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getline
from a PipeOmniscience has much to recommend it. Failing that, attention to details would be useful.
The output of a command can also be piped into getline
, using
‘command | getline’. In
this case, the string command is run as a shell command and its output
is piped into awk
to be used as input. This form of getline
reads one record at a time from the pipe.
For example, the following program copies its input to its output, except for
lines that begin with ‘@execute’, which are replaced by the output
produced by running the rest of the line as a shell command:
{ if ($1 == "@execute") { tmp = substr($0, 10) # Remove "@execute" while ((tmp | getline) > 0) print close(tmp) } else print }
The close()
function is called to ensure that if two identical
‘@execute’ lines appear in the input, the command is run for
each one.
See section Closing Input and Output Redirections.
Given the input:
foo bar baz @execute who bletch
the program might produce:
foo bar baz arnold ttyv0 Jul 13 14:22 miriam ttyp0 Jul 13 14:23 (murphy:0) bill ttyp1 Jul 13 14:23 (murphy:0) bletch
Notice that this program ran the command who
and printed the result.
(If you try this program yourself, you will of course get different results,
depending upon who is logged in on your system.)
This variation of getline
splits the record into fields, sets the
value of NF
, and recomputes the value of $0
. The values of
NR
and FNR
are not changed.
RT
is set.
According to POSIX, ‘expression | getline’ is ambiguous if
expression contains unparenthesized operators other than
‘$’—for example, ‘"echo " "date" | getline’ is ambiguous
because the concatenation operator is not parenthesized. You should
write it as ‘("echo " "date") | getline’ if you want your program
to be portable to all awk
implementations.
NOTE: Unfortunately,
gawk
has not been consistent in its treatment of a construct like ‘"echo " "date" | getline’. Most versions, including the current version, treat it as ‘("echo " "date") | getline’. (This is also how BWKawk
behaves.) Some versions instead treat it as ‘"echo " ("date" | getline)’. (This is howmawk
behaves.) In short, always use explicit parentheses, and then you won’t have to worry.
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