求解释awk getline的用法

1. cat file
   
2. 1
   
3. 2
   
4. 3
   
5. 4
   
6. awk '{"echo ok" | getline; print}' file
   
7. ok
   
8. 2
   
9. 3
   
10. 4

1. ok
   
2. ok
   
3. ok
   
4. ok

1. awk 'BEGIN{while("echo ok" | getline) print ++i}'
   
2. 1

1. awk 'BEGIN{while("time" | getline)print ++i}'
   
2. real    0m0.000s
   
3. user    0m0.000s
   
4. sys     0m0.000s

1. $ awk 'BEGIN{while("ls -1" | getline)print ++i}'
   
2. 1
   
3. 2
   
4. 3
   
5. 4
   
6. 5
   
7. 6

QUOTE:

When the output of a command is piped to getline and it contains multiple lines, getline reads a
line at a time. The first time getline is called it reads the first line of output. If you call it again, it
reads the second line. To read all the lines of output, you must set up a loop that executes getline
until there is no more output. For instance, the following example uses a while loop to read each line
of output and assign it to the next element of the array, who_out:
while ("who" | getline)
who_out[++i] = $0
Each time the getline function is called, it reads the next line of output. The who command,
however, is executed only once.

1. [root@station3 ~]# awk 'BEGIN{while("(time;)2>&1" | getline)print ++i}'
   
2. 1
   
3. 2
   
4. 3
   
5. 4

1. [root@station3 ~]# /usr/bin/time -p pwd 2>&1 1>/dev/null
   
2. real 0.00
   
3. user 0.00
   
4. sys 0.00
   
5. [root@station3 ~]# awk 'BEGIN{while("/usr/bin/time -p pwd 2>&1 1>/dev/null" | getline)print ++i}'
   
6. 1
   
7. 2
   
8. 3