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    % set hex 3A
    3A
    % scan $hex %x decimal
    1
    % set decimal
    58
'''Decimal to hex:'''
 format %4.4X $decimalNumber
Notice that the ".4" part gives leading zeroes, and does
'''not''' have to do with the fractional (right-of-the-point)
part of the number.

'''Character to hex''':
 format %4.4X [scan $c %c]

Notice that "[[scan $c %c]]" only does what one wants with newer
Tcl's, those since version 8.3.0+.

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[mfi]: Can someone suggest a pure Tcl replacement for xxd UNIX command
(creates a hex dump of a given string)?


[RS]: Sure enough:
 proc string2hex {string} {
    set where 0
    set res {}
    while {$where<[string length $string]} {
	set str [string range $string $where [expr $where+15]]
	if {![binary scan $str H* t] || $t==""} break
	regsub -all (....) $t {\1 } t4
	regsub -all (..) $t {\1 } t2
	set asc ""
	foreach i $t2 {
	    scan $i %2x c
	    append asc [expr {$c>=32 && $c<=127? [format %c $c]: "."}]
	}
	lappend res [format "%7.7x: %-42s %s" $where $t4  $asc]
	incr where 16
    }
    join $res \n
 }
See also [Dump a file in hex and ASCII]

[LH}: Here's something similar, but speeds things up by using 
external storage for a character map that is generated one time:



proc xxd2 {charMapname convertString} {
    upvar $charMapname mycharMap

    #generate a character map for displaying hex equiv of chars [0..127]
    if { 0 == [string length $mycharMap] } {
	# init charmap
	for {set i 0 } { $i < 128 } { incr i } { 
	    append mycharMap "[format " \\%03o {%02x }" $i $i ]"
	}
    }
 
    string map $mycharMap $convertString

}

#example:

set hexcharMap ""  ;# global variable

xxd2 hexcharMap "abcdz"           ;# slow, even if initialized

string map $hexcharMap "abcdez"   ;# much faster

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[Arts and crafts of Tcl-Tk programming]