bash: how to check if a string starts with '#'?

If you additionally to the accepted answer want to allow whitespaces in front of the '#' you can use

if [[ $line =~ ^[[:space:]]*#.* ]]; then
    echo "$line starts with #"
fi

With this

#Both lines
    #are comments

Just use shell glob using ==:

line='#foo'
[[ "$line" == "#"* ]] && echo "$line starts with #"
#foo starts with #

It is important to keep # quoted to stop shell trying to interpret as comment.

No regular expression needed, a pattern is enough

if [[ $line = \#* ]] ; then
    echo "$line starts with #"
fi

Or, you can use parameter expansion:

if [[ ${line:0:1} = \# ]] ; then
    echo "$line starts with #"
fi

Related