Understanding Awk in the UNIX Shell - Commands and variables
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The commands go between the {} brackets. Now you can enumerate them in a single line using the ";" separator character or a single command in a single line, as the new line character is also a command separator character. A command structure (awk script) usually looks as follows:
BEGIN {intro commands}
Pattern {Commands to be executed at every record if the pattern matches}
END {closing commands}
If you do not need to use any of these you can just skip them and not include them in the script. The character "#" is for comments. Any text found after this character will not be considered in the interpretation of the script. The awk variables are very similar to those present in the C language from the point of view of handling them.
However, here we do not need to declare them. They are created in the moment of declaration and the type is dependent on the environment. If you assign a number to it, they will be numbers (always represented as a float number); if you assign a string to it, then they will be of a string type.
Here is a list of internal variables that are created automatically at the start of the script (and you do not need to declare/modify them unless you want to change them); this will make your programming easier:
Variable
What it contains?
Default value
ARGC
Contains the number of arguments on the command line (Command Line Argument Count )
None
FILENAME
The input file's name (FILENAME)
None
FS
The field separator characters. If this is empty, every character will be in a separate column. (Field Separator)
Space and tab => [t]+
RS
The record separator characters. (Record Separator)
Newline character=> n
IGNORECASE
If not zero at the regular expression pattern matching, an ignore case compare will be made.
0
NF
How many fields are in the input record?
(Number of Fields in the current input record)
none
NR
How many lines have already been processed?
(Numbers of records seen so far)
None
OFMT
How should it format the numbers that will be printed?
(The output format for numbers)
"%.6g"
OFS
Theoutput field separator character.
Space
ORS
Output Record Separator.
With this we separate records that are printed out. If you change this to an empty string, the output will be not separated.
New line.
When you declare some string constants, you can use the escape characters of the C language:
Escape character
What it means?
Backslash
b
Backspace
f
Form-feed
n
Newline
r
Carriage return
t
Horizontal Tab
v
Vertical Tab
0xn
Where n is a number.
Hex numbers.
c
Literally the c character.
For example ? => ?
0n
Where n is a number.
Octal Number.
With this I will stop. I have given you quite a chunk of information. I will leave you a little time to read it in detail and comprehend every piece of it. Nevertheless, we are still far from the end. So make sure you come back for the next article, where I will treat the other, equally important parts of awk -- like the functions, external commands, arrays, and so forth. We will also put all this together and solve a couple of problems to demonstrate the power of the language.
Before I say farewell I would like to encourage you to rate the article. If you have any kinds of questions do not hesitate to ask them. Remember the blog commenting at the end of this article was constructed for this sole purpose. I also welcome any comments that are in the spirit of constructive criticism. Live With Passion!
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