Awk from entry to earth (14) awk output redirection

up to now , our AWK Programs output results to standard output On .

although AWK The program did not open the file / Read and write files and other corresponding functions , but AWK The program also supports Redirect operation , That is, it supports the output of data to file On .

AWK The redirection operation of usually occurs in print or printf Statement behind .

AWK Redirection and shell The redirection in the command is similar , It's just that they're written in AWK In the program .

Redirection operator >

Redirection operator > Is to put AWK The output of the program is redirected to a file .

But here's the thing ： Redirection operator > The original contents of the file will be deleted first, and then the data will be written , That is to say, the original content no longer exists .

Redirection operator > The syntax format of the operator is as follows

print DATA > output-file

Redirection operator > Will be able to print The output result of is written to a file output-file in

If output-file non-existent , Redirection operator > The file will be created first .

Redirection operator > The contents of the file will be deleted first, and then output , however , For the same AWK Subsequent in the program Redirection operator > Then the original content will not be deleted , Instead, add .

in other words , Redirection operator > When you open a file, you will delete the content first . hinder Redirection operator > Because the file has been opened , No more open operations , The original content will not be deleted

Example 1

First we use Redirection operator > Create a file and write data

[www.twle.cn]$ echo " Old data ： Welcome to the simple tutorial " > /tmp/message.txt
[www.twle.cn]$ cat /tmp/message.txt

Run the top awk command , The output is as follows

 Old data ： Welcome to the simple tutorial 

next , We'll use it again Redirection operator > Write some data into the file , Look at the results

Example 2

[www.twle.cn]$ awk 'BEGIN { print " The new data ： Welcome to the simple tutorial " > "/tmp/message.txt" }'
[www.twle.cn]$ cat /tmp/message.txt

Run the top awk command , The output is as follows

 The new data ： Welcome to the simple tutorial 

Data append operator >>

Data append operation is to output the output to the end of the file , Instead of overwriting the original content .

Data append operator >> The syntax of is as follows

print DATA >> output-file

Data append operator >> Will be able to print Output result of Additional To output-file End of file , The original data is still .

If output-file file does not exist , Then the file will be created first, and then the data will be entered .

Example

We use Redirection operator > Create a file and write some data

[www.twle.cn]$ echo " Old data ： Welcome to the simple tutorial " > /tmp/hello.txt 
[www.twle.cn]$ cat /tmp/hello.txt

Run the top awk command , The output is as follows

 Old data ： Welcome to the simple tutorial 

So let's use Data append operator >> Go to the one just now hello.txt Add some data to the file

Example 2

[www.twle.cn]$ awk 'BEGIN { print " The new data ： Welcome to the simple tutorial " >> "/tmp/hello.txt" }'
[www.twle.cn]$ cat /tmp/hello.txt

Run the top awk command , The output is as follows

 Old data ： Welcome to the simple tutorial 
 The new data ： Welcome to the simple tutorial 

The Conduit |

AWK It also supports pipes and pipe operators |.

That is, we can use pipes instead of files to send the output to another program .

Pipe operators | Will open a pipe ,AWK Write the output result to another process through this pipeline to execute the command .

The pipeline redirection parameter command is actually AWK expression .

AWK Middle pipe | The syntax of is as follows

print items | command

Example

below , We use pipes | Redirect output to external tr command

[www.twle.cn]$ awk 'BEGIN { print "hello, world !!!" | "tr [a-z] [A-Z]" }'

Run the top awk command , The output is as follows

HELLO, WORLD !!!

Two way communication pipeline |&

AWK have access to |& Communicate with external processes .

|& Will open a two-way communication pipeline ,AWK The program can read and write to the open pipeline , External programs can also read and write to the open pipeline .

AWK in |& The syntax format of the two-way pipeline is as follows

print items |& command

Example

BEGIN {
   cmd = "tr [a-z] [A-Z]"
   print "hello, world !!!" |& cmd
   close(cmd, "to")

   cmd |& getline out
   print out;
   close(cmd);
}

Run the top awk command , The output is as follows

HELLO, WORLD !!!

Does the above program look confused ？？

This is probably the most complicated program we have ever seen , Now let's disassemble it step by step !!!

1. first line cmd = "tr [a-z] [A-Z] Defines a system command , We will use this command to open a Two way communication pipeline

2. The second line print "hello, world !!!" |& cmd .

   Let's see first |& cmd , This sentence means to use |& The output redirection operator opens a Two way communication pipeline Connect current AWK The program and the command just defined .

   then print "hello, world !!!" |& cmd Is to put print The output of the function is redirected to the just defined Two way communication pipeline .

   To be frank , Namely print Function is tr The command provides input .

3. The third line close(cmd, "to") Used to close the just opened Two way communication pipeline Input process of .

4. In the fourth row cmd |& getline out hold tr The execution result of the command uses the pipeline operator &| Redirect to getline function .getline The function stores the result in out variable .

5. The last line close(cmd) Used to close the just opened Two way communication pipeline