Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting
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# Appendix D. Parsing and Managing Pathnames

Emmanual Rouat contributed the following example of parsing and transforming filenames and, in particular, pathnames. It draws heavily on the functionality of sed.

       1 #!/usr/bin/env bash
       2 #-----------------------------------------------------------
       3 # Management of PATH, LD_LIBRARY_PATH, MANPATH variables...
       4 # By Emmanuel Rouat <no-email>
       5 # (Inspired by the bash documentation 'pathfuncs' and on
       6 # discussions found on stackoverflow:
       7 # http://stackoverflow.com/questions/370047/
       8 # http://stackoverflow.com/questions/273909/#346860 )
       9 # Last modified: Sat Sep 22 12:01:55 CEST 2012
      10 #
      11 # The following functions handle spaces correctly.
      12 # These functions belong in .bash_profile rather than in
      13 # .bashrc, I guess.
      14 #
      15 # The modular aspect of these functions should make it easy
      16 # to expand them to handle path substitutions instead
      17 # of path removal etc....
      18 #
      19 # See http://www.catonmat.net/blog/awk-one-liners-explained-part-two/
      20 # (item 43) for an explanation of the 'duplicate-entries' removal
      21 # (it's a nice trick!)
      22 #-----------------------------------------------------------
      23
      24 # Show $@ (usually PATH) as list.
      25 function p_show() { local p="$@" && for p; do [[ ${!p} ]] &&
      26 echo -e ${!p//:/\\n}; done }
      27
      28 # Filter out empty lines, multiple/trailing slashes, and duplicate entries.
      29 function p_filter()
      30 { awk '/^[ \t]*$/ {next} {sub(/\/+$/, "");gsub(/\/+/, "/")}!x[$0]++' ;}
      31
      32 # Rebuild list of items into ':' separated word (PATH-like).
      33 function p_build() { paste -sd: ;}
      34
      35 # Clean $1 (typically PATH) and rebuild it
      36 function p_clean()
      37 { local p=${1} && eval ${p}='$(p_show ${p} | p_filter | p_build)' ;}
      38
      39 # Remove $1 from $2 (found on stackoverflow, with modifications).
      40 function p_rm()
      41 { local d=$(echo $1 | p_filter) p=${2} &&
      42   eval ${p}='$(p_show ${p} | p_filter | grep -xv "${d}" | p_build)' ;}
      43
      44 #  Same as previous, but filters on a pattern (dangerous...
      45 #+ don't use 'bin' or '/' as pattern!).
      46 function p_rmpat()
      47 { local d=$(echo $1 | p_filter) p=${2} && eval ${p}='$(p_show ${p} |
      48   p_filter | grep -v "${d}" | p_build)' ;}
      49
      50 # Delete $1 from $2 and append it cleanly.
      51 function p_append()
      52 { local d=$(echo $1 | p_filter) p=${2} && p_rm "${d}" ${p} &&
      53   eval ${p}='$(p_show ${p} d | p_build)' ;}
      54
      55 # Delete $1 from $2 and prepend it cleanly.
      56 function p_prepend()
      57 { local d=$(echo $1 | p_filter) p=${2} && p_rm "${d}" ${p} &&
      58   eval ${p}='$(p_show d ${p} | p_build)' ;}
      59
      60 # Some tests:
      61 echo
      62 MYPATH="/bin:/usr/bin/:/bin://bin/"
      63 p_append "/project//my project/bin" MYPATH
      64 echo "Append '/project//my project/bin' to '/bin:/usr/bin/:/bin://bin/'"
      65 echo "(result should be: /bin:/usr/bin:/project/my project/bin)"
      66 echo $MYPATH
      67
      68 echo
      69 MYOTHERPATH="/bin:/usr/bin/:/bin:/project//my project/bin"
      70 p_prepend "/project//my project/bin" MYOTHERPATH
      71 echo "Prepend '/project//my project/bin' \
      72 to '/bin:/usr/bin/:/bin:/project//my project/bin/'"
      73 echo "(result should be: /project/my project/bin:/bin:/usr/bin)"
      74 echo $MYOTHERPATH
      75
      76 echo
      77 p_prepend "/project//my project/bin" FOOPATH  # FOOPATH doesn't exist.
      78 echo "Prepend '/project//my project/bin' to an unset variable"
      79 echo "(result should be: /project/my project/bin)"
      80 echo $FOOPATH
      81
      82 echo
      83 BARPATH="/a:/b/://b c://a:/my local pub"
      84 p_clean BARPATH
      85 echo "Clean BARPATH='/a:/b/://b c://a:/my local pub'"
      86 echo "(result should be: /a:/b:/b c:/my local pub)"
      87 echo $BARPATH

***

David Wheeler kindly permitted me to use his instructive examples.

       1 Doing it correctly: A quick summary
       2 by David Wheeler
       3 http://www.dwheeler.com/essays/filenames-in-shell.html
       4
       5 So, how can you process filenames correctly in shell? Here's a quick
       6 summary about how to do it correctly, for the impatient who "just want the
       7 answer". In short: Double-quote to use "$variable" instead of $variable,
       8 set IFS to just newline and tab, prefix all globs/filenames so they cannot
       9 begin with "-" when expanded, and use one of a few templates that work
      10 correctly. Here are some of those templates that work correctly:
      11
      12
      13  IFS="$(printf '\n\t')"
      14  # Remove SPACE, so filenames with spaces work well.
      15
      16  #  Correct glob use:
      17  #+ always use "for" loop, prefix glob, check for existence:
      18  for file in ./* ; do          # Use "./*" ... NEVER bare "*" ...
      19    if [ -e "$file" ] ; then    # Make sure it isn't an empty match.
      20      COMMAND ... "$file" ...
      21    fi
      22  done
      23
      24
      25
      26  # Correct glob use, but requires nonstandard bash extension.
      27  shopt -s nullglob  #  Bash extension,
      28                     #+ so that empty glob matches will work.
      29  for file in ./* ; do        # Use "./*", NEVER bare "*"
      30    COMMAND ... "$file" ...
      31  done
      32
      33
      34
      35  #  These handle all filenames correctly;
      36  #+ can be unwieldy if COMMAND is large:
      37  find ... -exec COMMAND... {} \;
      38  find ... -exec COMMAND... {} \+ # If multiple files are okay for COMMAND.
      39
      40
      41
      42  #  This skips filenames with control characters
      43  #+ (including tab and newline).
      44  IFS="$(printf '\n\t')"
      45  controlchars="$(printf '*[\001-\037\177]*')"
      46  for file in $(find . ! -name "$controlchars"') ; do
      47    COMMAND "$file" ...
      48  done
      49
      50
      51
      52  #  Okay if filenames can't contain tabs or newlines --
      53  #+ beware the assumption.
      54  IFS="$(printf '\n\t')"
      55  for file in $(find .) ; do
      56    COMMAND "$file" ...
      57  done
      58
      59
      60
      61  # Requires nonstandard but common extensions in find and xargs:
      62  find . -print0 | xargs -0 COMMAND
      63
      64  # Requires nonstandard extensions to find and to shell (bash works).
      65  # variables might not stay set once the loop ends:
      66  find . -print0 | while IFS="" read -r -d "" file ; do ...
      67    COMMAND "$file" # Use quoted "$file", not $file, everywhere.
      68  done
      69
      70
      71
      72  #  Requires nonstandard extensions to find and to shell (bash works).
      73  #  Underlying system must include named pipes (FIFOs)
      74  #+ or the /dev/fd mechanism.
      75  #  In this version, variables *do* stay set after the loop ends,
      76  #  and you can read from stdin.
      77  #+ (Change the 4 to another number if fd 4 is needed.)
      78
      79  while IFS="" read -r -d "" file <&4 ; do
      80    COMMAND "$file"   # Use quoted "$file" -- not $file, everywhere.
      81  done 4< <(find . -print0)
      82
      83
      84  #  Named pipe version.
      85  #  Requires nonstandard extensions to find and to shell's read (bash ok).
      86  #  Underlying system must include named pipes (FIFOs).
      87  #  Again, in this version, variables *do* stay set after the loop ends,
      88  #  and you can read from stdin.
      89  # (Change the 4 to something else if fd 4 needed).
      90
      91  mkfifo mypipe
      92
      93  find . -print0 > mypipe &
      94  while IFS="" read -r -d "" file <&4 ; do
      95    COMMAND "$file" # Use quoted "$file", not $file, everywhere.
      96  done 4< mypipe

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