Set Env Declare Export Different

First, you must understand that environment variables and shell variables are not the same thing.

Then, you should know that shells have attributes which govern how it works. These attributes are not environment nor shell variables.

Now, on to answering your question.

• env: without any options, shows current environment variables with their values; However can be used to set environment variable for a single command with the -i flag
• set: without options, the name and value of each shell variable are displayed* ~ from running man set in rhel; can also be used to set shell attribute. This command DOES NOT set environment nor shell variable.
• declare: without any options, the same as env; can also be used to set shell variable
• export: makes shell variables environment variables

In short:

• set doesn’t set shell nor environment variables. 例如：set -e
• env can set environment variables for a single command. 直接设置环境变量，不能将shell vairables导出成environment variable
• declare sets shell variables. 例如：declare -a s='hello shell'
• export makes shell variables environment variables. 例如：可以直接将shell vairables导出成environment variable, export s

参考

• What is the difference between set, env, declare and export when setting a variable in a Linux shell?
• Bash 脚本 set 命令教程