Shell Search Path

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Sources:

  • Ian! D. Allen. Week05 Shell Search Path. CST8207.

Shell Script Searching Path

Shell will find and execute command. The searching sequence of command name is:

  1. 将该command name看作一个文件的相对/绝对路径, 并执行该文件, 例如/bin/ls或./ls.
  2. 由 alias 找到该指令来执行
  3. 由 bash 内置的( builtin)指令来执行.
  4. 通过 PATH 这个变量的顺序搜寻到的第一个指令来执行.

在某一步骤找到command name后就会停止查找, 并且尝试执行.

因此, 如果一个command name不是一个可执行的路径, 也不是一个alias, 又不是一个builtin( 例如alias), 则shell会在搜环境变量$PATH中搜索该command name, 如果找到, 就执行; 找不到, 则报错"command not found".

  • 由于PATH 是个环境变量, 因此它在任意的子session中都有效.

  • Command not found == not found in PATH. 这只是意味着在PATH中找不到该command name. 可以将该command的路径添加到PATH来让shell能搜索到它.

  • 可以看到, 如果一个command name是路径, 那么在step1就找到了, 不会尝试后面的步骤. 一个command name被视作路径名 iff 该command name带有slash( / ).

    • 对于当前目录下的文件file, 它的路径名是./file, 而不是file. 后者不会让Shell将其视作路径.

  • Builtin commands: Your shell has some built-in commands that are executed directly by the shell itself and not looked up in PATH, e.g. echo, cd, umask, pwd, history

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    $ type echo
    echo is a shell builtin

Append path to PATH

将命令的路径加入环境变量PATH的方法:

声明环境变量PATH, 值为之前的PATH的值(采用$进行赋值), 再append 一个:, 因为环境变量之间通过冒号隔开, 再append要添加的路径, 这里是/home/to/operation_tools.

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export PATH=$PATH:/home/to/operation_tools

Bad Practices

以下是一些错误的对PATH进行设置的例子.

Current directory in PATH

A leading or trailing colon in PATH, or two adjacent colons (::), indicate that the current directory is one of the directories that the shell should try when looking for executable command names. This is a security risk and you must not do it:

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$ echo "$PATH"
:/bin/:/usr/bin                          # EXTREME SECURITY RISK!

$ echo "$PATH"
/bin/::/usr/bin                          # MODERATE SECURITY RISK!

$ echo "$PATH"
/bin/:/usr/bin:                          # SECURITY RISK!

Do not put the current directory (or any relative path directory) in PATH – it is a security risk. You might accidentally execute a malicious program in the current directory.

For example: If you put the current directory at the start of your PATH, e.g. PATH=:/bin:/usr/bin, then when you type ls the first program that is tried is ./ls (the program named ls in the current directory) which might be a malicious program.