Wednesday, August 12, 2015

use getline to capture system command output in awk

I just learnt this today: awk also has its pipe (|) and getline, just like unix. If I want to call a system command in awk and capture its output (Note: system() won't work as it only return the exit status), I can use pipe the output to getline. 

For example,

$cat > test.txt
aa bb cc
11 22 33
44 55 cc

$awk 'BEGIN{cmd="grep cc test.txt | sort -k1,1 | head -n1 | cut -f2 -d\" \""; cmd | getline a; print a}'


$awk 'BEGIN{cmd="grep cc test.txt | sort -k1,1 | head -n1 | cut -f2 -d\" \""; system(cmd);}'

Note that if only use cmd, it won't print out anything, because cmd itself won't switch to console (unlike system). 

$awk 'BEGIN{cmd="grep cc test.txt | sort -k1,1 | head -n1 | cut -f2 -d\" \""; cmd;}'

If you want to output the multiple lines and process them in awk, you can do

$awk 'BEGIN{cmd="grep cc test.txt | sort -k1,1 | cut -f2 -d\" \""; while( (cmd | getline a) >0) print a;}'


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