Check if shell variable contains an absolute path

I want to check if a shell variable contains an absolute path. I don't care if the path exists or not—if it doesn't I'm going to create it—but I do want to ensure that I'm dealing with an absolute pathname. My code looks something like the following: myfunction() { [ magic test to see if "$1" is an absolute path ] || return 1 mkdir -p "$(dirname "$1")" || return 1 commands >> "$1" } Or, the use case where the absolute path to be verified is intended to be a directory: anotherfunction() { [ same magic test ] || return 1 mkdir -p "$1" dostuff >> "$1/somefile" } If this were awk I would do the check like so: myvar ~ /^\// There *must* be a clean way to do this with the shell's string handling, but I'm having trouble coming up with it. (Mentioning a bash-specific solution would be fine but I'd like to know how to do this portably, also. POSIX string handling seems like it should be sufficient for this.)