what is +$ in this command: [[ $1 =~ ^[0-9]+$ ]]
2
The + applies to the [0-9] and not the $.
+
[0-9]
$
The intended command was:
[[ $1 =~ ^[0-9]+$ ]]
It checks if $1 only contains digits, e.g. 123 or 9 (but not 123f or foo or empty string).
$1
123
9
123f
foo
It breaks down as:
[[
=~
^[0-9]+$
^
[0-9]+
]]
+ in regexp matches for “1 or more times the preceding pattern” and $ signifies the end of string anchor.
^ is beginning of string anchor (the natural complement to $), and [0-9] matches any single digit (in the range of 0 to 9).
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Answers
The
+applies to the[0-9]and not the$.The intended command was:
It checks if
$1only contains digits, e.g.123or9(but not123forfooor empty string).It breaks down as:
[[, start of a Bash extended test command$1, the first parameter=~, the Bash extended test command regex match operator^[0-9]+$, the regex to match against:^, anchor matching the start of the line[0-9]+, one or more digits[0-9], a digit+, one or more of the preceding atom$, anchor matching the end of the line]]to terminate the test command+in regexp matches for “1 or more times the preceding pattern” and$signifies the end of string anchor.^is beginning of string anchor (the natural complement to$), and[0-9]matches any single digit (in the range of 0 to 9).