m4, 54 bytes

Result

WA

Duration

684ms

Code [DL]

syscmd(`sed "1h
${:1
x
s/ $//
t1
x
s/K /KK/
t1
h}
g"')

stdin

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      K                                           

stdout

stderr

sh: syntax error: bad substitution

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7ffd60958860 /* 7 vars */) = 0
12    execve("/usr/bin/m4", ["/usr/bin/m4", "/volume/CODE"], 0x55b969490d40 /* 7 vars */ <unfinished ...>
11    execve("/bin/cat", ["cat", "-"], 0x55b969490d30 /* 7 vars */) = 0
11    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=11, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
12    <... execve resumed> )            = 0
13    execve("/bin/sh", ["sh", "-c", "sed \"1h\n${:1\nx\ns/ $//\nt1\nx\ns/K /"...], 0x7ffd1dd6bcc0 /* 7 vars */) = 0
13    +++ exited with 2 +++
12    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=13, si_uid=0, si_status=2, si_utime=0, si_stime=0} ---
12    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
10    +++ exited with 0 +++