Rating:
It looks like a lot of characters are filtered out.
Running this reveals chars which are left:
;1234567890-=qwertyuiop[]\asdfghjkl;'zxcvbnm,./`~!@#$%^&*()_+QWERTYUIOP{}|ASDFGHJKL:"ZXCVBNM<>?
bash: -c: line 0: syntax error near unexpected token `;'
bash: -c: line 0: `;-;#$^_+QWERTYUIOP{}|ASDFGHJKL:ZXCVBNM<>'
Let's check the environment variable `_`:
> `${_}`
declare -x A="T"
declare -x AB="HI"
declare -x ABC="ISN"
declare -x ABCD="OTTH"
declare -x ABCDE="EFLAG"
declare -x ABCDEF="BUTMAY"
declare -x ABCDEFG="BEITCAN"
declare -x ABCDEFGH="HELPGETT"
declare -x ABCDEFGHI="INGFLAG:D"
declare -x OLDPWD
declare -x PWD="/home/special"
declare -x SHELL=""
declare -x SHLVL="1"
declare -x _="export"
Finally: ${PWD:${#ABCDEF}:${#A}}${PWD:${#A}:${#A}}
^ parses to 'sh'
Flag{B4sh_subst1tut1on_is_gud!}
