What's the use of +$ in the give command?

Question

what is +$ in this command: [[ $1 =~ ^[0-9]+$ ]]

Answer 1

The + applies to the [0-9] and not the $.

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).

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

Answer 2

+ 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).