Subcontractors – Bc, Xargs, Find

Subcontractors – bc , xargs , find David Morgan © David Morgan 2011- 14 bc – math help for the shell interactive or programatic can accept its commands from stdin can accept an entire bc program’s worth © David Morgan 2011-14 1 bc – math help for shell bc, commanded interactively by user does what shell cannot e.g. high-precision math ( & much more ) bc, commanded non-interactively by shell shell, by extension, does high-precision can embed bc code wholesale as here document © David Morgan 2011-14 bc – shell script or bc interpreter script © David Morgan 2011-14 2 bc – math help for shell bc code, not bash shell code background: http://jeremykun.com/tag/simpsons-rule http://www.zweigmedia.com/RealWorld/integral/integral.html © David Morgan correct! 2011-14 bc – interactive math help for shell Wicked Cool Shell Scripts , Dave Taylor, p29 © David Morgan 2011-14 3 xargs produce list of vi’s backup files long list ‘em with xargs remove ‘em with xargs gone © David Morgan 2011-14 xargs filename too long to type command to avoid typing it what’s in the file? …oops cat needs filename as argument, not input xargs composes correct cat syntax by pasting its input onto cat’s command line produces: cat /root/class… etc etc produces: cat /etc/resolv.conf /root/class… etc etc xargs composes incorrect cp syntax by pasting its input onto cp’s command line produces: cp . /root/class… etc etc cp differs from cat, needs insertion from xargs not pasting control argument placement with xargs’ –I (xargs stuff goes where the special© char David appears, _ in this case) Morgan 2011-14 4 xargs and efficiency sleeptime totals 15 sec run sleeps simultaneously took 15 sec took 5 sec significant for compute-heavy commands distributes them across multi-cores – simultaneous execution © David Morgan 2011-14 xargs and efficiency commonly used with find to process files functionally equivalent – find . -name "*" -exec rm {} \; – find . -name "*" | xargs rm computationally different – -exec -rm creates one process per file – xargs rm creates one process per group of files © David Morgan 2011-14 5 xargs and efficiency 12 times longer to delete 50 files © David Morgan 2011-14 xargs spaces in filenames space and newline are delimiters to xargs find output-delimits with 0a but can use 00 instead of 0a and xargs can input-delimit on 00 instead of 20 (space) © David Morgan 2011-14 6 find searches for files in a directory tree described by an expression expression consists of elements – options – tests – actions each element returns boolean result find evaluates as many elements of its expression as needed to know expression’s outcome © David Morgan 2011-14 Most common use for <all files in a set of files> if <something about the file> do <something with the file> next but the operation details are more complex than that © David Morgan 2011-14 7 find example expression find . -maxdepth 1 -size +1000000c -print an optiona test an action find files 1) in the current directory (no subdirectory search) 2) bigger than a million bytes © David 3) and print their names Morgan 2011-14 Some example elements find . options* tests actions maxdepth name print mount atime+n ls etc size +n exec executable ok type etc empty false etc © David * find’s options, not shell command options Morgan 2011-14 8 Some example elements find . options tests actions what it returns: true always true or false true or false what it does: influence nothing their particular find behavior action © David Morgan 2011-14 Operational logic "[evaluates] the given expression from left to right... until the outcome is known (the left hand side is false for and operations, true for or ), at which point find moves on to the next file name.“ - “find” man page © David Morgan 2011-14 9 Operational logic because -name “A*” is false for B* files because printing happens before -name “A*” evaluation from the 2 nd -print (2 nd print doesn’t happen for B* files) © David Morgan 2011-14 exec action – arbitrary response for qualifying files needs to be terminated with ; uses {} as placeholder for current file need to escape these from shell a “finder” script command: find . -type f –exec grep –l “$1” {} \; print names of all files in current directory containing a given string © David Morgan 2011-14 10.

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