need a shell script to convert big endian to little endian - bash

Question

I need a shell script program to print the hexadecimal number from big endian to little endian
For example
Input: my virtual address = 00d66d7e
Output: 7e6dd600
How can I can I create this in a bash script?

Answer 1

Just had to do this... but from decimal to little endian.. adapting that here:
echo 00d66d7e | tac -rs .. | echo "$(tr -d '\n')"
achieves the desired result, for arbitrarily sized hexadecimal representations of unsigned integers.
(h/t 'tac -rs' MestreLion, very nice!)

Answer 2

For 32 bit addresses, assuming it's zero padded:
v=00d66d7e
echo ${v:6:2}${v:4:2}${v:2:2}${v:0:2}
# 7e6dd600

Answer 3

Based on Karoly's answer you could use the following script, reading an argument or piped input:
#!/bin/bash
# check 1st arg or stdin
if [ $# -ne 1 ]; then
if [ -t 0 ]; then
exit
else
v=`cat /dev/stdin`
fi
else
v=$1
fi
i=${#v}
while [ $i -gt 0 ]
do
i=$[$i-2]
echo -n ${v:$i:2}
done
echo
For e.g. you could save this script as endian.sh and make it executable with:
chmod u+x endian.sh
Then:
echo 00d66d7e | ./endian.sh
gives you:
7e6dd600
For a different length string:
echo d76f411475428afc90947ee320 | ./endian.sh
result would be:
20e37e9490fc8a427514416fd7
#Update: Modified the script to accept the input either as an argument or from stdin, addressing Freewind's request. So now:
./endian.sh d76f411475428afc90947ee320
also works and gives you:
20e37e9490fc8a427514416fd7

Answer 4

This works for dash (and many other shells) :
v=0x12345678
v2=$(( (v<<8 & 0xff00ff00) | (v>>8 & 0xff00ff) ))
v2=$(( (v2<<16 & 0xffff0000) | v2>>16 ))
printf '0x%08x\n' $v2
Result should be "0x78563412"
${v:6:2} is for bash.

Answer 5

In response to Freewind's comment request and building off of hutheano's great answer, I wrote my own bash script and I include a condensed version below. The full script can be downloaded here.
The following implementation accounts for odd length strings, 0x or \x prefixes, and multiple output formats and can be used like the following:
$ be2le d76f411475428afc90947ee320 0xaaff 0xffa '\x3'
20e37e9490fc8a427514416fd7
0xffaa
0xfa0f
\x03
be2le bash script
#!/bin/bash
args=()
format=preserve
delimiter="\n"
nonewline=false
join=false
strip=false
while (( "$#" )); do
case "$1" in
-h|--help) usage;;
-f) format=$2; shift 2;;
--format=*) format="${1#*=}"; shift;;
-d) delimiter=$2; shift 2;;
--delimiter=*) delimiter="${1#*=}"; shift;;
-n|--no-newline) nonewline=true; shift;;
-j|--join) join=true; shift;;
-s|--strip-null) strip=true; shift;;
-*|--*) echo "Error: unsupported flag $1 specified"; exit 1;;
*) args=( "${args[#]}" "$1" ); shift;;
esac
done
case "$format" in
preserve);;
int) prefix="0x";;
char) prefix="\x";;
raw) ;;
*) echo "Error: unsupported format $format"; exit 1;;
esac
n=0
parts=()
for arg in ${args[#]}; do
digest=""
prefix=""
# remove prefix if string begins with "0x"
if [[ $arg =~ ^[0\\]x ]]; then
if [ "$format" == "preserve" ]; then
prefix=${arg:0:2}
fi
arg=${arg:2}
fi
# zero-pad if string has odd length
if [ $[${#arg} % 2] != 0 ]; then
arg="0$arg"
fi
part=""
i=${#arg}
while [ $i -gt 0 ]; do
i=$[$i-2]
byte=${arg:$i:2}
if [ $strip == true ] && [ -z "$part" ] && [ $byte == "00" ]; then
continue
fi
case "$format" in
int) part="$part"'0x'"$byte ";;
char) part="$part\x$byte";;
raw) part="$part$(printf "%b" "\x$byte")";;
*) part="$part$byte";;
esac
done
digest="$prefix$digest$part"
parts=( "${parts[#]}" "$digest" )
n=$[$n+1]
done
if [ $join == true ]; then
case "$format" in
*) printf "%s" "${parts[#]}";;
esac
else
i=0
for part in "${parts[#]}"; do
if [[ $(($i + 1)) < $n ]]; then
printf "%s$delimiter" "$part"
else
printf "%s" "$part"
fi
i=$(($i+1))
done
fi
if [ $nonewline == false ]; then
echo
fi

Answer 6

This script is for flipping 16 bit data.
#!/bin/bash
if [ -t 0 ]; then exit; fi
data=`cat /dev/stdin | od -An -vtx1 | tr -d ' ' | tr -d '\n'`
length=${#data}
i=0
while [ $i -lt $length ]; do
echo -n -e "\x${data:$[$i+2]:2}"
echo -n -e "\x${data:$[$i]:2}"
i=$[$i+4]
done