Diff for "BashFAQ/071"

How do I convert an ASCII character to its decimal (or hexadecimal) value and back?

If you have a known octal or hexadecimal value (at script-writing time), you can just use printf:

   # POSIX
   printf '\x27\047\n'

This prints two literal ' characters (27 is the hexadecimal ASCII value of the character, and 47 is the octal value) and a newline.

If you need to convert characters (or numeric ASCII values) that are not known in advance (i.e., in variables), you can use something a little more complicated:

• The following example was submitted quite recently and needs to be cleaned up and validated. In particular, the function names with periods in them are abusing an undocumented, possibly unintended bash parsing glitch.

        ###############################################################
        #
        #       Note about Ext Ascii and UTF-8 encoding
        #
        # for values 0x00 - 0x7f identical
        # for values 0x80 - 0x00 conflict between UTF-8 & ExtAscii
        # for values 0x100 -0x7FFFFFFF Only UTF-8 UTF-32
        #
        # value         EAscii   UTF-8                          UTF-16  UTF-32
        # 0x20          "\x20"  "\x20"                          \u0020  \U00000020
        # 0x20          "\x7f"  "\x7f"                          \u007f  \U0000007f
        # 0x80          "\x80"  "\xc2\x80"                      \u0080  \U00000080
        # 0xff          "\xff"  "\xc3\xbf"                      \u00ff  \U000000ff
        # 0x100         N/A     "\xc4\x80"                      \u0100  \U00000100
        # 0x1000        N/A     "\xc8\x80"                      \u1000  \U00001000
        # 0xffff        N/A     "\xef\xbf\xbf"                  \uffff  \U0000ffff
        # 0x10000       N/A     "\xf0\x90\x80\x80"              N/A     \U00010000
        # 0xfffff       N/A     "\xf3\xbf\xbf\xbf"              N/A     \U000fffff
        # 0x10000000    N/A     "\xfc\x90\x80\x80\x80\x80"      N/A     \U10000000
        # 0x7fffffff    N/A     "\xfd\xbf\xbf\xbf\xbf\xbf"      N/A     \U7fffffff
        # 0x80000000    N/A     N/A                             N/A     N/A
        # 0xffffffff    N/A     N/A                             N/A     N/A

        ###########################################################################
        ## ord family
        ###########################################################################
        # ord        <Return Variable Name> <Char to convert> [Optional Format String]
        # ord.hex    <Return Variable Name> <Char to convert>
        # ord.oct    <Return Variable Name> <Char to convert>
        # ord.utf8   <Return Variable Name> <Char to convert> [Optional Format String]
        # ord.eascii <Return Variable Name> <Char to convert> [Optional Format String]
        # ord.echo                      <Char to convert> [Optional Format String]
        # ord.hex.echo                  <Char to convert>
        # ord.oct.echo                  <Char to convert>
        # ord.utf8.echo                 <Char to convert> [Optional Format String]
        # ord.eascii.echo               <Char to convert> [Optional Format String]
        #
        # Description:
        # converts character using native encoding to its decimal value and stores
        # it in the Variable specified
        #
        #       ord
        #       ord.hex         output in hex
        #       ord.hex         output in octal
        #       ord.utf8        forces UTF8 decoding
        #       ord.eascii      forces eascii decoding
        #       ord.echo        prints to stdout
        function ord {
                printf -v "${1?Missing Dest Variable}" "${3:-%d}" "'${2?Missing Char}"
        }
        function ord.oct {
                ord "${@:1:2}" "%c"
        }
        function ord.hex {
                ord "${@:1:2}" "%x"
        }
        function ord.utf8 {
                LC_CTYPE=en_US.UTF8 ord "${@}"
        }
        function ord.eascii {
                LC_CTYPE=C ord "${@}"
        }
        function ord.echo {
                printf "${2:-%d}" "'${1?Missing Char}"
        }
        function ord.oct.echo {
                ord.echo "${@:1:1}" "%o"
        }
        function ord.hex.echo {
                ord.echo "${@:1:1}" "%x"
        }
        function ord.utf8.echo {
                LC_CTYPE=en_US.UTF8 ord.echo "${@}"
        }
        function ord.eascii.echo {
                LC_CTYPE=C ord.echo "${@}"
        }

        ###########################################################################
        ## chr family
        ###########################################################################
        # chr.utf8   <Return Variale Name> <Integer to convert>
        # chr.eascii <Return Variale Name> <Integer to convert>
        # chr        <Return Variale Name> <Integer to convert>
        # chr.oct    <Return Variale Name> <Octal number to convert>
        # chr.hex    <Return Variale Name> <Hex number to convert>
        # chr.utf8.echo                  <Integer to convert>
        # chr.eascii.echo                <Integer to convert>
        # chr.echo                       <Integer to convert>
        # chr.oct.echo                   <Octal number to convert>
        # chr.hex.echo                   <Hex number to convert>
        #
        # Description:
        # converts decimal value to character representation an stores
        # it in the Variable specified
        #
        #       chr                     Tries to guess output format
        #       chr.utf8                forces UTF8 encoding
        #       chr.eascii              forces eascii encoding
        #       chr.echo                prints to stdout
        #
        function chr.utf8 {
                local val
                printf -v ${val} '\\U%08x' "${2?Missing Value}"
                printf -v ${1?Missing Dest Variable} ${val}
        }
        function chr.eascii {
                local val
                [ ${1} -lt 0x100 ] || return 1
                printf -v ${val} '\\x%02x' "${2?Missing Value}"
                printf -v ${1?Missing Dest Variable} ${val}
        }
        function chr {
                if [ ${1} -lt 0x80 ]; then
                        chr.eascii "${@}"
                elif [ ${1} -lt 0x100 ]; then
                        if [ "${LC_CTYPE:-${LC_ALL:-}}" = "C" ]; then
                                chr.eascii "${@}"
                        else
                                chr.utf8 "${@}"
                        fi
                else
                        chr.utf8 "${@}"
                fi
        }
        function chr.oct {
                chr "${1}" "0${2}"
        }
        function chr.hex {
                chr "${1}" "0x${2}"
        }
        function chr.utf8.echo {
                local val
                printf -v ${val} "%08x" "${1?Missing Value}"
                printf \\U${val}
        }
        function chr.eascii.echo {
                local val
                printf -v ${val} "%02x" "${1?Missing Value}"
                printf \\x${val}
        }
        function chr.echo {
                if [ ${1} -lt 0x80 ]; then
                        chr.eascii.echo "${@}"
                elif [ ${1} -lt 0x100 ]; then
                        if [ "${LC_CTYPE:-${LC_ALL:-}}" = "C" ]; then
                                chr.eascii.echo "${@}"
                        else
                                chr.utf8.echo "${@}"
                        fi
                else
                        chr.utf8.echo "${@}"
                fi
        }
        function chr.oct.echo {
                chr.echo "${1}" "0${2}"
        }
        function chr.hex.echo {
                chr.echo "${1}" "0x${2}"
        }
   chr.echo $(ord.echo A)    # -> A
   ord.echo $(chr.echo 65)   # -> 65

Here are some older, simpler functions that do similar things:

# POSIX
# chr() - converts decimal value to its ASCII character representation
# ord() - converts ASCII character to its decimal value

chr() {
  printf \\$(printf '%03o' $1)
}
  
# Another version doing the octal conversion with arithmetic 
# faster as it avoids a subshell
chr () {
 printf \\$(($1/64*100+$1%64/8*10+$1%8))
}

# Another version using a temporary variable to avoid subshell.
# This one requires bash 3.1.
chr() {
  local tmp
  printf -v tmp '%03o' "$1"
  printf \\"$tmp"
}

ord() {
  printf '%d' "'$1"
}

# hex() - converts ASCII character to a hexadecimal value
# unhex() - converts a hexadecimal value to an ASCII character

hex() {
   printf '%x' "'$1"
}

unhex() {
   printf \\x"$1"
}

# examples:

chr $(ord A)    # -> A
ord $(chr 65)   # -> 65

The ord function above is quite tricky.

• Tricky? Rather, it's using a feature that I can't find documented anywhere -- putting a single quote in front of an integer. Neat effect, but how on earth did you find out about it? Source diving? -- GreyCat

  • It validates The Single Unix Specification: "If the leading character is a single-quote or double-quote, the value shall be the numeric value in the underlying codeset of the character following the single-quote or double-quote." (see printf() to know more) -- mjf