Incrementing the New File Version Number Post: 302820843

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Top Forums UNIX for Dummies Questions & Answers Incrementing the New File Version Number Post 302820843 by Don Cragun on Thursday 13th of June 2013 02:09:31 PM

06-13-2013

Registered User

Quote:

Originally Posted by DGPickett

Fear of pipes can and should be overcome! But some tools like basename are silly, dated, when you can just ${full_path##*/} and save a fork() and exec().

I'm not sure if this comment was directed at my suggestion to eliminate the ls|head pipeline. For the record, I am not on a mission to get rid of pipelines. And, when speed isn't a critical issue in a shell script, I sometimes use a pipeline (even when avoiding it could produce faster code) if the pipeline is "easier to read and understand."

But, when globbing could yield a fork error due to ARG_MAX limits, and an ls -t | head -1 pipeline would sometimes yield the wrong file and result in the destruction of data, I much prefer a different solution that avoids both issues.

Note also that the basename and dirname utilities aren't quite as easy to replace as you're suggesting. The commands:

Code:

ddir=$(dirname "$1")
sdir=${1%/*}
bbase=$(basename "$1")
sbase=${1##*/}
printf "directory containing \"%s\" is \"%s\" or \"%s\"\n" "$1" "$ddir" "$sdir"
printf "last component of \"%s\" is \"%s\" or \"%s\"\n" "$1" "$bbase" "$sbase"

show that they produce identical results when this script is invoked with the name of my home directory:

Code:

directory containing "/Users/dwc" is "/Users" or "/Users"
last component of "/Users/dwc" is "dwc" or "dwc"

but, very different results if I invoke the same script with / as an operand:

Code:

directory containing "/" is "/" or ""
last component of "/" is "/" or ""

I can certainly modify the script above to check for the special case when the operand is root; but I will frequently use basename and dirname because I don't have to think about the special case (even though a fork and exec is involved). And, what is going on will be more obvious to naive readers of my code. If this is done once during the initialization of a long running script, I might not care about the small additional start up costs. However, if this were to be done every time in a tight loop processing thousands of files that might include "/", it would be a different issue.

Don Cragun

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