Expanding awk -v variable

Question

When I run awk '/2017-12-05T12:07:33.941Z/{y=1;next}y' file.json it works as expected, printing everything after the timestamp.

I'm trying to follow the syntax in this Q/A but my syntax isn't expanding right:

awk -v last_log="$last_log" '/{print last_log}/{y=1;next}y' file.txt

Also trying:

awk -v last_log="$last_log" '/$0 ~ last_log/{y=1;next}y' file.txt

Example, given the following input, return all logs after last processed( 2017-12-05T12:07:33.941Z):

{ "name": "PeriodicWork", "hostname": "myHost", "pid": 12189, "level": 20, "msg": "Executing [CheckFailedTask NodeId=8]", "time": "2017-12-05T10:07:33.941Z", "v": 0 }

{ "name": "PeriodicWork", "hostname": "myHost", "pid": 12188, "level": 50, "msg": "Executing [CheckFailedTask NodeId=8]", "time": "2017-12-05T12:07:33.941Z", "v": 0 }

{ "name": "PeriodicWork", "hostname": "myHost", "pid": 12187, "level": 40, "msg": "Executing [CheckFailedTask NodeId=8]", "time": "2017-12-05T12:57:33.941Z", "v": 0 }

`/{print last_log}/` will not expand your variable... use `$0 ~ last_log` — Sundeep, Dec 05 '17 at 15:27
@Sundeep ok just tried it and it didn't work. If I remove the `/`s how will it expand into the original command I posted `awk '/2017-12-05T12:07:33.941Z/{y=1;next}y' file.json`? — Philip Kirkbride, Dec 05 '17 at 15:30
`$0 ~ last_log` will do the regex matching... give us sample input and output so that we can test... if you want proof, check `seq 18 | awk -v r='4' '$0 ~ r{f=2; next} f && f--'` — Sundeep, Dec 05 '17 at 15:32

Stéphane Chazelas · Accepted Answer · 2017-12-13T16:53:23.523

3

Here, I don't think it was your intention to treat 2017-12-05T12:07:33.941Z as a regexp, (where . would match any character instead of just .).

For a substring match, as opposed to a regexp match, you can use index():

LAST_LOG="$last_log" awk 'index($0, ENVIRON["LAST_LOG"]) {y=1;next};y' file.txt

I prefer ENVIRON over -v as -v mangles the content of the variable if it contains backslashes (as already noted in the Q&A you referenced).

About why yours fail:

/{print last_log}/

as a condition, matches on records that match the {print last_log} regular expression, but that's not a valid regular expression ({ is a regexp operator that needs to be used as {2} or {1,5}...).

In:

/$0 ~ last_log/

again, that tries to match on $0 ~ last_log as a regexp. Here that means on lines that contain 0 ~ last_log after the end of the line ($), so will never match. You probably meant:

awk -v last_log="$last_log" '$0 ~ last_log {y=1;next};y' file.txt

That is where the condition is the $0 ~ last_log expression as opposed to just one regexp /foo/. /foo/ is short for $0 ~ /foo/, that is match the foo regexp against the full record.

You can do $0 ~ var, but you can't do var alone, as it wouldn't be a regexp matching, but an expression that resolves to true if var contains a number other than 0, or a non-empty non-numerical string (like for your y one)

edited Dec 13 '17 at 16:53

answered Dec 05 '17 at 15:35

Stéphane Chazelas

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thanks, that answers my specific question. Off hand any idea how to stop my format from breaking the `awk` syntax? I'm getting `awk: cmd. line:1: LAST_LOG=2017-12-05T12:07:33.941Z awk: cmd. line:1: ^ syntax error` – Philip Kirkbride Dec 05 '17 at 15:39
In this case I'll just convert the time to a unix timestamp that'll make it easier to process anyways. – Philip Kirkbride Dec 05 '17 at 15:44
ah, I guess I can't convert to a timestamp without a lot of processing because I don't control producing the log files. – Philip Kirkbride Dec 05 '17 at 15:47
@Philip, see edit. – Stéphane Chazelas Dec 05 '17 at 15:51
Sorry my mistake it was working, I broke something else. Thanks! – Philip Kirkbride Dec 05 '17 at 15:55

JJoao · Answer 2 · 2017-12-05T16:36:24.910

0

With awk intervals

From last_log to eof can also be obtained by "," operator;

 awk  '$0 ~ last, 0'  last="$last_log" file.txt

With last_log values like the one presented

 awk "/$last_log/,0" file.txt

Probably we need to add | tail -n +1

With Sed:

sed  "1,/$last_log/d"  file.txt

edited Dec 05 '17 at 16:36

answered Dec 05 '17 at 15:50

JJoao

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That prints the first matching line though which it seems the OP wanted to skip. – Stéphane Chazelas Dec 05 '17 at 15:52
@StéphaneChazelas, your are right. ` | tail -n +2` ? – JJoao Dec 05 '17 at 15:59
1

Or `awk 'NR==1, $0 ~ last{next}; 1'` though that wouldn't be simpler than the OP's `$0 ~ last {y=1;next};y` – Stéphane Chazelas Dec 05 '17 at 16:02
@StéphaneChazelas `awk "1,/$last_log/{next} 1" ` – JJoao Dec 05 '17 at 16:07
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Those are best avoided as it's a command injection vulnerability if the content of the variable is not under your control. It's just as dangerous as passing variable data to `eval`. – Stéphane Chazelas Dec 05 '17 at 16:08
@StéphaneChazelas, right again. The same problem appear with the other uses of `"$last_log"` – JJoao Dec 05 '17 at 16:15
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In the other cases (`LAST_LOG="$last_log" awk...` or `awk -v last_log="$last_log"`), except for bugs in awk implementations, you wouldn't get a command injection vulnerability. You could get a DoS in the regexp case for values of regexps that are very expensive like a `(x*){5000}` that eats all your memory and time of one CPU with GNU awk, but normally not arbitrary command execution. – Stéphane Chazelas Dec 05 '17 at 16:34
@StéphaneChazelas, thank you. (In this situation this does not constitutes a problem, but in other situations it may be dangerous) – JJoao Dec 05 '17 at 16:42

Expanding awk -v variable

2 Answers2

With awk intervals

With Sed: