AWK getline示例，请解释输出？

Question

I found this awk example with getline but not able to understand how it works.

seq 5 | awk 'BEGIN { getline; print "Read ahead first line", $0 } {print $0 }'

The output is:

Read ahead first line 1
2
3
4
5

Answer 1

seq 5 | awk 'BEGIN { getline; print "Read ahead first line", $0 } {print $0 }'

seq 5 you will get

$ seq 5
1
2
3
4
5

and from BEGIN { getline; print "Read ahead first line", $0 } BEGIN { getline; print "Read ahead first line", $0 } , awk will read first record from stdin thats is 1

Through pipe seq 5 | awk '{...}' seq 5 | awk '{...}' ( first program | second program )
Pipes are used to redirect a stream from one program to another. When a program's standard output is sent to another through a pipe

$ seq 5 | awk 'BEGIN { getline; print "Read ahead first line", $0 }'
Read ahead first line 1

and {print $0 } after BEGIN block will read remaining records that is 2 to 5, here is how without printing inside BEGIN block

$ seq 5 | awk 'BEGIN { getline} {print $0}'
2
3
4
5

Answer 2

seq 5 produces

1
2
3
4
5

awk 's BEGIN block is executed before the main block.
getline reads a line, which is printed in the BEGIN block as $0 .
Then the main block reads the remaining lines.

$ seq 5 | awk 'BEGIN { getline; print "BEGIN read", $0} {print "main read", $0 }'
BEGIN read 1
main read 2
main read 3
main read 4
main read 5