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#Bash + GNU utilities, 204

fold -$1|sed 2~2{s/.\\+/printf\ %$1's "`echo "&"|rev`"/e y/'`printf %s {A..Z} {a..z}`"&_?!\"'.,/∀qƆpƎℲפHIſʞ˥WNOԀQɹS┴∩ΛMX⅄Zɐqɔpǝɟƃɥᴉɾʞlɯuodbɹsʇnʌʍxʎz⅋‾¿¡„,˙'/ }" 

N is given on the command line and S is given via STDIN:

$ echo "The quick brown fox jumps over the lazy dog." | ./boustrophedon.sh 30 The quick brown fox jumps over ˙ƃop ʎzɐl ǝɥʇ $ 

###Explanation

• fold -N splits the input into lines of length N.
• The rest of the processing is done by sed, line-by-line:
  • 2~2 matches every other line, starting at line 2
  • s/.+/printf %'N's "`echo "&"|rev`"/e uses GNU Sed's exec feature to call a shell to reverse the line and left-pad it with up to N spaces
  • y/ABC.../∀qƆ.../ transform characters

Note ABC... is generated using a bash expansion and printf. Also some fancy quoting for all the different characters.

#Bash + GNU utilities, 204

fold -$1|sed 2~2{s/.\\+/printf\ %$1's "`echo "&"|rev`"/e y/'`printf %s {A..Z} {a..z}`"&_?!\"'.,/∀qƆpƎℲפHIſʞ˥WNOԀQɹS┴∩ΛMX⅄Zɐqɔpǝɟƃɥᴉɾʞlɯuodbɹsʇnʌʍxʎz⅋‾¿¡„,˙'/ }" 

N is given on the command line and S is given via STDIN:

$ echo "The quick brown fox jumps over the lazy dog." | ./boustrophedon.sh 30 The quick brown fox jumps over ˙ƃop ʎzɐl ǝɥʇ $ 

###Explanation

• fold -N splits the input into lines of length N.
• The rest of the processing is done by sed, line-by-line:
  • 2~2 matches every other line, starting at line 2
  • s/.+/printf %'N's "`echo "&"|rev`"/e uses GNU Sed's exec feature to call a shell to reverse the line and left-pad it with up to N spaces
  • y/ABC.../∀qƆ.../ transform characters

Note ABC... is generated using a bash expansion and printf. Also some fancy quoting for all the different characters.

#Bash + GNU utilities, 204

fold -$1|sed 2~2{s/.\\+/printf\ %$1's "`echo "&"|rev`"/e y/'`printf %s {A..Z} {a..z}`"&_?!\"'.,/∀qƆpƎℲפHIſʞ˥WNOԀQɹS┴∩ΛMX⅄Zɐqɔpǝɟƃɥᴉɾʞlɯuodbɹsʇnʌʍxʎz⅋‾¿¡„,˙'/ }" 

N is given on the command line and S is given via STDIN:

$ echo "The quick brown fox jumps over the lazy dog." | ./boustrophedon.sh 30 The quick brown fox jumps over ˙ƃop ʎzɐl ǝɥʇ $ 

###Explanation

• fold -N splits the input into lines of length N.
• The rest of the processing is done by sed, line-by-line:
  • 2~2 matches every other line, starting at line 2
  • s/.*/printf %'N's "`echo "&"|rev`"/e uses GNU Sed's exec feature to call a shell to reverse the line and left-pad it with up to N spaces
  • y/ABC.../∀qƆ.../ transform characters

Note ABC... is generated using a bash expansion and printf. Also some fancy quoting for all the different characters.

#Bash + GNU utilities, 204

fold -$1|sed 2~2{s/.\\+/printf\ %$1's "`echo "&"|rev`"/e y/'`printf %s {A..Z} {a..z}`"&_?!\"'.,/∀qƆpƎℲפHIſʞ˥WNOԀQɹS┴∩ΛMX⅄Zɐqɔpǝɟƃɥᴉɾʞlɯuodbɹsʇnʌʍxʎz⅋‾¿¡„,˙'/ }" 

N is given on the command line and S is given via STDIN:

$ echo "The quick brown fox jumps over the lazy dog." | ./boustrophedon.sh 30 The quick brown fox jumps over ˙ƃop ʎzɐl ǝɥʇ $ 

###Explanation

• fold -N splits the input into lines of length N.
• The rest of the processing is done by sed, line-by-line:
  • 2~2 matches every other line, starting at line 2
  • s/.+/printf %'N's "`echo "&"|rev`"/e uses GNU Sed's exec feature to call a shell to reverse the line and left-pad it with up to N spaces
  • y/ABC.../∀qƆ.../ transform characters

Note ABC... is generated using a bash expansion and printf. Also some fancy quoting for all the different characters.

#Bash + GNU utilities, 205

fold -$1|sed -r 2~2{s/.+/printf\ %$1's "`echo "&"|rev`"/e y/'`printf %s {A..Z} {a..z}`"&_?!\"'.,/∀qƆpƎℲפHIſʞ˥WNOԀQɹS┴∩ΛMX⅄Zɐqɔpǝɟƃɥᴉɾʞlɯuodbɹsʇnʌʍxʎz⅋‾¿¡„,˙'/ }" 

N is given on the command line and S is given via STDIN:

$ echo "The quick brown fox jumps over the lazy dog." | ./boustrophedon.sh 30 The quick brown fox jumps over ˙ƃop ʎzɐl ǝɥʇ $ 

###Explanation

• fold -N splits the input into lines of length N.
• The rest of the processing is done by sed, line-by-line:
  • 2~2 matches every other line, starting at line 2
  • s/.*/printf %'N's "`echo "&"|rev`"/e uses GNU Sed's exec feature to call a shell to reverse the line and left-pad it with up to N spaces
  • y/ABC.../∀qƆ.../ transform characters

Note ABC... is generated using a bash expansion and printf. Also some fancy quoting for all the different characters.

#Bash + GNU utilities, 204

fold -$1|sed 2~2{s/.\\+/printf\ %$1's "`echo "&"|rev`"/e y/'`printf %s {A..Z} {a..z}`"&_?!\"'.,/∀qƆpƎℲפHIſʞ˥WNOԀQɹS┴∩ΛMX⅄Zɐqɔpǝɟƃɥᴉɾʞlɯuodbɹsʇnʌʍxʎz⅋‾¿¡„,˙'/ }" 

N is given on the command line and S is given via STDIN:

$ echo "The quick brown fox jumps over the lazy dog." | ./boustrophedon.sh 30 The quick brown fox jumps over ˙ƃop ʎzɐl ǝɥʇ $ 

###Explanation

• fold -N splits the input into lines of length N.
• The rest of the processing is done by sed, line-by-line:
  • 2~2 matches every other line, starting at line 2
  • s/.*/printf %'N's "`echo "&"|rev`"/e uses GNU Sed's exec feature to call a shell to reverse the line and left-pad it with up to N spaces
  • y/ABC.../∀qƆ.../ transform characters

Note ABC... is generated using a bash expansion and printf. Also some fancy quoting for all the different characters.