第一次更新BigSur，在命令行运行gcc出现了xcrun: error: invalid active developer path, missing xcrun的报错。
经搜索，需要重新安装命令行工具Command Line Tools，是因为更新系统时原来的 Command Line Tools 被自动卸载了，所以重新安装即可。
在终端中执行如下指令即可
Command Line Tools包含的工具很多，如下所示
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| ar
as
asa
bison
BuildStrings
c++
c89
c99
cc
clang
clang++
cmpdylib
codesign_allocate
CpMac
cpp
ctags
ctf_insert
DeRez
dsymutil
dwarfdump
dyldinfo
flex
flex++
g++
gatherheaderdoc
gcc
gcov
GetFileInfo
git
git-cvsserver
git-receive-pack
git-shell
git-upload-archive
git-upload-pack
gm4
gnumake
gperf
hdxml2manxml
headerdoc2html
indent
install_name_tool
ld
lex
libtool
lipo
lldb
lorder
m4
make
MergePef
mig
mkdep
MvMac
nasm
ndisasm
nm
nmedit
otool
pagestuff
projectInfo
ranlib
rebase
redo_prebinding
ResMerger
resolveLinks
Rez
RezDet
RezWack
rpcgen
segedit
SetFile
size
SplitForks
strings
strip
svn
svnadmin
svndumpfilter
svnlook
svnrdump
svnserve
svnsync
svnversion
unifdef
unifdefall
UnRezWack
unwinddump
what
xml2man
yacc
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