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I am checking a bash script which contains something like: export PATH=${PATH:+$PATH:}/usr/bin I assume it has same output as export PATH=$PATH:/usr/bin. I did a echo ${PATH:+$PATH:} it just output the value of $PATH plus a ':'. but if I modify it to ${PSOMETHING:+$PATH:}, it outputs nothing.

my question what's the different between using $PATH: and ${PATH:+$PATH:}? any benefit? how to understand the syntax?

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1 Answer 1

up vote 3 down vote accepted

See Parameter Expansion in man bash:

${parameter:+word}

Use Alternate Value. If parameter is null or unset, nothing is substituted, other- wise the expansion of word is substituted.

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I just found it on tldp.org/LDP/abs/html/parameter-substitution.html thanks anyways –  Fivesheep Nov 4 '12 at 7:56

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