我不明白这个参数扩展：$ {p // [0-9] /}

Question

In Linux /etc/init.d/functions script I found the following parameter expansions that I don't quite understand:

${p//[0-9]/}  replace all instances of any number to/by what?

${1##[-+]}  This seems to remove all the longest left instances of minuses and pluses?

${LSB:-}  This seems to say that if LSB is not set then set nothing? in other words do nothing?

Answer 1

These are instances of bash Shell Parameter Expansion ; see http://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html

Note: ksh and zsh support the expansions in your question, too (I'm unclear on the full extent of the overlap in functionality), whereas sh (POSIX-features-only shells), does NOT support the string-replacement expansion, ${p//[0-9]/} .

---

${p//[0-9]/}

Removes all digits : replaces all ( // ) instances of digits ( [0-9] ) with an empty string - ie, it removes all digits (what comes after the last / is the replacement string, which is empty in this case).

${1##[-+]}

Strips a single leading - or + , if present : Technically, this removes the longest prefix ( ## ) composed of a single - or + character from parameter $1 . Given that the search pattern matches just a single character, there is no need to use ## for the longest prefix here, and # - for the shortest prefix - would do.

${LSB:-} 

A no-op designed to prevent the script from breaking when run with the -u ( nounset ) shell attribute : Technically, this expansion means: In case variable $LSB is either not set or empty , it is to be replaced with the string following :- , which, in this case, is also empty.

While this may seem pointless at first glance, it has its purpose, as Sigi points out:

"
The ${LSB:-} construct makes perfect sense if the shell is invoked with the -u option (or set -u is used), and the variable $LSB might actually be unset. You then avoid the shell bailing out if you reference $LSB as ${LSB:-} instead. Since it's good practice to use set -u in complex scripts, this move comes in handy quite often.
"

Answer 2

${p//[0-9]/} # removes digits from anywhere in `$p`

${1##[-+]} # removes + or - from start in $1

${LSB:-} # not really doing anything