Features#

They allow you to run programs, give them input, and inspect their output in a semi-structured way

Usage#

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date # 显示时间
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echo hello # 输入参数‘hello’并输出
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echo "hello world"
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echo $PATH # 输出环境变量￥PATH
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# /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/snap/bin

When we execute the echo command, the shell realizes that it needs to run the echo program; it will then search for that program in the directories separated by : in the $PATH, performing a name-based search for the program. When the program is found, it is executed. To determine exactly which program a given program name refers to, you can use the which command. We can also bypass the $PATH by directly specifying the path to the program we want to run.

Navigating in the shell#

The paths in the shell are a set of directories separated by / on Linux and macOS, and by \\ on Windows. / is the root directory.

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pwd # 获取当前路径
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cd /home # 根目录下的home文件夹
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cd ./home # 当前目录下的home文件夹
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cd .. # 上级目录

Generally speaking, when we run a program, if we do not specify a path, the program will be executed from the current directory.

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ls # 查看目录文件
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ls --help
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ls -l /home
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# drwxr-xr-x  4 root    root    4096 11月 30 21:01 data

First, the line’s first character d indicates that data is a directory. Then the next nine characters, three characters at a time, form a group. (rwx). They represent the permissions that the file owner (root), user group (root), and everyone else have. A - indicates that the corresponding permission is not granted.

From the above information, only the file owner can modify (w), the data folder (for example, adding or removing files within the folder).

To enter a folder, a user needs the folder and its parent folders’ “execute” permission (represented by x). To list its contents, the user must have read permission (r) on that folder. For files, the meaning of the permissions is similar. Note that the programs in the last group of the /bin directory have x permission for everyone, meaning anyone can execute these programs.

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mv test ./data/test.txt
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cp test.c test01.c
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mkdir test
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man ls

Redirection of input/output in Programs#

In the shell, a program has two main “streams”: its input stream and its output stream. When a program tries to read information, it reads from the input stream; when a program prints information, it outputs to the output stream. Typically, a program’s input/output streams are your terminal. That is, your keyboard as input and the display as output. However, we can also redirect these streams!

The simplest redirection is < file and > file. These two forms can redirect the program’s input and output streams respectively to a file:

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echo hello > hello.txt
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cat hello.txt
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cat < hello.txt
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cat < hello.txt > hello2.txt
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cat hello2.txt
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ls -l / | tail -n1
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curl --head --silent baidu.com | grep --ignore-case content-length | cut --delimiter=' ' -f2

Root user#

Permission denied

sudo