[Linux 文章收集] Awk Introduction Tutorial – 7 Awk Print Examples

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Awk Introduction and Printing Operations 
Awk is a programming language which allows easy manipulation of structured data and the generation of formatted reports. Awk stands for the names of its authors “Aho, Weinberger, and Kernighan” 

The Awk is mostly used for pattern scanning and processing. It searches one or more files to see if they contain lines that matches with the specified patterns and then perform associated actions. Some of the key features of Awk are: 

* Awk views a text file as records and fields.
* Like common programming language, Awk has variables, conditionals and loops
* Awk has arithmetic and string operators.
* Awk can generate formatted reports

Awk reads from a file or from its standard input, and outputs to its standard output. Awk does not get along with non-text files. The Syntax: 

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1. awk '/search pattern1/ {Actions}  
2.      /search pattern2/ {Actions}' file  

In the above awk syntax: 

* search pattern is a regular expression.
* Actions – statement(s) to be performed.
* several patterns and actions are possible in Awk.
* file – Input file.
* Single quotes around program is to avoid shell not to interpret any of its special characters.

Awk Working Methodology 
Awk reads the input files one line at a time. 

1. For each line, it matches with given pattern in the given order, if matches performs the corresponding action.
2. If no pattern matches, no action will be performed.
3. In the above syntax, either search pattern or action are optional, But not both.
4. If the search pattern is not given, then Awk performs the given actions for each line of the input.
5. If the action is not given, print all that lines that matches with the given patterns which is the default action.
6. Empty braces with out any action does nothing. It wont perform default printing operation.
7. Each statement in Actions should be delimited by semicolon.

Let us create employee.txt file which has the following content, which will be used in the examples mentioned below. 
- examplee.txt 

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1. 100  Thomas  Manager    Sales       $5,000  
2. 200  Jason   Developer  Technology  $5,500  
3. 300  Sanjay  Sysadmin   Technology  $7,000  
4. 400  Nisha   Manager    Marketing   $9,500  
5. 500  Randy   DBA        Technology  $6,000  

Awk Usage Examples 
Awk Example 1. Default behavior of Awk 
By default Awk prints every line from the file. 

# awk '{print;}' employee.txt
100 Thomas Manager Sales $5,000
200 Jason Developer Technology $5,500
300 Sanjay Sysadmin Technology $7,000
400 Nisha Manager Marketing $9,500
500 Randy DBA Technology $6,000

In the above example pattern is not given. So the actions are applicable to all the lines. Action print with out any argument prints the whole line by default. So it prints all the lines of the file with out fail. Actions has to be enclosed with in the braces. 

Awk Example 2. Print the lines which matches with the pattern. 

# awk '/Thomas|Nisha/' employee.txt
100 Thomas Manager Sales $5,000
400 Nisha Manager Marketing $9,500

In the above example it prints all the line which matches with the ‘Thomas’ or ‘Nisha’. It has two patterns. Awk accepts any number of patterns, but each set (patterns and its corresponding actions) has to be separated by newline. 

Awk Example 3. Print only specific field. 
Awk has number of built in variables. For each record i.e line, it splits the record delimited by whitespace character by default and stores it in the $n variables. If the line has 4 words, it will be stored in $1, $2, $3 and $4. $0 represents whole line. NF is a built in variable which represents total number of fields in a record. 

# awk '{print $2,$5;}' employee.txt
Thomas $5,000
Jason $5,500
Sanjay $7,000
Nisha $9,500
Randy $6,000
# awk '{print $2,$NF;}' employee.txt
Thomas $5,000
Jason $5,500
Sanjay $7,000
Nisha $9,500
Randy $6,000
# awk '{print $NF;}' employee.txt
$5,000
$5,500
$7,000
$9,500
$6,000

In the above example $2 and $5 represents Name and Salary respectively. We can get the Salary using $NF also, where $NF represents last field. In the print statement ‘,’ is a concatenator. 

Awk Example 4. Initialization and Final Action 
Awk has two important patterns which are specified by the keyword called BEGIN and END. Syntax: 

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1. BEGIN { Actions}  
2. {ACTION} # Action for everyline in a file  
3. END { Actions }  
4.   
5. # is for comments in Awk  

* Actions specified in the BEGIN section will be executed before starts reading the lines from the input.
* END actions will be performed after completing the reading and processing the lines from the input.

# awk 'BEGIN {print "Name\tDesignation\tDepartment\tSalary";}
> {print $2,"\t",$3,"\t",$4,"\t",$NF;}
> END{print "Report Generated\n--------------";}' employee.txt
Name Designation Department Salary
Thomas Manager Sales $5,000
Jason Developer Technology $5,500
Sanjay Sysadmin Technology $7,000
Nisha Manager Marketing $9,500
Randy DBA Technology $6,000
Report Generated
--------------

In the above example, it prints headline and last file for the reports. 

Awk Example 5. Find the employees who has employee id greater than 200 

# awk '$1 > 200' employee.txt
300 Sanjay Sysadmin Technology $7,000
400 Nisha Manager Marketing $9,500
500 Randy DBA Technology $6,000

In the above example, first field ($1) is employee id. So if $1 is greater than 200, then just do the default print action to print the whole line. 

Awk Example 6. Print the list of employees in Technology department 
Now department name is available as a fourth field, so need to check if $4 matches with the string “Technology”, if yes print the line. 

# awk '$4 ~/Technology/' employee.txt
200 Jason Developer Technology $5,500
300 Sanjay Sysadmin Technology $7,000
500 Randy DBA Technology $6,000

Operator ~ is for comparing with the regular expressions. If it matches the default action i.e print whole line will be performed. 

Awk Example 7. Print number of employees in Technology department 
The below example, checks if the department is Technology, if it is yes, in the Action, just increment the count variable, which was initialized with zero in the BEGIN section. 

# awk 'BEGIN { count=0;}
> $4 ~ /Technology/ { count++; }
> END { print "Number of employees in Technology Dept =",count;}' employee.txt
Number of employees in Tehcnology Dept = 3

Then at the end of the process, just print the value of count which gives you the number of employees in Technology department. 

Recommended Reading 
Sed and Awk 101 Hacks, by Ramesh Natarajan. I spend several hours a day on UNIX / Linux environment dealing with text files (data, config, and log files). I use Sed and Awk for all my my text manipulation work. Based on my Sed and Awk experience, I’ve written Sed and Awk 101 Hacks eBook that contains 101 practical examples on various advanced features of Sed and Awk that will enhance your UNIX / Linux life. Even if you’ve been using Sed and Awk for several years and have not read this book, please do yourself a favor and read this book. You’ll be amazed with the capabilities of Sed and Awk utilities. 
 


Supplement 
* Awk User-defined Variables with 3 Practical Examples 
* 8 Powerful Awk Built-in Variables – FS, OFS, RS, ORS, NR, NF, FILENAME, FNR 
* 7 Powerful Awk Operators Examples (Unary, Binary, ...itional, Reg-Ex Awk Operators) 
* 4 Awk If Statement Examples ( if, if else, if else if, : ? ) 
* Caught In the Loop? Awk While, Do While, For Loop,...Break, Continue, Exit Examples 
* Awk Tutorial