I'm using ubuntu 20.04 via windows 10 wsl. When I try to compile sqlite3.c in bash I get these errors.

tom@sp4:~/sqlite$ ls temp
shell.c  sqlite3.c  sqlite3.h  sqlite3ext.h
tom@sp4:~/sqlite$ cd temp
tom@sp4:~/sqlite/temp$ gcc -I ~/sqlite sqlite3.c -o sqlite.o
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/Scrt1.o: in function `_start':
(.text+0x24): undefined reference to `main'
/usr/bin/ld: /tmp/ccVahWO8.o: in function `pthreadMutexAlloc':
sqlite3.c:(.text+0x45bf): undefined reference to `pthread_mutexattr_init'
/usr/bin/ld: sqlite3.c:(.text+0x45d0): undefined reference to `pthread_mutexattr_settype'
/usr/bin/ld: sqlite3.c:(.text+0x45ef): undefined reference to `pthread_mutexattr_destroy'
/usr/bin/ld: /tmp/ccVahWO8.o: in function `pthreadMutexTry':
sqlite3.c:(.text+0x46c4): undefined reference to `pthread_mutex_trylock'
/usr/bin/ld: /tmp/ccVahWO8.o: in function `sqlite3ThreadCreate':
sqlite3.c:(.text+0x954b): undefined reference to `pthread_create'
/usr/bin/ld: /tmp/ccVahWO8.o: in function `sqlite3ThreadJoin':
sqlite3.c:(.text+0x95e1): undefined reference to `pthread_join'
/usr/bin/ld: /tmp/ccVahWO8.o: in function `unixDlOpen':
sqlite3.c:(.text+0x124d1): undefined reference to `dlopen'
/usr/bin/ld: /tmp/ccVahWO8.o: in function `unixDlError':
sqlite3.c:(.text+0x124f4): undefined reference to `dlerror'
/usr/bin/ld: /tmp/ccVahWO8.o: in function `unixDlSym':
sqlite3.c:(.text+0x12547): undefined reference to `dlsym'
/usr/bin/ld: /tmp/ccVahWO8.o: in function `unixDlClose':
sqlite3.c:(.text+0x12581): undefined reference to `dlclose'
collect2: error: ld returned 1 exit status
tom@sp4:~/sqlite/temp$

Can anyone tell me why?

gcc -I ~/sqlite sqlite3.c -o sqlite.o
gcc tried to make an executable file.
If you want an object file you have to pass a -c option to stop gcc rigth after assembling the object file.
gcc -c -I ~/sqlite sqlite3.c -o sqlite.o

Similar to above why does

gcc -I ~/sqlite sqlite3.c shell.c -o sqlite3

produce errors. Does shell.c have other dependencies apart from sqlite3.c?

tom@sp4:~/sqlite$ ls
LICENSE.md          art            config.sub    fts5.h         lemon          manifest.uuid  parse.h        sqlite.pc.in   sqlite3ext.h
Makefile            autoconf       configure     fts5parse.c    lempar.c       mkkeywordhash  parse.out      sqlite3.1      sqlite3session.h
Makefile.in         compat         configure.ac  fts5parse.h    libsqlite3.la  mkso.sh        parse.sql      sqlite3.c      src
Makefile.linux-gcc  config.guess   contrib       fts5parse.out  libtool        mksourceid     parse.y        sqlite3.h      test
Makefile.msc        config.h       doc           fts5parse.sql  ltmain.sh      mptest         shell.c        sqlite3.lo     tool
README.md           config.h.in    ext           fts5parse.y    magic.txt      opcodes.c      spec.template  sqlite3.o      tsrc
VERSION             config.log     fossil        install-sh     main.mk        opcodes.h      sqlite.fossil  sqlite3.pc     vsixtest
aclocal.m4          config.status  fts5.c        keywordhash.h  manifest       parse.c        sqlite.o       sqlite3.pc.in

tom@sp4:~/sqlite$ gcc -I ~/sqlite sqlite3.c shell.c -o sqlite3

/usr/bin/ld: /tmp/ccYlh56d.o: in function `pthreadMutexAlloc':
sqlite3.c:(.text+0x45c4): undefined reference to `pthread_mutexattr_init'
/usr/bin/ld: sqlite3.c:(.text+0x45d5): undefined reference to `pthread_mutexattr_settype'
/usr/bin/ld: sqlite3.c:(.text+0x45f4): undefined reference to `pthread_mutexattr_destroy'
/usr/bin/ld: /tmp/ccYlh56d.o: in function `pthreadMutexTry':
sqlite3.c:(.text+0x46c9): undefined reference to `pthread_mutex_trylock'
/usr/bin/ld: /tmp/ccYlh56d.o: in function `sqlite3ThreadCreate':
sqlite3.c:(.text+0x9556): undefined reference to `pthread_create'
/usr/bin/ld: /tmp/ccYlh56d.o: in function `sqlite3ThreadJoin':
sqlite3.c:(.text+0x95ec): undefined reference to `pthread_join'
/usr/bin/ld: /tmp/ccYlh56d.o: in function `unixDlOpen':
sqlite3.c:(.text+0x1251d): undefined reference to `dlopen'
/usr/bin/ld: /tmp/ccYlh56d.o: in function `unixDlError':
sqlite3.c:(.text+0x12540): undefined reference to `dlerror'
/usr/bin/ld: /tmp/ccYlh56d.o: in function `unixDlSym':
sqlite3.c:(.text+0x12593): undefined reference to `dlsym'
/usr/bin/ld: /tmp/ccYlh56d.o: in function `unixDlClose':
sqlite3.c:(.text+0x125cd): undefined reference to `dlclose'
collect2: error: ld returned 1 exit status

tom@sp4:~/sqlite$

Because you need link the pthread and dl libaries.

gcc -I ~/sqlite sqlite3.c shell.c -o sqlite3 -ldl -pthread

Thanks Keith and anon.