Gister to POST or PATCH a single file as public gist. PATCH does gist ID lookup: no need to remember gist ID's

pgist.sh

#!/bin/sh
# --------------------------------------------------------
# Shell gister to POST or PATCH a single file as public gist.
# PATCH does gist ID lookup: no need to remember gist ID's
# --------------------------------------------------------
# usage: $ pgist file_to_post_or_patch.extension
#
# installation: $ curl -O https://gist.githubusercontent.com/ProBackup-nl/3971a45b21749cfff6c0069d3dad1dde/raw/pgist.sh && chmod 755 pgist.sh && mv pgist.sh /opt/usr/sbin/pgist
#
# tested with Busybox (ash shell) + Entware (opkg)
# Limitation #1: 300 files per gist <- Github API
# Limitation #2: 30 (maybe 100) gists per user <- Github API && pagination is not implemented in this script
# Limitation #3: requires your gist filenames to be user unique (= not any of your gists can have an identical filename; otherwise filename to gist matching fails)

# ToDo: test large files; expect 4 MiB to fail.

# Dependencies
which jq > /dev/null || { echo 'Error: dependency jq not found. Install it with: ''$ opkg install jq''. Exiting'; exit 1 ; }
[ -z "$GITHUB_TOKEN" ] && { echo 'Error: GITHUB_TOKEN is empty. For example ''export GITHUB_TOKEN=replace_this_with_your_token'' in your ''~/.profile''. Create a personal access token <https://developer.github.com/v3/guides/getting-started/#oauth>  Exiting'; exit 2; }
[ -z "$1" ] && { echo 'Error: no file supplied as argument. Exiting'; exit 3 ; }
which sed > /dev/null || { echo 'Error: dependency ''sed'' not found. Install it. Exiting'; exit 4 ; }
which awk > /dev/null || { echo 'Error: dependency ''awk'' not found. Install it. Exiting'; exit 5 ; }
which basename > /dev/null || { echo 'Error: dependency ''basename'' not found. Install it. Exiting'; exit 6 ; }
which curl > /dev/null || { echo 'Error: dependency ''curl'' not found. Install it with ''$ opkg install curl ca-certificates''. Exiting'; exit 7 ; }
# ToDo: ca-certificates /opt/etc/ssl/certs/

# Supply filename on the command-line as first argument
[ -f $1 ] && readonly FNAME=$1 || { echo "Error: file '$1' not found. Exiting" ; exit 8 ; } 
readonly BNAME=`basename $FNAME`

# Lookup: authenticated user's gists | select matching filename | return parent gist id
# api.github.com/gists?filename=$FNAME doesn't filter results
# curl -I will show headers
readonly GIST_ID=`curl --silent -H "Authorization: token $GITHUB_TOKEN" https://api.github.com/gists?per_page=100 |jq --raw-output '.[]|select(.files["'$BNAME'"])|.id'`

# Count results
id_count=0 ; for i in $GIST_ID ; do let id_count++ ; done

if [ $id_count -gt 1 ]; then
	echo "Error: Duplicate filename found. The file names of your gists do need to be unique. Exiting"
	exit 10
else
	# POST or PATCH

	# Sanitize file content
	#    Replace \ with \\
	#    Remove \r (from Windows end-of-lines),
	#    Replace tabs by \t
	#    Replace " with \"
	#    Replace EOL by \n (note: awk is not bullet-proof on busybox; awk '{ printf($0 "\\n") }' fails; slow od alternative <https://stackoverflow.com/questions/1251999/how-can-i-replace-a-newline-n-using-sed#answer-23166624>)
	[ -n "$FNAME" ] && readonly CONTENT=$(sed -e 's/\\/\\\\/g' -e 's/\r//' -e's/\t/\\t/g' -e 's/"/\\"/g' "${FNAME}" | awk '{ printf("%s\\n", $0) } END { print "" }') || { echo 'Error: at this point the filename can''t be empty. Exiting' ; exit 11 ; }
	#echo $CONTENT

	if [ $id_count = 1 ] ; then
		echo PATCH
		# Use curl to send an explicit PATCH request to an existing gist
		readonly TDATA="{ \"files\": { \"${BNAME}\": { \"content\": \"${CONTENT}\" } } }"
       		curl --silent --request PATCH -d "$TDATA" -H "Authorization: token $GITHUB_TOKEN" https://api.github.com/gists/$GIST_ID

	elif [ $id_count = 0 ] ; then
		echo POST
		read -p 'Give a description: ' DESCRIPTION
		# Use curl to send an implicit POST request to create a new gist
		curl -d "{ \"description\": \"${DESCRIPTION}\", \"public\": true, \"files\": { \"${BNAME}\": { \"content\": \"${CONTENT}\" } } }" -H "Authorization: token $GITHUB_TOKEN" https://api.github.com/gists
	else
		echo "Logic error. Exiting"
		exit 20
	fi
fi

echo 'Done'

As a result of being unable to update an existing gist file with the answers of "upload-a-file-to-a-gist-with-bash" without installing significant dependencies (like ruby) on a mips router, this script found its existence.
Notes: Tested with scripts that use Heredoc notation. Not expected to handle with 4MiB large files.