1. Metacharacters

?: one random character
[]: one character in the square bracket
cd -: change the directory which you worked before using last 'cd'.
;: execute orderly arranged commands divided by semicolon.
``(back quote): replace the string in the back quote with the result of the string.

2. stdin & stdout & stderr

CMD > FILE: overwrite FILE with stdout
set -o noclobber for preventing from overwriting
set +o noclobber for release set -o noclobber
cat > FILE: save stdout to FILE
CMD >> FILE: append stdout to FILE
CMD 2> FILE: overwrite stderr to FILE
CMD > FILE1 2> FILE2: the execution of CMD is success, stdout is saved in FILE1, else stderr is saved in FILE2.
If you don't want to save stderr, replace stderr file with /dev/null.
/dev/null is like trash bin, but unlike trash bin, nothing is left there.
CMD < FILE: read the content of FILE and execute CMD with it.

Shell variable can be used only in the shell in which the variable are defined.(can't be used in subshell)
Environment variable can be used in a subshell as well as the shell by which the subshell inherited.
set: print shell variables and environment variables.
env: print only environment variables.
export: change the shell variable to a env variable.
unset: release assigned variable.

Environment Setting File: the file in which commands are saved that are automatically executed whenever user logins.
/etc/profile: System Wide User Profiles that contain Linux system wide environment and other startup scripts.
~/.bash_profile, ~/.profile, ~/.bashrc: Bash Startup Files that contain commands that are to be executed on shell startup.
Actually, ~/.bash_profile is equal to ~/.profile, and bash shell takes ~/.bash_profile as a priority.
whereas ~/.bash_profile or ~/.profile is executed only when user logins, ~/.bashrc is executed only when subshell is executed.
Threrefore, if you want to work always in the same environment, you should put all variables into ~/.bashrc and execute the file in ~/.bash_profile.
source {file}: apply user start file.