Bash – syntax error in conditional expression

Wrapping the whole into a function seems to do the trick:

#!/bin/bash -e

main () {
readonly a=(1 2)
    # A syntax error is here:
    if (( "${a[#]}" == 2 )); then
        echo ok
    else
        echo not ok
    fi
    echo status $?
    echo 'Bad: has not aborted execution on syntax error!'
}

main "$@"

Result:

$ ./sh-on-syntax-err 
$ ./sh-on-syntax-err line 6: #: syntax error: operand expected (error token is "#")
$ 

Though I have no clue why - maybe someone else can explain?

Keywords like if, then, else, fi, for, case and so on need to be in a place where the shell expects a command name. Otherwise they are treated as ordinary words. For example,

echo if

just prints if, it doesn't start a conditional instruction.

Thus, in the line

if [ -r "$NAME" -af "$NAME" ] then

the word then is an argument of the command [ (which it would complain about if it ever got to run). The shell keeps looking for the then, and finds a fi in command position. Since there's an if that's still looking for its then, the fi is unexpected, there's a syntax error.

You need to put a command terminator before then so that it's recognized as a keyword. The most common command terminator is a line break, but before then, it's common to use a semicolon (which has exactly the same meaning as a line break).

if [ -r "$NAME" -af "$NAME" ]; then

or

if [ -r "$NAME" -af "$NAME" ]
then

Once you fix that you'll get another error from the command [ because it doesn't understand -af. You presumably meant

if [ -r "$NAME" -a -f "$NAME" ]; then

Although the test commands look like options, you can't bundle them like this. They're operators of the [ command and they need to each be a separate word (as do [ and ]).

By the way, although [ -r "$NAME" -a -f "$NAME" ] works, I recommend writing either

[ -r "$NAME" ] && [ -f "$NAME" ]

or

[[ -r $NAME && -f $NAME ]]

It's best to keep [ … ] conditionals simple because the [ command can't distinguish operators from operand easily. If $NAME looks like an operator and appears in a position where the operator is valid, it could be parsed as an operator. This won't happen in the simple cases seen in this answer, but more complex cases can be risky. Writing this with separate calls to [ and using the shell's logical operators avoids this problem.

The second syntax uses the [[ … ]] conditional construct which exists in bash (and ksh and zsh, but not plain sh). This construct is special syntax, whereas [ is parsed like any other command, thus you can use things like && inside and you don't need to quote variables except in arguments to some string operators (=, ==, !=, =~) (see When is double-quoting necessary? for details).