Updated SC2051 (markdown)

SC2051.md

@@ -1,25 +1,37 @@
-## Bash doesn't support variables in brace range expansions.
+## Bash doesn't support variables in brace range expansions.
-
+
-### Problematic code:
+### Problematic code:
-
+
-    for i in {1..$n}
+    for i in {1..$n}
-    do
+    do
-      echo "$i"
+      echo "$i"
-    done
+    done
-
+
-### Correct code:
+### Correct code:
-
+
-    for ((i=0; i<n; i++))
+    for ((i=0; i<n; i++))
-    do
+    do
-      echo "$i"
+      echo "$i"
-    done
+    done
-
+
-### Rationale:
+### Rationale:
-
+
-In Bash, brace expansion happens before variable expansion. This means that brace expansion will not account for variables.
+In Bash, brace expansion happens before variable expansion. This means that brace expansion will not account for variables.
-
+
-Use an arithmetic for loop instead.
+For integers, use an arithmetic for loop instead. For zero-padded numbers or letters, use of eval may be warranted:
-
+
-### Contraindications
+    from="a" to="m"
-
+    for c in $(eval "echo {$from..$to}"); do echo "$c"; done
+
+or more carefully (if `from`/`to` could be user input, or if the brace expansion could have spaces or globs):
+
+    from="a" to="m"
+    while IFS= read -d '' -r c
+    do
+      echo "Read $c"
+    done <  <(eval "printf '%s\0' $(printf "{%q..%q}.jpg" "$from" "$to")")
+    
+
+### Contraindications
+
 None (if you're writing for e.g. zsh, make sure the shebang indicates this so shellcheck won't warn)