Heading
+------------------------- | ---
+`expression`              | Matches when the expression is true
+`/regexpr/`               | Matches when the current input line contains a substring matched by __regexpr__
+`expression~/regpexpr/`   | Matches if the string value of __expression__ contains matched by __regexpr__
+`expression!~/regpexpr/`  | Matches if the string value of __expression does not__ contain a substring matched by regexpr
+`beginpat,endpat`         | Matches starting at the begin pattern and stop matching after end pattern.
+--- | ---
+
+
+
+```bash
+awk -F, 'NF>3' unequalnumcol.csv # Print only rows with more
+                                 # than three columns
+awk '/G/,/T/' sortednames.txt    # Print names starting with
+                                 # G till (including first) T
+```
+See also [regular expressions](https://devhints.io/regexp)
+
+
+Special variables
+-----------------
+
+| -------- | -------------------------------------------- | --------- |
+| Variable | Description                                  | Default   |
+| -------- | -------------------------------------------- | --------- |
+| ARGC     | Number of commandline arguments              |           |
+| ARGV     | Array of commandline arguments               |           |
+| FILENAME | Name of current input file                   |           |
+| FNR      | Record number in current file                |           |
+| FS       | Input field seperator                        | _(space)_ |
+| NF       | Number of fields in current record           |           |
+| NR       | Number of records read so far                |           |
+| OFMT     | Output format for numbers                    |  `%.6g`   |
+| OFS      | Output field seperator                       | _(space)_ |
+| ORS      | Output record seperator                      |  `\n`     |
+| RLENGTH  | Length of string matched by `match` function |           |
+| RS       | Input record seperator                       | `\n`      |
+| RSTART   | Start of string matched by `match` function  |           |
+| SUBSEP   | Array subscript seperator                    | `\034`    |
+{: .-headers}
+
+See also in the GNU manual on [special variables](https://www.gnu.org/software/gawk/manual/html_node/Built_002din-Variables.html)
+
+
+Operators
+---------
+
+| Type           | Description                  |
+| -------------- | ---------------------------- |
+| arithmetic     | `+ - * / % ^`                |
+| assignment     | `= += -= *= /= %= ^= ++ -- ` |
+| logical        | `\|\| && !`                  |
+| matching       | `~ !~`                       |
+| relational     | `< <= == != > >=`            |
+| concatenation  | _(no explicit operator)_     |
+| unary          | `+ - `                       |
+| conditional    | `?:`                         |
+| grouping       | `( )`                        |
+{: .-headers}
+
+
+Functions
+---------
+
+### String functions
+
+| Function        | Description                                          |
+| --------------- | ----------------------------------------------------- |
+| `gsub(r,s)`     | substitute __s__ for __r__ globally in $0, return number of substitutions made        |
+| `gsub(r,s,t)`   | substitute __s__ for __r__ globally in string __t__, return number of substitutions made|
+| `index(s,t)`    | return the first position of string __t__ in __s__, or 0 if __t__ is not present        |
+| `length(s)`     | return number of characters __s__                                                   |
+| `match(s,r)`    | test whether __s__ contains a sbstring matched by __r__, return index or 0. Set special variables RSTART and RLENGTH |
+{: .-headers}
+
+### Math functions
+
+| Function        | Return value                                              |
+| --------------- | --------------------------------------------------------- |
+| `atan2(y,x)`    | arctangent of __y__/__x__                                 |
+| `cos(x)`        | cosine of __x__ , with __x__ in radians                   |
+| `exp(x)`        | exponential function of __x__                             |
+| `int(x)`        | integer part of __x__, truncated towards 0 when __x < 0__ |
+| `log(x)`        | natural (base __e__) logarithm of __x__                   |
+| `rand()`        | random number __r__, where __0 <= r < 1__                 |
+| `sin(x)`        | sine of __x__ , with __x__ in radians                     |
+| `sqrt(x)`       | square root of __x__                                      |
+| `srand(x)`      | set __x__ as new seed for __rand()__                      |
+{: .-headers}
+
+Output Formating
+----------------
+
+### Print function
+
+
+
+| Format       | Description                                                                  |
+| ------------ | ---------------------------------------------------------------------------- |
+| `%c`         | character                                                                    |
+| `%[-][n]c`   | string, optionally with leading `[-]` using `[n]` positions                  |
+| `%[-][n]d`   | decimal integer                                                              |
+| `%[-][n]o`   | octal integer                                                                |
+| `%[-][n]x`   | hexadecimal integer                                                          |
+| `%[-][n]u`   | unsigned decimal integer                                                     |
+| `%[-][n.m]f` | floating number                                                              |
+| `%[-][n.m]e` | scientific notation                                                          |
+| `%[-][n.m]g` | Format __e__ or __f__, whichever is shorter with non-significant zeros suppressed |
+{: .-headers}
+
+```bash
+awk 'BEGIN{printf("pi = %2.3e\n",22/7)}'
+awk 'BEGIN{printf("%c %d %o %x",65,65,65,65)}'
+```
+
+### Escape sequences
+
+| Format   | Description                                                        |
+| -------- | ------------------------------------------------------------------ |
+| `\r`     | carriage return                                                    |
+| `\n`     | newline                                                            |
+| `\t`     | tab                                                                |
+| `\f`     | formfeed                                                           |
+| `\ddd`   | octal value `ddd`, where `ddd` is 1 to 3 digits between 0 and 7    |
+| `\c`     | any other character `c` literally: `\\` for backslash, `\"` for `"`|
+{: .-headers}
+
+```bash
+awk 'BEGIN{printf("\110\145\154\154\157\n")}'
+```