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bash/rna-transcription/README.md

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+# RNA Transcription
+
+Given a DNA strand, return its RNA complement (per RNA transcription).
+
+Both DNA and RNA strands are a sequence of nucleotides.
+
+The four nucleotides found in DNA are adenine (**A**), cytosine (**C**),
+guanine (**G**) and thymine (**T**).
+
+The four nucleotides found in RNA are adenine (**A**), cytosine (**C**),
+guanine (**G**) and uracil (**U**).
+
+Given a DNA strand, its transcribed RNA strand is formed by replacing
+each nucleotide with its complement:
+
+* `G` -> `C`
+* `C` -> `G`
+* `T` -> `A`
+* `A` -> `U`
+
+
+Run the tests with:
+
+```bash
+bats rna_transcription_test.sh
+```
+
+After the first test(s) pass, continue by commenting out or removing the
+`[[ $BATS_RUN_SKIPPED == true ]] || skip` 
+annotations prepending other tests.
+
+To run all tests, including the ones with `skip` annotations, run:
+
+```bash
+BATS_RUN_SKIPPED=true bats rna_transcription_test.sh
+```
+
+## Source
+
+Hyperphysics [http://hyperphysics.phy-astr.gsu.edu/hbase/Organic/transcription.html](http://hyperphysics.phy-astr.gsu.edu/hbase/Organic/transcription.html)
+
+
+## External utilities
+`Bash` is a language to write "scripts" -- programs that can call
+external tools, such as
+[`sed`](https://www.gnu.org/software/sed/),
+[`awk`](https://www.gnu.org/software/gawk/),
+[`date`](https://www.gnu.org/software/coreutils/manual/html_node/date-invocation.html)
+and even programs written in other programming languages, 
+like [`Python`](https://www.python.org/).
+This track does not restrict the usage of these utilities, and as long
+as your solution is portable between systems and does not require
+installation of third party applications, feel free to use them to solve
+the exercise.
+
+For an extra challenge, if you would like to have a better understanding
+of the language, try to re-implement the solution in pure `Bash`,
+without using any external tools. Note that there are some types of
+problems that bash cannot solve, such as performing floating point
+arithmetic and manipulating dates: for those, you must call out to an
+external tool.
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others
+have completed the exercise.

bash/rna-transcription/rna_transcription.sh

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+#!/usr/bin/env bash
+#
+# External tools: none.
+#
+# V1: initial version
+
+shopt -s extglob
+
+# set to mask to enable logs. 0: none, 255: all
+(( debug=2#00000000 ))
+#(( debug=2#00001111 ))
+# $1: log level (mask), then strings to display.
+debug () {
+    (( debug & $1 )) && shift && echo Line ${BASH_LINENO[0]}: "${@}" >&2
+}
+
+die () {
+    echo "${@}" >&2
+    exit 1
+}
+usage() {
+    die "usage: rna_transcription.sh [seq1] [...]"
+}
+
+declare -A complement=([G]='C'
+                       [C]='G'
+                       [T]='A'
+                       [A]='U')
+debug 1 keys/vals=${!complement[@]}/${complement[@]}
+# we will accept multiple args (out of topic for exercise), meaning that:
+#  rna_transcription.sh GCTA
+# is equivalent to
+#  rna_transcription.sh G C T A
+# or
+#  rna_transcription.sh G CTA
+dna="${@}"
+dna="${dna// /}"                        # remove spaces
+(($# > 0 )) && [[ "$dna" != +([GCTA]) ]] && die "Invalid nucleotide detected."
+
+((ldna=${#dna}))
+debug 1 "dna=$dna, length=$ldna"
+rna=""                                  # rna sequence
+
+for ((i=0; i<ldna; ++i)); do
+    seq=${dna:$i:1}
+    rna+="${complement[$seq]}"
+    debug 2 "rna=$rna"
+done
+
+echo "$rna"                             # not perfect, empty line if no args.
+exit 0
+
+# emacs/vim settings.
+# Local Variables:
+# sh-basic-offset: 4
+# indent-tabs-mode: nil
+# comment-column: 40
+# fill-column: 80
+# End:
+# vim: set tabstop=4 expandtab: