$ {MYPATH //：//：// bin：} / bin在bash shell中是什么意思？

Question

I've been seeing a case of appending a particular dir named 'bin' under $MYPATH (which could contain multiple values in format of a:b:c ) to $PATH , the solution is ${MYPATH//://bin:}/bin ,for example:

> MYPATH=/local/a:/local/b
> echo ${MYPATH//://bin:}/bin
/local/a/bin:/local/b/bin
> echo ${MYPATH}/bin  # as a bad example, this is not what we want
/local/a:/local/b/bin

as you see, ${MYPATH//://bin:}/bin works pretty well.

My question is where is such parameter expansion documented, I can't find any doc(include 'man bash', seems it's not the "Substring Expansion" which in format of {parameter:offset:length} ) on explaining the syntax.

Could anyone explain it to me or point me the doc of this feature?

Answer 1

This is called Parameter expansion. In this case, it replaces all : with /bin:

${MYPATH//://bin:}
  ^^^^^^  ^ 
   |    ^^| ^^^^^
   |    | | replacement
   |    | match string
   var  match all

Too many slashes are misleading. See better examples:

MYPATH="hello1hello2"
$ echo ${MYPATH//hello/bye}
bye1bye2

or:

d="a:b:c"
$ echo ${d//://bin:}
a/bin:b/bin:c

Note the difference if we just use one slash:

$ echo ${MYPATH/hello/bye}
bye1hello2

More info in Parameter expansion#Search and replace .

Answer 2

Substring Expansion

This is a confusing, although valid, implementation of the substring expansion form of:

${parameter//pattern/string}

The doubled forward slash will replace all copies of pattern of with string , rather than just the first match. In your example:

echo ${MYPATH//://bin:}/bin

the expansion says:

1. Match all copies of pattern : .
2. Replace each match with the string /bin: .
3. Append a literal /bin to the result of the substring expansion, since it's outside the parameter expansion.