m4, 70 bytes

Result

AC

Duration

706ms

Code [DL]

m4wrap(`syscmd(`sed "1h
\${:1
x
s/ \$//
t1
x
:2
s/K /KK/
t2
h
}
g"')')

stdin

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          K                                       

stdout

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          KKKKKKKKKKKKKKK

stderr

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7ffc554e7ec0 /* 7 vars */) = 0
11    execve("/bin/cat", ["cat", "-"], 0x55a1d96e9d30 /* 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"], 0x55a1d96e9d40 /* 7 vars */) = 0
13    execve("/bin/sh", ["sh", "-c", "sed \"1h\n\\${:1\nx\ns/ \\$//\nt1\nx\n:2\n"...], 0x7ffd79e23570 /* 7 vars */) = 0
13    execve("/bin/sed", ["sed", "1h\n${:1\nx\ns/ $//\nt1\nx\n:2\ns/K /KK"...], 0x55dc9ad42d20 /* 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 +++