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Task/Sierpinski-triangle/UNIX-Shell/sierpinski-triangle.sh

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+#!/bin/bash
+
+### BASH (pure-bash)
+### https://rosettacode.org/wiki/Bourne_Again_SHell
+### Ported from bash+sed+tr version
+### Tested with bash versions 3.2.57 and 5.2.9
+### This version completely avoids any number-theoretic workarounds.
+### Instead, it repeatedly replaces characters by "blocks of characters".
+### The strategy is in no way bash-specific,
+### it would work with any other language just as well,
+### but is particularly well suited for Bash Parameter Expansion
+###     ${parameter/pattern/string}
+### syntax used for pure-bash global-pattern-substitution.
+### (Search "man bash" output for "Parameter Expansion" for additional details
+###  on the
+###     ${parameter/pattern/string}
+###  and
+###     ${parameter:-word}
+###  syntax)
+
+# Basic principle:
+#
+#
+#  x ->  dxd       d -> dd      s -> s
+#        xsx            dd           s
+#
+# In the end all 'd' and 's' are removed.
+function rec(){
+  if [ $1 == 0 ]
+  then
+    echo "x"
+  else
+    rec $[ $1 - 1 ] | while read line ; do
+      A="$line" ; A="${A//d/dd}" ; A="${A//x/dxd}" ; echo "$A"
+      A="$line" ; A="${A//d/dd}" ; A="${A//x/xsx}" ; echo "$A"
+    done
+  fi
+}
+
+### If the script has no arguments, then the default is n=4
+### Else n is the first argument to the script
+export n="${1:-4}"
+
+B="$(rec "$n")" ; B="${B//d/ }" ; B="${B//s/ }" ; B="${B//x/*}"
+echo "$B"