Shell Programming and Scripting

[root@Imperfecto_1 ~]# cat infile
a
b
c c
d d d g
e
f
[root@Imperfecto_1 ~]# awk '{ for(i=1;i<=NF;i++){a[$i]} } END{for (i in a) {print i}}' infile
a
b
c
d
e
f
g

man typeset
-a     Each name is an array variable (see Arrays above).

-A     Declares vname to be an associative array.
-a     Declares vname to be an indexed array.

awk ' {
        v = $0
        gsub (/[ \t]*/, x, v)
        for ( i = 1; i <= length(v); i++ )
        {
                d = substr ( v, i, 1 )
                A[d]
        }
} END {
        for ( k in A )
                print k
} ' file

$ cat input.txt
a
b hi
cc
d d d g
 e
f

$ sed "s/\(.\)/\1\n/g" input.txt | sort | uniq | grep -v "^ *$"
a
b
c
d
e
f
g
h
i