Use shell commands in the awk Program

The awk program allows calling shell commands and provides pipelines to solve the problem of data transmission between awk and the system. Therefore, awk is easy to use system resources. There are two main methods:

A. Syntax] awk output command | "commands accepted by shell" (for example, Print $1, $2 | "sort-K 1 ")

[B. Syntax] "commands accepted by shell" | awk input command (for example, "ls" | Getline)

Note:

The awk input command only has one Getline command.

The awk output Commands include print and printf.

In the syntax, the data output by awk will be transferred to the shell for processing by the Shell Command. in the preceding example, the data output by print is sorted by the Shell Command "sort-K 1" and then sent to the screen (stdout ).

In the previous awk program, "Print $1, $2" may be executed multiple times, and the output results will be saved in pipe first. When the program ends, "Sort-K 1 ".

Note the following two points:

1. No matter how many times Print $1 and $2 are executed, the execution time of "sort-K 1" is "awk program end time ",

2. "sort-K 1" is executed once ".

 

In the B syntax, awk will first call the shell command. the execution result is sent to the awk program through pipe. In the preceding example, awk first let the shell execute "ls". After the shell executes the command, it stores the result in pipe. The awk command Getline then reads data from pipe.

Note the following when using this syntax: in the preceding example, awk "calls shell to execute" ls "immediately, and the execution frequency is one time.

Getline may be executed multiple times (if pipe contains multiple rows of data ).

Example: sort the process PID

PS-Ef | awk '{print $2 | "sort-n "}'

Store the pid value of the awk result to pipe and send it to "sort-n" at the end of the program for sorting and output it to the standard output, in addition, you can call close ("sort-N") to close the pipe before "sort-N" and immediately submit "sort-N" to shell for execution.

Example: print the number of online users

Awk 'in in {While ("who" | Getline) n ++; print n }'

Print the current number of online users

Note: awk does not have to process files. Begin will be executed once before any input files are enabled. awk regards the string before the "|" pipeline symbol as a shell command, the command is sent to shell, and the execution result is sent to the awk program by pipe. The Getline is the input command provided by awk.

Syntax	Where to read data	After the data is read
Getline var <File	Specified file	VaR (if VaR is omitted, it indicates that it is placed in $0)
Getline VaR	Pipe variable	VaR (if VaR is omitted, it indicates that it is placed in $0)

When pattern is begin or end, Getline reads data from stdin. Otherwise, it reads data from the data file being processed by awk.

Getline reads a row of data at a time. If the read succeeds, return 1. If the read fails, return-1. If the file ends (EOF), return 0 (false)