vim, 29 bytes

Result

WA

Duration

913ms

Code [DL]

:%s/.*\(.\)\(\1\| \).*/\1/
ZZ

Disassembly

:%s/.*\(.\)\(\1\| \).*/\1/<NL>ZZ

stdin

761 115
422 227
435 779
951 412
549 995
377 167
675 643
615 518
196 671
377 779
533 687
833 344
137 332
947 671
758 252
239 289
822 215
988 388
151 165
911 884
792 694
969 441
151 568
287 876
355 158
911 998
747 461
312 121
341 885
438 833
214 113
725 847

stdout

1
2
7
1
9
7
5
5
6
7
3
4
3
7
8
9
2
8
1
8
2
4
1
7
5
9
7
2
8
3
1
5

stderr

strace

11    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7fffc352d4d0 /* 9 vars */) = 0
12    execve("/bin/cat", ["cat", "-"], 0x562858fb4d00 /* 9 vars */) = 0
12    +++ exited with 0 +++
11    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
13    execve("/usr/bin/vim", ["/usr/bin/vim", "-N", "-u", "NONE", "-i", "NONE", "-s", "/volume/CODE", "/tmp/out"], 0x5628593f4288 /* 10 vars */) = 0
13    +++ exited with 0 +++
11    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=13, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
14    execve("/bin/cat", ["cat", "/tmp/out"], 0x562858fb4c98 /* 9 vars */) = 0
14    +++ exited with 0 +++
11    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=14, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
15    execve("/bin/rm", ["rm", "/tmp/out"], 0x562858fb4c98 /* 9 vars */) = 0
15    +++ exited with 0 +++
11    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=15, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
11    +++ exited with 0 +++