m4, 56 bytes

Result

AC

Duration

689ms

Code [DL]

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

stdin

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                               K                  

stdout

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                               KKKKKKKKK

stderr

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7fffdc3ad650 /* 7 vars */) = 0
11    execve("/bin/cat", ["cat", "-"], 0x565529084d30 /* 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("/usr/bin/m4", ["/usr/bin/m4", "/volume/CODE"], 0x565529084d40 /* 7 vars */) = 0
13    execve("/bin/sh", ["sh", "-c", "sed \"1h\n\\${:1\nx\ns/ \\$//\nt1\nx\ns/K"...], 0x7ffc35d89660 /* 7 vars */) = 0
13    execve("/bin/sed", ["sed", "1h\n${:1\nx\ns/ $//\nt1\nx\ns/K /KK/\nt"...], 0x562c949f2d08 /* 7 vars */) = 0
13    +++ exited with 0 +++
12    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=13, si_uid=0, si_status=0, 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 +++