0. System Information
OS info
Let's starting gaining some knowledge of the OS running
(cat /proc/version || uname -a ) 2>/dev/null
lsb_release -a 2>/dev/null # old, not by default on many systems
cat /etc/os-release 2>/dev/null # universal on modern systems
cat /etc/issue
cat /etc/*-release
cat /etc/lsb-release
cat /etc/redhat-release
Kernel version
cat /proc/version
uname -a
uname -mrs
rpm -q kernel
dmesg | grep Linux
ls /boot | grep vmlinuz
Path
If you have written permissions on any folder inside the PATH variable you may be able to hijacking some libraries or binaries:
echo $PATH
Env info
Interesting information, passwords or API keys in the environment variables?
(env || set) 2>/dev/null
Kernel exploits
Check the kernel version and if there is some exploit that can be used to escalate privileges
cat /proc/version
uname -a
searchsploit "Linux Kernel"