In short, you tried to nest string quoted with single-quotes. That doesn't work. What follows is what bash see and how to fix the problem without changing what Perl sees.

To form the string from the literal 

'if (-d "/usr") { print 'yes';}'

[download]

bash extracts what's in single quotes and concatenates it with what's not in quotes. That means the following are concatenated together:

  1. if (-d "/usr") { print (not subject to interpolation and unescaping)
  2. yes (subject to interpolation and unescaping)
  3. ;} (not subject to interpolation and unescaping)

Let's use Perl to see how bash sees the command: 

$ perl -le'print "[$_]" for @ARGV' -- \
    perl -le 'if (-d "/usr") { print 'yes';}'
[perl]
[-le]
[if (-d "/usr") { print yes;}]

[download]

Compare with: 

'if (-d "/usr") { print '\''yes\''';}'

[download]
  1. if (-d "/usr") { print (not subject to interpolation and unescaping)
  2. \' (subject to interpolation and unescaping)
  3. yes (not subject to interpolation and unescaping)
  4. \' (subject to interpolation and unescaping)
  5. ;} (not subject to interpolation and unescaping) 
$ perl -le'print "[$_]" for @ARGV' -- \
    perl -le 'if (-d "/usr") { print '\''yes'\'';}'
[perl]
[-le]
[if (-d "/usr") { print 'yes';}]

[download]

In reply to Re: if -d by ikegami
in thread if -d by Anonymous Monk

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