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I was skimming through an /etc/rc.d/init.d/sendmail file (I know this is hardly ever used, but I'm studying for an exam), and I've become a bit confused about the && and the || operators. I've read where they can be used in statements such as:

if [ test1 ] && [ test2 ]; then
     echo "both tests are true"
elif [ test1 ] || [ test2 ]; then
     echo "one test is true"
fi

However, this script shows single line statements such as:

[ -z "$SMQUEUE" ] && SMQUEUE="QUEUE"
[ -f /usr/sbin/sendmail ] || exit 0

These seem to be using the && and || operators to elicit responses based on tests, but I haven't been able to dig up documenation regarding this particular use of these operators. Can anyone explain what these do in this particular context?

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3 Answers 3

up vote 30 down vote accepted

The right side of && will only be evaluated if the exit status of the left side is zero. || is the opposite: it will evaluate the right side only if the left side exit status is nonzero. You can consider [ ... ] to be a program with a return value. If the test inside evaluates to true, it returns zero; it returns nonzero otherwise.

Examples:

$ false && echo howdy!

$ true && echo howdy!
howdy!
$ true || echo howdy!

$ false || echo howdy!
howdy!

Extra notes:

If you do which [, you might see that [ actually does point to a program! It's usually not actually the one that runs in scripts, though; run type [ to see what actually gets run. If you wan to try using the program, just give the full path like so: /bin/[ 1 = 1.

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3  
When you see "X or Y", you test X. If it's true, you already know the answer to "X or Y" (it's true), so no need to test Y. If it's true, you don't know the answer to "X or Y", so you do need to test Y. When you see "X and Y", you test X. If it's false, you already know the answer to "X and Y" (it's false), so no need to test Y. If X is true, then you do need to test Y to see if "X and Y" is true. –  David Schwartz Nov 16 '11 at 3:41
1  
Instead of "if the left side returns zero", I would write "if the left side command's exit status is zero". I find "return" a bit ambiguous as the output and the exit status can both be considered as "return values" –  glenn jackman Nov 16 '11 at 14:36

to expand on @Shawn-j-Goff's answer from above, && is a logical AND, and || is a logical OR.

See this part of the Advanced Bash Scripting Guide. Some of the contents from the link for user reference as below.

&& AND

if [ $condition1 ] && [ $condition2 ]
#  Same as:  if [ $condition1 -a $condition2 ]
#  Returns true if both condition1 and condition2 hold true...

|| OR

if [[ $condition1 && $condition2 ]]    # Also works.
#  Note that && operator not permitted inside brackets
#+ of [ ... ] construct.

if [ $condition1 ] || [ $condition2 ]
# Same as:  if [ $condition1 -o $condition2 ]
# Returns true if either condition1 or condition2 holds true...

if [[ $condition1 || $condition2 ]]    # Also works.
#  Note that || operator not permitted inside brackets
#+ of a [ ... ] construct.
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There is a notion of "short cutting".

When (expr1 && expr2) is evaluated - expr2 is only evaluated if exp1 evaluates to "true". This is because both expr1 AND expr2 have to be true for (expr1 && expr2) to be true. If expr1 evaluates to "false" expr2 is NOT evalued (short cut) because (expr1 && expr2) is already "flase".

Try the following - assume file F1 exists & file F2 does not exist:

( [ -s F1 ] && echo "File Exists" )  # will print "File Exists" - no short cut
( [ -s F2 ] && echo "File Exists" )  # will NOT print "File Exists" - short cut

Similarly for || (or) - but short cutting is reversed.

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