Fix setting of Rstudio preferences

Move the preference setting logic to a function for readability.

Use jq rather than grep to check for existing keys for readability.

Perform modifications on a string copy of the preference file
rather than piping straight back to the file.
Should anything go awry, this should help prevent damaging the existing
preferences file.

If, for whatever reason, the preferences file is blank at the time of
running this script, replace blank string with an empty dictionary `{}`
to allow the following `jq` processes to succeed.

Only write back to prefs file once modifications complete to avoid
writing blank/invalid data to file.

devcontainer/postCreate.sh

@@ -7,7 +7,16 @@ set -euo pipefail
 #set R working directory
 ! grep -q "$1" $R_HOME/etc/Rprofile.site && sudo tee -a $R_HOME/etc/Rprofile.site <<< "setwd(\"$1\")"
 #set RStudio working directory
-! grep -q "$1" ~/.config/rstudio/rstudio-prefs.json && cat ~/.config/rstudio/rstudio-prefs.json | jq ". + {\"initial_working_directory\":\"$1\"}" >  ~/.config/rstudio/rstudio-prefs.json
+set_rstudio_pref() {
+    key="$1"
+    value="$2"
+    orig_prefs=$(jq -ne "input ? // {}" < ~/.config/rstudio/rstudio-prefs.json)
+    new_prefs=$(jq "if has(\"$key\") then . else . + {\"$key\": $value} end" <<< "$orig_prefs")
+    echo "$new_prefs" > ~/.config/rstudio/rstudio-prefs.json
+}
+
+#set RStudio working directory
+set_rstudio_pref "initial_working_directory" "\"$1\""
 
 #download and extract latest ehrql source
 wget https://github.com/opensafely-core/ehrql/archive/main.zip -P .devcontainer