Created SC2031 (markdown)

SC2031.md

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+# var was modified in a subshell. That change might be lost.
+
+### Problematic code:
+
+There are many ways of accidentally creating subshells, but a common one is piping to a loop:
+
+    n=0
+    printf "%s\n" {1..10} | while read i; do (( n+=i )); done
+    echo $n
+
+### Correct code:
+
+    # Bash specific:
+    n=0
+    while read i; do (( n+=i )); done <   <(printf "%s\n" {1..10})
+    echo $n
+
+In `sh`, a temp file can be used instead of process substitution. 
+
+### Rationale:
+
+Variables set in subshells are not available outside the subshell. This is a wide topic, and better described on the [Wooledge Bash Wiki](http://mywiki.wooledge.org/BashFAQ/024).
+
+Here are some constructs that cause subshells (shellcheck may not warn about all of them). In each case, you can replace `subshell1` by a command or function that sets a variable, e.g. simply `var=foo`, and the variable will appear to be unset after the command is run. Similarly, you can replace `regular` with `var=foo`, and it will be set afterwards:
+    
+Pipelines:
+
+    subshell1 | subshell2 | subshell3    # Bash, Dash, Ash
+    subshell1 | subshell2 | regular      # Ksh, Zsh
+
+Command substitution:
+
+    regular "$(subshell1)" "`subshell2`"
+
+Process substitution: 
+
+    regular <(subshell1) >(subshell2)
+
+Some forms of grouping:
+
+    ( subshell )
+    { regular; }
+
+Backgrounding:
+
+    subshell1 &
+    subshell2 &
+
+Anything executed by external processes:
+
+    find . -exec subshell1 {} \;
+    find . -print0 | xargs -0 subshell2
+    sudo subshell3
+    su -c subshell4
+
+This applies not only to setting variables, but also setting shell options and changing directories. 
+
+### Contraindications
+
+You can ignore this error if you don't care that the changes aren't reflected, because work on the value branches and shouldn't be recombined.