Syntax error in [[ conditional expression with bash

Question

When I execute this

regex='^[-a-z0-9]+$'
string='abcd1--'
if [[ $string =~ $regex ] -a ![ grep - "--" ]]
then 
    echo "valid"
else
    echo "not valid"
fi

I get

~$ sh t.sh 
t.sh: line 3: syntax error in conditional expression
t.sh: line 3: syntax error near `]'
t.sh: line 3: `if [[ $string =~ $regex ] -a [ grep - "--" ]]'
~$

It is suppose to return not valid.

Can someone figure out what's wrong?

You have to execute the script using bash, not sh (which usually is (d)ash). There is no =~ operator using the test utility (which is being used by (d)ash). — Gandaro
– Gandaro, Commented Feb 28, 2012 at 14:49

Mat · Accepted Answer · 2012-02-28 14:45:35Z

10

You're mixing [ and [[ syntax in a strange way.

Try:

if [[ ( $string =~ $regex ) && !( $string =~ "--" ) ]]

and check bash's man page.

answered Feb 28, 2012 at 14:45

Mat

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7 Comments

Benoit Over a year ago

note, /usr/bin/[ is not part of bash, and its special syntax is accessible with man test not man bash.

Mat Over a year ago

bash does use a built-in [ if it can. (See the test expr part of the bash man page.)

Benoit Over a year ago

oh, you're right, there is a builtin test command, I did not know it. thanks for pointing it out.

Sandra Schlichting Over a year ago

When I search for [[ in the bash manpage, I don't get any matches. What is the difference of [, (, and [[ ?

Mat Over a year ago

gnu.org/software/bash/manual/… - [ is an "alias" for test, ( is just a grouping parenthesis, [[ is another form of conditional construct.

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