Subcontractors – Bc, Xargs, Find

Subcontractors – bc , xargs , find

David Morgan

bc – math help for the shell

interactive or programatic can accept its commands from stdin can accept an entire bc program’s worth

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

bc – shell script or bc interpreter script

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

3 xargs

produce list of vi’s backup files

long list ‘em with xargs

remove ‘em with xargs gone

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

5 xargs and efficiency

12 times longer to delete 50 files

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)

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

if do 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

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

9 Operational logic

because -name “A*” is false for B* files

because printing happens before -name “A*” evaluation

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” {} \;