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chapter: Pathames
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‎pages/parsing_pathnames.poly.pm

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#lang pollen
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◊page-init{}
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◊define-meta[page-title]{Parsing Pathnames}
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◊define-meta[page-description]{Parsing and Managing Pathnames}
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This is an example of parsing and transforming filenames and, in
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particular, pathnames. It draws heavily on the functionality of
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◊command{sed}.
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◊example{
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#!/usr/bin/env bash
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#-----------------------------------------------------------
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# Management of PATH, LD_LIBRARY_PATH, MANPATH variables...
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# By Emmanuel Rouat <no-email>
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# (Inspired by the bash documentation 'pathfuncs' and on
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# discussions found on stackoverflow:
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# http://stackoverflow.com/questions/370047/
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# http://stackoverflow.com/questions/273909/#346860 )
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# Last modified: Sat Sep 22 12:01:55 CEST 2012
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#
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# The following functions handle spaces correctly.
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# These functions belong in .bash_profile rather than in
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# .bashrc, I guess.
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#
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# The modular aspect of these functions should make it easy
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# to expand them to handle path substitutions instead
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# of path removal etc....
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#
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# See http://www.catonmat.net/blog/awk-one-liners-explained-part-two/
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# (item 43) for an explanation of the 'duplicate-entries' removal
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# (it's a nice trick!)
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#-----------------------------------------------------------
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# Show $@ (usually PATH) as list.
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function p_show() { local p="$@" && for p; do [[ ${!p} ]] &&
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echo -e ${!p//:/\\n}; done }
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# Filter out empty lines, multiple/trailing slashes, and duplicate entries.
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function p_filter()
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{ awk '/^[ \t]*$/ {next} {sub(/\/+$/, "");gsub(/\/+/, "/")}!x[$0]++' ;}
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# Rebuild list of items into ':' separated word (PATH-like).
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function p_build() { paste -sd: ;}
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# Clean $1 (typically PATH) and rebuild it
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function p_clean()
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{ local p=${1} && eval ${p}='$(p_show ${p} | p_filter | p_build)' ;}
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# Remove $1 from $2 (found on stackoverflow, with modifications).
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function p_rm()
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{ local d=$(echo $1 | p_filter) p=${2} &&
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eval ${p}='$(p_show ${p} | p_filter | grep -xv "${d}" | p_build)' ;}
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# Same as previous, but filters on a pattern (dangerous...
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#+ don't use 'bin' or '/' as pattern!).
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function p_rmpat()
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{ local d=$(echo $1 | p_filter) p=${2} && eval ${p}='$(p_show ${p} |
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p_filter | grep -v "${d}" | p_build)' ;}
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# Delete $1 from $2 and append it cleanly.
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function p_append()
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{ local d=$(echo $1 | p_filter) p=${2} && p_rm "${d}" ${p} &&
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eval ${p}='$(p_show ${p} d | p_build)' ;}
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# Delete $1 from $2 and prepend it cleanly.
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function p_prepend()
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{ local d=$(echo $1 | p_filter) p=${2} && p_rm "${d}" ${p} &&
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eval ${p}='$(p_show d ${p} | p_build)' ;}
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# Some tests:
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echo
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MYPATH="/bin:/usr/bin/:/bin://bin/"
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p_append "/project//my project/bin" MYPATH
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echo "Append '/project//my project/bin' to '/bin:/usr/bin/:/bin://bin/'"
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echo "(result should be: /bin:/usr/bin:/project/my project/bin)"
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echo $MYPATH
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echo
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MYOTHERPATH="/bin:/usr/bin/:/bin:/project//my project/bin"
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p_prepend "/project//my project/bin" MYOTHERPATH
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echo "Prepend '/project//my project/bin' \
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to '/bin:/usr/bin/:/bin:/project//my project/bin/'"
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echo "(result should be: /project/my project/bin:/bin:/usr/bin)"
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echo $MYOTHERPATH
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echo
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p_prepend "/project//my project/bin" FOOPATH # FOOPATH doesn't exist.
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echo "Prepend '/project//my project/bin' to an unset variable"
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echo "(result should be: /project/my project/bin)"
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echo $FOOPATH
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echo
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BARPATH="/a:/b/://b c://a:/my local pub"
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p_clean BARPATH
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echo "Clean BARPATH='/a:/b/://b c://a:/my local pub'"
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echo "(result should be: /a:/b:/b c:/my local pub)"
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echo $BARPATH
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}
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How can you process filenames correctly in shell?
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◊example{
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Doing it correctly: A quick summary
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by David Wheeler
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http://www.dwheeler.com/essays/filenames-in-shell.html
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So, how can you process filenames correctly in shell? Here's a quick
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summary about how to do it correctly, for the impatient who "just want the
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answer". In short: Double-quote to use "$variable" instead of $variable,
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set IFS to just newline and tab, prefix all globs/filenames so they cannot
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begin with "-" when expanded, and use one of a few templates that work
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correctly. Here are some of those templates that work correctly:
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IFS="$(printf '\n\t')"
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# Remove SPACE, so filenames with spaces work well.
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# Correct glob use:
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#+ always use "for" loop, prefix glob, check for existence:
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for file in ./* ; do # Use "./*" ... NEVER bare "*" ...
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if [ -e "$file" ] ; then # Make sure it isn't an empty match.
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COMMAND ... "$file" ...
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fi
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done
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# Correct glob use, but requires nonstandard bash extension.
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shopt -s nullglob # Bash extension,
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#+ so that empty glob matches will work.
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for file in ./* ; do # Use "./*", NEVER bare "*"
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COMMAND ... "$file" ...
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done
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# These handle all filenames correctly;
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#+ can be unwieldy if COMMAND is large:
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find ... -exec COMMAND... {} \;
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find ... -exec COMMAND... {} \+ # If multiple files are okay for COMMAND.
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# This skips filenames with control characters
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#+ (including tab and newline).
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IFS="$(printf '\n\t')"
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controlchars="$(printf '*[\001-\037\177]*')"
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for file in $(find . ! -name "$controlchars"') ; do
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COMMAND "$file" ...
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done
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# Okay if filenames can't contain tabs or newlines --
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#+ beware the assumption.
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IFS="$(printf '\n\t')"
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for file in $(find .) ; do
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COMMAND "$file" ...
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done
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# Requires nonstandard but common extensions in find and xargs:
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find . -print0 | xargs -0 COMMAND
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# Requires nonstandard extensions to find and to shell (bash works).
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# variables might not stay set once the loop ends:
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find . -print0 | while IFS="" read -r -d "" file ; do ...
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COMMAND "$file" # Use quoted "$file", not $file, everywhere.
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done
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# Requires nonstandard extensions to find and to shell (bash works).
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# Underlying system must include named pipes (FIFOs)
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#+ or the /dev/fd mechanism.
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# In this version, variables *do* stay set after the loop ends,
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# and you can read from stdin.
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#+ (Change the 4 to another number if fd 4 is needed.)
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while IFS="" read -r -d "" file <&4 ; do
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COMMAND "$file" # Use quoted "$file" -- not $file, everywhere.
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done 4< <(find . -print0)
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# Named pipe version.
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# Requires nonstandard extensions to find and to shell's read (bash ok).
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# Underlying system must include named pipes (FIFOs).
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# Again, in this version, variables *do* stay set after the loop ends,
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# and you can read from stdin.
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# (Change the 4 to something else if fd 4 needed).
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mkfifo mypipe
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find . -print0 > mypipe &
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while IFS="" read -r -d "" file <&4 ; do
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COMMAND "$file" # Use quoted "$file", not $file, everywhere.
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done 4< mypipe
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}

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