The echo is dangerous if any if the first variable expands to a string starting with a hyphen or if any of the variables expand to a string containing any backslash characters. Otherwise:

Code:

echo "$a$b$c"

is fine. But, if you're reading data from a user supplied source, never assume that you know what the data will contain, and, as Robin suggested, use printf instead.

But:

Code:

printf "${a}${b}${c}"

is also dangerous (and doesn't add the trailing <newline> character that echo would provide). The dangers are the same as with echo but has an additional problem if the expansion of any of those variables contain a <percent-sign> character. The safe way to print three variables (with no added space between them and with a trailing <newline>) would be:

Code:

printf '%s%s%s\n' "$a" "$b" "$c"

You only need to include braces around variable names if text following the variable name could be part of a valid variable name. For instance if you wanted to print the expansion of the variable b followed by the character b you could use either of the following:

Code:

printf '%sb\n' "$b"
printf '%s\n' "${b}b"

but not either of:

Code:

echo "$bb"
printf '%s\n' "$bb"

since both of those you result in the expansion of the variable bb instead of the variable b. Shell variable names can contain letters, digits, and underscores where the 1st character is not a digit.

Positional parameters look like variable names just containing digits, but the rules are slightly different. Single digit positional parameters never require (but do allow) braces but multi-digit positional parameters always require braces. The expansion of $11 (or of ${1}1) will result in the expansion of the 1st positional parameter followed by the digit 1. To get the expansion of the 11th positional parameter, you must use ${11}.