Shell implementation of cpio

cpio.sh

#!/bin/sh -e
# See http://people.freebsd.org/~kientzle/libarchive/man/cpio.5.txt
#
# Missing features:
#  - hard links
#  - block/character devices
#  - cpio does not erase existing files by default

set -e

DEBUG=''

usage() {
    printf "Usage: %s: -i [-d] [-m] [-v] \n" $0
    exit 2
}

EXTRACT=''
VERBOSE=''
MAKEDIR=''
PRESERVEMTIME=''
while getopts dimv name
do
    case $name in
    d) MAKEDIR='y';;
    i) EXTRACT='y';;
    m) PRESERVEMTIME='y';;
    v) VERBOSE='y';;
    ?) usage;;
    esac
done

if [ -z "$EXTRACT" ]; then
    usage
fi

settrap() {
    trap "trap - INT HUP 0; $1 exit 2" INT HUP
    trap "trap - INT HUP 0; $1" 0
}

# Print an error if we do not exit on our own terms
settrap 'echo "Error during extraction.";'

# readnpad n
# Read n bytes, and discard padding, such that n+pad is a multiple of 4
readnpad() {
    head -c "$1"
    head -c "$(( (4 - ($1 % 4)) % 4 ))" > /dev/null
}

# readnpadheader n
# Read n bytes, and discard padding, such that header size is a multiple of 4
# Also discards null-termination
readnpadheader() {
    local n="$(($1-1))"
    # 110 bytes of header plus terminating \0
    local extra=111
    local pad="$(( (4 - (($n + $extra) % 4)) % 4 + 1 ))"
    head -c "$n"
    head -c "$pad" > /dev/null
}

while true; do
    # FIXME: endianness?!?!? Format is middle-weird endian:
    # The four-byte integer is stored with the most-significant 16 bits
    # first followed by the least-significant 16 bits.  Each of the two
    # 16 bit values are stored in machine-native byte order.

    eval "`head -c 110 | awk '{
        if (substr($0, 0, 6) != "070701") {
            print "Invalid magic" > "/dev/stderr"
            exit 1
        }
        i=0;
        print "ino=\\"$((0x" substr($0, 7+i++*8, 8) "))\\""
        print "mode=\\"$((0x" substr($0, 7+i++*8, 8) "))\\""
        print "uid=\\"$((0x" substr($0, 7+i++*8, 8) "))\\""
        print "gid=\\"$((0x" substr($0, 7+i++*8, 8) "))\\""
        print "nlink=\\"$((0x" substr($0, 7+i++*8, 8) "))\\""
        print "mtime=\\"$((0x" substr($0, 7+i++*8, 8) "))\\""
        print "filesize=\\"$((0x" substr($0, 7+i++*8, 8) "))\\""
        print "devmajor=\\"$((0x" substr($0, 7+i++*8, 8) "))\\""
        print "devminor=\\"$((0x" substr($0, 7+i++*8, 8) "))\\""
        print "rdevmajor=\\"$((0x" substr($0, 7+i++*8, 8) "))\\""
        print "rdevminor=\\"$((0x" substr($0, 7+i++*8, 8) "))\\""
        print "namesize=\\"$((0x" substr($0, 7+i++*8, 8) "))\\""
        print "check=\\"$((0x" substr($0, 7+i++*8, 8) "))\\""
    }'`"

    type="$((($mode & 0170000) >> 12))"
    filename="`readnpadheader $namesize`"

    if [ "$type" -eq 0 -a "$filename" = "TRAILER!!!" ]; then
        # We reached the end of the archive (isn't that awesome?)
        if [ -n "$DEBUG" ]; then
            echo "EOF"
        fi
        # Do not print error
        settrap 'true;'
        exit 0
    fi

    if [ -n "$VERBOSE" ]; then
        echo "$filename"
    fi

    if [ -n "$MAKEDIR" ]; then
        mkdir -p "`dirname $filename`"
    fi

    case $type in
    4)
        if [ ! -d "$filename" ]; then
            mkdir "$filename"
        fi
        ;;
    8)
        readnpad $filesize > "$filename"
        ;;
    10)
        target="`readnpad $filesize`"
        ln -sTf "$target" "$filename"
        ;;
    *)
       echo "Type $type unsupported" 1>&2
       exit 1
       ;;
    esac

    # Crude octal conversion (also contains SUID/SGID bits)
    perm="$((($mode & 07000) >> 9))$((($mode & 00700) >> 6))$((($mode & 00070) >> 3))$(($mode & 00007))"

    # Do not update ownership and permissions for symlinks
    if [ "$type" -ne 10 ]; then
        chown $uid:$gid "$filename"
        chmod $perm "$filename"
    fi
    if [ -n "$PRESERVEMTIME" ]; then
        touch -c -h -d "@$mtime" "$filename"
    fi
done

Performance

time sudo cpio -idm < ../bash-4.2-37.4.mga3.x86_64.cpiodec

Proper cpio:
real    0m0.031s
user    0m0.003s
sys     0m0.027s

Using dd:
real    0m3.277s
user    0m0.103s
sys     0m0.187s

Using head -c:
real    0m3.651s
user    0m0.080s
sys     0m0.270s

Using gawk to decode header:
real    0m1.634s
user    0m0.343s
sys     0m0.187s

And mawk:
real    0m1.320s
user    0m0.053s
sys     0m0.177s

Not there yet...

How does head -c # compare in speed?

Remove unnecessary variable generation from awk (ino, etc), and use awk's int() function to convert the numbers to hex. Type and perm can also be calculated inside awk; not sure how that affects performance.
awk could also print out a combined uidgid variable?