golang, 56 bytes

Result

WA

Duration

989ms

Code [DL]

s[];main(){for(;gets(&s);)putchar(s[1]-s[2]?s[4]:s[1]);}

stdin

737 738
215 119
322 884
733 336
582 583
615 337
828 288
255 517
399 797
665 561
522 227
418 918
638 486
755 776
199 368
744 786
639 994
682 424
369 776
517 782
135 331
688 485
153 433
329 821
334 715
423 356
851 379
645 453
655 574
649 474
726 277
876 663

stdout

stderr

can't load package: package main: 
/tmp/code.go:1:1: expected 'package', found s
/root/script: line 7: /tmp/code: not found
rm: can't remove '/tmp/code': No such file or directory

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7fff7e60e600 /* 7 vars */) = 0
11    execve("/usr/bin/realpath", ["realpath", "/volume/CODE"], 0x55b367514d18 /* 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("/bin/ln", ["ln", "-sf", "/volume/CODE", "/tmp/code.go"], 0x55b367514d38 /* 7 vars */) = 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} ---
13    execve("/usr/bin/go", ["/usr/bin/go", "build", "-o", "/tmp/code", "/tmp/code.go"], 0x55b3683b3028 /* 7 vars */) = 0
15    +++ exited with 1 +++
16    +++ exited with 1 +++
14    +++ exited with 1 +++
17    +++ exited with 1 +++
13    +++ exited with 1 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=13, si_uid=0, si_status=1, si_utime=0, si_stime=1} ---
18    execve("/bin/cat", ["cat", "-"], 0x55b367514d08 /* 7 vars */ <unfinished ...>
19    execve("/tmp/code", ["/tmp/code"], 0x55b367514cf0 /* 7 vars */) = -1 ENOENT (No such file or directory)
18    <... execve resumed>)             = 0
18    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=18, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
19    +++ exited with 127 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=19, si_uid=0, si_status=127, si_utime=0, si_stime=0} ---
20    execve("/bin/rm", ["rm", "/tmp/code.go", "/tmp/code"], 0x55b367514cf0 /* 7 vars */) = 0
20    +++ exited with 1 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=20, si_uid=0, si_status=1, si_utime=0, si_stime=0} ---
10    +++ exited with 1 +++