Complicated bash variable syntax

Question

In one bash script i found the next construction:

if [[ "${xvar[id]:0:${#cnt}}" != "$cnt" ]]; then

Can someone explain what the above condition does?

Another way to help understand is that it behaves identically to if [[ ${xvar[id]} = ${cnt}* ]]; then. — chepner
– chepner, Commented Sep 24, 2012 at 0:49

Community · Accepted Answer · 2017-05-23 12:12:23Z

4

The complicated expression is: ${xvar[id]:0:${#cnt}}.

$xvar must be an array, possibly associative. If it is associative, the part ${xvar[id]} refers to the element of the array identified by the string 'id'; if not, then it refers to the element indexed by variable $id (you're allowed to omit the nested $), as noted by chepner in a comment.

The ${xxx:0:${#cnt}} part of the expression refers to a substring from offset 0 to the length of the variable $cnt (so ${#cnt} is the length of the string in the variable $cnt).

All in all, the test checks whether the first characters of ${xvar[id]} are the same as the value of $cnt, so is the value in $cnt a prefix of the value in ${xvar[id]}.

edited May 23, 2017 at 12:12

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answered Sep 23, 2012 at 23:33

Jonathan Leffler

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2 Comments

kobame Over a year ago

OMG. The substring part was the problematic one. Zilion thanx. ;)

chepner Over a year ago

xvar could be an indexed array as well; id could be a simple integer-valued parameter.

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