Bash command to print string in unambiguous form

You can access the just-executed command line with the history built-in. (I have no idea why history 1 prints the just-executed command line but fc -nl -1 prints the previous commmand, as does fc -nl 0.)

PROMPT_COMMAND='echo -en "\033[38;5;2m"; history 1; echo -en "\033[0m\n"'

This prints a number before the command text. Here's a version that removes the number. (It may be incorrect if you go beyond 99999 history lines, I don't know how bash formats the number then.)

prompt_function () {
  local prompt_history="$(history 1)"
  prompt_history=${prompt_history:7}
  echo -En $'\033[38;5;2m'"$prompt_history"$'\033[0m\n'
}
PROMPT_COMMAND=prompt_function

(Note that echo -en ..."$prompt_history"... would expand backslashes in the command line, so I use echo -E and let the shell expansion generate the control characters with $''.).

if ($mystr:q =~ *'\
'*) echo yes

should work in some implementations and versions of csh (like the csh and tcsh ones found on Debian). In some others (like the one found on Solaris 10), you may have better luck with

set nl = '\
'
if ($mystr:q =~ *$nl:q*) echo yes

Most people have given up trying to write reliable scripts with csh by now. Why would you use csh in this century?

This code works for me (outputs no) in tcsh 6.17.00 (Astron) 2009-07-10 (x86_64-unknown-linux) options wide,nls,dl,al,kan,rh,color,filec

set mystr = '1234ABC\
 -------\
FOOBAR'
if ($mystr:q !~ *'\
'*) then
  echo yes
else
  echo no
endif

Note that if you do:

set var = `some command`

csh stores each word (blank separated) of the output of some command in several elements of the var array.

With:

set var = "`some command`"

it stores each non-empty line in elements of the array.

It looks like one cannot¹ store the output of a command whole into a variable in (t)csh, so your only option would be:

set var = "`some command`" # note that it removes the empty lines
if ($#var == 1)...

¹ Strictly speaking, that's not true, one could do something like:

set x = "`some command | paste -d. /dev/null -`"
set var = ""
set nl = '\
'

foreach i ($x:q)
  set i = $i:s/.//:q
  set var = $var:q$i:q$nl:q
end

^{(of course, it may not work in all csh implementations/versions)}