In Bash, how to find the lowest-numbered unused file descriptor?

Question

In a Bash-script, is it possible to open a file on \"the lowest-numbered file descriptor not yet in use\"?

I have looked around for how to do this, but it seems that Bas

Answer 1

I revised my original answer and now have a one line solution for the original post.
The following function could live in a global file or sourced script (e.g. ~/.bashrc):

# Some error code mappings from errno.h
readonly EINVAL=22   # Invalid argument
readonly EMFILE=24   # Too many open files

# Finds the lowest available file descriptor, opens the specified file with the descriptor
# and sets the specified variable's value to the file descriptor.  If no file descriptors
# are available the variable will receive the value -1 and the function will return EMFILE.
#
# Arguments:
#   The file to open (must exist for read operations)
#   The mode to use for opening the file (i.e. 'read', 'overwrite', 'append', 'rw'; default: 'read')
#   The global variable to set with the file descriptor (must be a valid variable name)
function openNextFd {
    if [ $# -lt 1 ]; then
        echo "${FUNCNAME[0]} requires a path to the file you wish to open" >&2
        return $EINVAL
    fi

    local file="$1"
    local mode="$2"
    local var="$3"

    # Validate the file path and accessibility
    if [[ "${mode:='read'}" == 'read' ]]; then
        if ! [ -r "$file" ]; then
            echo "\"$file\" does not exist; cannot open it for read access" >&2
            return $EINVAL
        fi
    elif [[ !(-w "$file") && ((-e "$file") || !(-d $(dirname "$file"))) ]]; then
        echo "Either \"$file\" is not writable (and exists) or the path is invalid" >&2
        return $EINVAL
    fi

    # Translate mode into its redirector (this layer of indirection prevents executing arbitrary code in the eval below)
    case "$mode" in
        'read')
            mode='<'
            ;;
        'overwrite')
            mode='>'
            ;;
        'append')
            mode='>>'
            ;;
        'rw')
            mode='<>'
            ;;
        *)
            echo "${FUNCNAME[0]} does not support the specified file access mode \"$mode\"" >&2
            return $EINVAL
            ;;
    esac

    # Validate the variable name
    if ! [[ "$var" =~ [a-zA-Z_][a-zA-Z0-9_]* ]]; then
        echo "Invalid variable name \"$var\" passed to ${FUNCNAME[0]}" >&2
        return $EINVAL
    fi

    # we'll start with 3 since 0..2 are mapped to standard in, out, and error respectively
    local fd=3
    # we'll get the upperbound from bash's ulimit
    local fd_MAX=$(ulimit -n)
    while [[ $fd -le $fd_MAX && -e /proc/$$/fd/$fd ]]; do
        ((++fd))
    done

    if [ $fd -gt $fd_MAX ]; then
        echo "Could not find available file descriptor" >&2
        $fd=-1
        success=$EMFILE
    else
        eval "exec ${fd}${mode} \"$file\""
        local success=$?
        if ! [ $success ]; then
            echo "Could not open \"$file\" in \"$mode\" mode; error: $success" >&2
            fd=-1
        fi
    fi

    eval "$var=$fd"
    return $success;
}

One would use the foregoing function as follows to open files for input and output:

openNextFd "path/to/some/file" "read" "inputfile"
# opens 'path/to/some/file' for read access and stores
# the descriptor in 'inputfile'

openNextFd "path/to/other/file" "overwrite" "log"
# truncates 'path/to/other/file', opens it in write mode, and
# stores the descriptor in 'log'

And one would then use the preceding descriptors as usual for reading and writing data:

read -u $inputFile data
echo "input file contains data \"$data\"" >&$log