How to find next available file descriptor in Bash?

Question

How can I figure out if a file descriptor is currently in use in Bash? For example, if I have a script that reads, writes, and closes fd 3, e.g.

exec 3<          


        

Answer 1

I decided to summarize the brilliant answer given by @paxdiablo into the single shell function with two auxiliary ones:

fd_used_sym() {
    [ -e "/proc/$$/fd/$1" ]
}

fd_used_rw() {
    : 2>/dev/null >&$1 || : 2>/dev/null <&$1
}

fd_free() {
    local fd_check
    if [ -e "/proc/$$/fd" ]
    then
        fd_check=fd_used_sym
    else
        fd_check=fd_used_rw
    fi

    for n in {0..255}
    do
        eval $fd_check $n || {
            echo "$n"
            return
        }
    done
}

There is sort of simplifications -- escape from auxiliary functions without losing the main functionality:

fd_free() {
    local fd_check
    if [ -e "/proc/$$/fd" ]
    then
        fd_check='[ -e "/proc/$$/fd/$n" ]'
    else
        fd_check=': 2>/dev/null >&$n || : 2>/dev/null <&$n'
    fi

    for n in {0..255}
    do
        eval $fd_check || {
            echo "$n"
            return
        }
    done
}

Both functions check availability of the file descriptor and output the number of the first found free file descriptor. Benefits are following:

• both check ways are implemented (via /proc/$$/fd/X and R/W to a particular FD)
• builtins are used only