CODEWITHSUNDEEP
linuxbash
St.Antario
St.Antario

Reputation: 27425

Why does space in Bash string subscripts matter here?

I'm experimenting with bash scripting and noticed the following behavior:

file="test.jar"
echo "${file: -4}" #prints .jar
echo "${file:-4}"  #prints test.jar

Very confusing behavior actually. Can someone explain why the second case prints the whole test.jar?

Upvotes: 2

Views: 56

Answers (2)

Benjamin W.
Benjamin W.

Reputation: 52361

This is a compromise due to the timeline of features being added to Bash.

  • The ${parameter:-word} or ${parameter-word} syntax for "replace parameter with word if parameter is null or unset (:-) / null (-)" was around for pretty much always; the - version was already in the Version 7 Bourne Shell.
  • The ${parameter:offset:length} and ${parameter:offset} syntax for "substring of parameter starting at offset (with optional length length)" was introduced in Bash 2.0 (no conflict so far).

  • Negative offsets and length specifications for the substring construct were introduced in Bash 4.2. This leads to a problem:

    $ string=01234567890abcdefgh
$ echo ${string:7:-2}           # Not ambiguous
7890abcdef
$ echo ${string:-7}             # Interpreted as "undefined/null, or..."
01234567890abcdefgh
$ echo ${string: -7}            # Interpreted as offset from the end
bcdefgh
$ echo ${string:(-7)}           # Interpreted as offset from the end
bcdefgh

The space before - or the parentheses around the negative offset are there to tell the expansion apart from the :- (default value) expansion.

If the space is not there, the expansion ${file:-4} is interpreted as "print 4 if the parameter file is null or unset, and the expansion of file otherwise".

References:

Upvotes: 1

Jean-Baptiste Yunès
Jean-Baptiste Yunès

Reputation: 36431

This is due to inconsistent syntax. {"string":-} means default (all the string), whatever follows -. So you need either a space or parenthesis:

{"string": -4}
{"string":(-4)}

Read bash string manipulation.

Upvotes: 3

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