How to write binary data in Bash

Question

Let us say there is a variable in bash script with value \"001\" how can i write this binary data into a file as bits (as \"001\" not \"1\") echo writes it as string but i w

Answer 1

Using bc (in CentOS 7.4):

VALUE="3F"
INPUT_BASE=16
OUTPUT_BASE=2
printf "%b\n" $(bc <<< "ibase=$INPUT_BASE; obase=$OUTPUT_BASE; $VALUE")

RESULT: 111111

If leading zeros are needed then:

VALUE="3F"
INPUT_BASE=16
OUTPUT_BASE=2
printf "%16b\n" $(bc <<< "ibase=$INPUT_BASE; obase=$OUTPUT_BASE; $VALUE") | sed 's^ ^0^g'

RESULT: 0000000000111111

Note the use of 16 in the printf format; change value to limit leading-zero count

Answer 2

A couple of more general functions to output integers to a file:

le16 () { # little endian 16 bit;  1st param: integer to 2nd param: file 
  v=`awk -v n=$1 'BEGIN{printf "%04X", n;}'`
  echo -n -e "\\x${v:2:2}\\x${v:0:2}" >> $2
}

le32 () { # 32 bit version
  v=`awk -v n=$1 'BEGIN{printf "%08X", n;}'`
  echo -n -e "\\x${v:6:2}\\x${v:4:2}\\x${v:2:2}\\x${v:0:2}" >> $2
}

Answer 3

Just use the %b in the printf:

printf "%b" "\012"
printf "%b" "\x0a"

%b - Print the associated argument while interpreting backslash escapes in there

so, above both prints binary value for the decimal 10.

printf "%b" "\x0a" | od -bc

output

0000000   012                                                            
          \n      

you can even mix

printf "%b" "\12\xa\n" | od -bc

0000000   012 012 012                                                    
          \n  \n  \n            

Answer 4

You can write arbitrary bytes in hex or octal with:

printf '\x03' > file   # Hex
printf '\003' > file   # Octal

If you have binary, it's a bit tricker, but you can turn it into octal with:

printf '%o\n' "$((2#00000011))"

which of course can be nested in the above:

binary=00000011
printf "\\$(printf '%o' "$((2#$binary))")" > file

Note that this only works with up to 8 bits. If you want to write longer values, you have to split it up into groups of 8.

Answer 5

Most often you want to write a pattern (for instance a 32 bit pattern).

# Write 32 bits 5 times to a file 
for i in {1..5}
do
   printf '\x12\x34\xAC\xCD' >> binaryfile.bin
done

Another example:

for i in {1..255}
do
    hx=$( printf "%x" $i )
    output="\x$hx\x00\x00\x00"
    printf "%b" $output >> binaryfile.bin 
done

Answer 6

I needed this recently, someone might find it useful :

#!/bin/bash

# generate bitmasks for a C array

# loop counter
NUMBER=0
MAX_BITMASK_LENGTH=8
# max loop value : 2^8 = 255
MAXVAL=$((2**MAX_BITMASK_LENGTH))
# line number
LINE=0

# echo start of C array
echo "    const long int bitmasks[${MAX_BITMASK_LENGTH}] = {"

# loop over range of values
while [ ${NUMBER} -le ${MAXVAL} ] ; do

    # use bc to convert to binary
    BINARY=`echo -e "obase=2\n${NUMBER}"| bc`

    # print current number, linenumber, binary
    printf "%12s, /* (%i) %8s */\n" ${NUMBER} ${LINE} ${BINARY}

    # increase loop value to next value
    NUMBER=$(((NUMBER*2)+1))

    # increment line number
    LINE=$((LINE+1))
done

# echo end of C array
echo "        };"

This outputs:

$ ./generate_bitmasks.sh
    const long int bitmasks[8] = {
           0, /* (0)        0 */
           1, /* (1)        1 */
           3, /* (2)       11 */
           7, /* (3)      111 */
          15, /* (4)     1111 */
          31, /* (5)    11111 */
          63, /* (6)   111111 */
         127, /* (7)  1111111 */
         255, /* (8) 11111111 */
        };