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I have been trying to find online what these if statement mean. Below are a few.

if [ "${snums[k]}" != "NA" -a ! -f "${aliases[k]}.4dfp.img" ]; then
if [ -f "$dcmfile" -a -n "`cat $dcmfile`" ]; then

The -a used in these two examples do not make senses to me.

  • A good reason you may not have seen -a or -o is that they have been declared "obsolescent" by POSIX. Script authors are strongly encouraged to use the shell's && and || instead – Fox Jul 10 '18 at 23:20
  • Oh i see. I always thought -a was for checking to see if a file exists. – Aaron Tanenbaum Jul 11 '18 at 15:41
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-a is logical AND. If both the operands are true, then the condition becomes true otherwise false.

-n Checks if the given string operand size is non-zero; if it is nonzero length, then it returns true.

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    And -f checks if the following word is a regular file. Also, you can use -s to check if a file is not empty. – l0b0 Jul 10 '18 at 21:27

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