diff options
| -rwxr-xr-x | tmpoverlay | 124 |
1 files changed, 124 insertions, 0 deletions
diff --git a/tmpoverlay b/tmpoverlay new file mode 100755 index 0000000..313309d --- /dev/null +++ b/tmpoverlay @@ -0,0 +1,124 @@ +#!/bin/sh + +usage() { + cat << EOF +usage: tmpoverlay [OPTIONS] SOURCE [DESTINATION] + +Create a tmpfs-backed overlayfs for SOURCE, which is one or more directories +separated by colons. If DESTINATION is specified, mount the result at +DESTINATION. Otherwise, mount it on top of the last SOURCE. + +To free the memory, simply unmount the DESTINATION. + +tmpoverlay is not fully race-safe. It tries to match the ownership and +permissions of the tmpfs to the final SOURCE, but this is done in a step before +mounting. Therefore, any permission changes immediately before mounting may be +hidden by the mount. + +options: + -h print this help + -k don't clean up temporary directory + -o additional overlayfs mount options + -n pass -n to mount (don't write to /etc/mtab) + -t additional tmpfs mount options + -T temporary directory + -v verbose mode + +examples: + tmpoverlay / /tmp/my_root # make a thin copy of root + tmpoverlay /etc # make read-only /etc writable +EOF +} + +log() { + fmt=$1 + shift + printf "tmpoverlay: $fmt\n" "$@" >&2 +} + +verb() { + if [ -n "$verbose" ]; then + log "$@" + fi +} + +die() { + log "$@" + exit 1 +} + +args=$(getopt -n tmpoverlay hko:nt:T:v "$@") || { usage >&2; exit 1; } +eval set -- "$args" +unset args + +unset no_mtab extra_overlayfs_opts tmpfs_opts tmpdir verbose +while true; do + case "$1" in + '-h') usage; exit 0;; + '-o') + [ -n "$2" ] && extra_overlayfs_opts="$extra_overlayfs_opts,$2" + shift 2 + ;; + '-n') no_mtab=1; shift;; + '-t') + [ -n "$2" ] && tmpfs_opts="${tmpfs_opts:+$tmpfs_opts,}$2" + shift 2 + ;; + '-T') + [ -d "$2" ] || die 'tmpdir "%s" is not a directory' "$2" + tmpdir=$2 + shift 2 + ;; + '-v') verbose=1; shift;; + '--') shift; break;; + *) die "getopt failure" + esac +done + +case "$#" in + 1) lowerdir=$1 dir=${1##*:};; + 2) lowerdir=$1 dir=$2;; + *) usage >&2; exit 1;; +esac + +ld= +set -f +savedIFS=$IFS IFS=: +for d in $lowerdir; do + d=$(realpath -e "$d") + [ -d "$d" ] || die 'lowerdir "%s" is not a directory' "$d" + ld=$ld:$d +done +IFS=$savedIFS +set +f +ld=${ld#:} +[ -d "$dir" ] || die 'destination "%s" is not a directory' "$dir" + +err() { + if [ -n "$mounted_tmpdir" ]; then + umount "$tmpdir" || exit 1 + fi + if [ -n "$tmpdir" ] && [ -n "$created_tmpdir" ]; then + rm -r "$tmpdir" + fi + exit 1 +} + +if [ -z "$tmpdir" ]; then + tmpdir=$(umask 077; mktemp -d --tmpdir tmpoverlay.XXXXXXXXXX) || exit 1 + created_tmpdir=1 + verb 'created tmpdir: %s' "$tmpdir" +fi +mount -t tmpfs ${tmpfs_opts:+-o "$tmpfs_opts"} ${verbose:+-v} tmpfs "$tmpdir" || err +mounted_tmpdir=1 +upperdir="$tmpdir/upper" +workdir="$tmpdir/work" +mkdir "$upperdir" "$workdir" || err +chown --reference="${ld##*:}" "$upperdir" "$workdir" +chmod --reference="${ld##*:}" "$upperdir" "$workdir" + +overlayfs_opts="lowerdir=$ld,upperdir=$upperdir,workdir=$workdir$extra_overlayfs_opts" +mount -t overlay -o "$overlayfs_opts" ${no_mtab:+-n} ${verbose:+-v} overlay "$dir" || err + +umount "$tmpdir" || err +rm -r "$tmpdir" || err |