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"Just sit right back and you'll hear a tale
a tale of a fateful trip,
that started from this simple term,
aboard this single slip."

Please, dear reader, learn from my mistake. (It will likely be less painful than repeating it yourself.)

Earlier today I did something I have done dozens of times before-created a command-line perl script (a "one-liner", for some very long definition of a "line") to create a file containing a subset of messages from a set of compressed logs, with the intent of pulling them back to my machine for additional processing. I launched it in a screen session, saw it was running and would take some time to begin generating output, and stepped away for lunch. I was rather surprised, however, when I returned to find it throwing "No space left on device" messages-especially when I looked to find that the partition's previous 40GB of free space was now zero. After cleaning up the space, I began trying to find the cause.

In digging through my code, I almost missed the issue-a line of the form if ( $str -~ m/some pattern/ ) { print $str; }. Notice the issue? An off-by-one-key error from a en.US keyboard, where [-/_] and [=/+] are beside one another. This was compounded by skipping the well-espoused logic of use strict; use warnings; because it was a "one-liner" (where "-Mstrict -Mwarnings" would have added a mere 19 characters in length). Had I have done so, instead of:

# made-up example with actual issue $ perl -le 'my $str = q{zxcv}; if ( $str -~ m/some pattern/i ) { print + $str; }' zxcv
I would have seen:
# made-up example with actual issue, with strict and warnings $ perl -Mstrict -Mwarnings -le 'my $str = q{zxcv}; if ( $str -~ m/some + pattern/i ) { print $str; }' Use of uninitialized value $_ in pattern match (m//) at -e line 1. Argument "zxcv" isn't numeric in subtraction (-) at -e line 1. zxcv
Seeing unexpected output like that (that early) would likely have resulted in my cancelling the command to investigate, and thus likely not filling up 40GB of storage.

I know this will probably be like the tone after the television broadcast day ended to a sleeping viewer, but maybe, just maybe, it will help someone. In the end, learning from others' mistakes is often less painful (but not necessarily as memorable).

(And yes, I do use the the duo on most every script I write-just not always on "one-liners".)

In reply to Off by one key... or yet another example of why strict and warnings are always a good idea (even for "one-liners") by atcroft

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