What does the “=~” operator do in shell scripts?

Question

It seems that it is sort of comparison operator, but what exactly it does in e.g. the following code (taken from https://github.com/lvv/git-prompt/blob/master/git-prompt.sh#

Answer 1

It matches regular expressions

Refer to following example from http://tldp.org/LDP/abs/html/bashver3.html#REGEXMATCHREF

#!/bin/bash

input=$1


if [[ "$input" =~ "[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]" ]]
#                 ^ NOTE: Quoting not necessary, as of version 3.2 of Bash.
# NNN-NN-NNNN (where each N is a digit).
then
  echo "Social Security number."
  # Process SSN.
else
  echo "Not a Social Security number!"
  # Or, ask for corrected input.
fi

Answer 2

Like Ruby, it matches where the RHS operand is a regular expression.

Answer 3

It's a regular expression matching. I guess your bash version doesn't support that yet.

In this particular case, I'd suggest replacing it with simpler (and faster) pattern matching:

[[ $LC_CTYPE == *UTF* && $TERM != "linux" ]]

(note that * must not be quoted here)

Answer 4

It's a bash-only addition to the built-in [[ command, performing regexp matching. Since it doesn't have to be an exact match of the full string, the symbol is waved, to indicate an "inexact" match.

In this case, if $LC_CTYPE CONTAINS the string "UTF".

More portable version:

if test `echo $LC_CTYPE | grep -c UTF` -ne 0 -a "$TERM" != "linux"
then
  ...
else
  ...
fi