scnzzh@ZUBT:~$ re='^[0-9]+$' scnzzh@ZUBT:~$ a=001 scnzzh@ZUBT:~$ b=a12 scnzzh@ZUBT:~$ if [ $a =~ $re ]; then echo 'yyy'; fi bash: [: =~: binary operator expected scnzzh@ZUBT:~$ if [[ $a =~ $re ]]; then echo 'yyy'; fi yyy scnzzh@ZUBT:~$ if [[ $b =~ $re ]]; then echo 'yyy'; else echo 'nnn'; fi nnn
scnzzh@ZUBT:~$ if [ 'a'=~'b' ]; then echo 'yyy'; fi yyy scnzzh@ZUBT:~$ if [[ 'a' =~ 'b' ]]; then echo 'yyy'; fi scnzzh@ZUBT:~$
-
What is the operator
=~called?I'm not sure it has a name. The bash documentation just calls it the
=~operator. -
Is it only used to compare the right side against the left side?
The right side is considered an extended regular expression. If the left side matches, the operator returns
0, and1otherwise. -
Why are double square brackets required when running a test?
Because
=~is an operator of the[[ expression ]]compound command.
scnzzh@ZUBT:~$ if [[ 'a' =~ 'a' && 'a' == 'a' ]]; then echo 'yyy'; fi yyy scnzzh@ZUBT:~$ if [[ 'a' =~ 'a' && 'a' == 'b' ]]; then echo 'yyy'; fi scnzzh@ZUBT:~$ if [[ 'a' =~ 'a' || 'a' == 'b' ]]; then echo 'yyy'; fi yyy scnzzh@ZUBT:~$
scnzzh@ZUBT:~$ re='^[0-9]+$' scnzzh@ZUBT:~$ a=001 scnzzh@ZUBT:~$ if [[ $a =~ $re ]]; then echo 'yyy'; fi yyy scnzzh@ZUBT:~$ a='001' scnzzh@ZUBT:~$ if [[ $a =~ $re ]]; then echo 'yyy'; fi yyy scnzzh@ZUBT:~$ if [[ '001' =~ '^[0-9]+$' ]]; then echo 'yyy'; fi scnzzh@ZUBT:~$ if [[ '001' =~ $re ]]; then echo 'yyy'; fi yyy scnzzh@ZUBT:~$ if [[ $a =~ '^[0-9]+$' ]]; then echo 'yyy'; fi scnzzh@ZUBT:~$ if [[ '001' =~ '^[0-9]+$' ]]; then echo 'yyy'; fi scnzzh@ZUBT:~$
[[ ]] is a shell keyword, which means it is part of shell syntax. Inside this construct, some reserved characters change meaning. For example, ( ) means parenthesis like other programming language (not launching a subshell to execute what's inside the parentheses). Another example is that < and > means less than and greater than, not shell redirection. This allow more "natural" appearance of logical expressions, but it can be confusing for novice bash programmers.