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# Enter the following command in a terminal to print all the environment variables:
printenv

# To show a list including the "shell variables" you can enter the next command:
# This will show you not only the shell variables, but the environment variables too.
( set -o posix ; set ) | less

# PROMPT_COMMAND
# 在 bash 里设置环境变量 PROMPT_COMMAND，这个命令会在用户提示符之前被执行，可以用来记录用户操作历史 
# Bash provides an environment variable called PROMPT_COMMAND. The contents of this variable are executed as a regular Bash command just before Bash displays a prompt.
If set, the value is executed as a command prior to issuing each primary prompt.

# HISTSIZE
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