current community

your communities

more stack exchange communities

Stack Exchange
tour help
careers 2.0
Take the 2-minute tour ×
Stack Overflow is a question and answer site for professional and enthusiast programmers. It's 100% free, no registration required.
up vote 1 down vote favorite
1

I have the following line in my shell script:

if [ -n "${USE_X:+1}" ]; then

I cannot figure out what the ":+1" part means. Any ideas?

share|improve this question

3 Answers 3

up vote 5 down vote accepted

Have a look here. That url provides the following explanation:

${parameter:+alt_value}

If parameter set, use alt_value, else use null string.

and has the following example:

echo
echo "###### \${parameter:+alt_value} ########"
echo

a=${param4:+xyz}
echo "a = $a"      # a =

param5=
a=${param5:+xyz}
echo "a = $a"      # a =
# Different result from   a=${param5+xyz}

param6=123
a=${param6:+xyz}
echo "a = $a"      # a = xyz
share|improve this answer
up vote 4 down vote

basically if $USE_X is set, the statement is evaluated to 1, otherwise null. Probably similar to

if [ -z $USE_X ];
then
    echo 1
else
    echo ""
fi

from http://tldp.org/LDP/abs/html/parameter-substitution.html#PATTMATCHING :

${parameter+alt_value}, ${parameter:+alt_value}
If parameter set, use alt_value, else use null string.

Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, see below.
share|improve this answer
up vote 2 down vote

since aioobe already answered the question itself, here's a way to search a long manpage like Bash's using a regex, using this question as an example:


/\{.*:\+

The first forward-slash puts less (the manpage viewer) into search mode; the regex says to search for a left bracket, followed by any amount of stuff, then a colon followed by a plus sign. The bracket and plus need to be escaped because they have special meaning to the regex parser.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.