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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5
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 recognizes that it needs to run the echo program. It then searches the sequence of directories in $PATH, separated by :, for a program with that name. Once found, the program is executed. To determine which specific program a program name refers to, you can use the which program. We can also bypass $PATH by directly specifying the path to the program we want to execute.

Navigating in the Shell#

A path in the shell is a sequence 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, when we run a program without specifying a path, the program executes in 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 first character on this line, d, indicates that data is a directory. The next nine characters are divided into groups of three (rwx). They represent the permissions of the file owner (root), the group (root), and everyone else, respectively. A - indicates that the corresponding user does not have that permission. From the information above, only the file owner can modify (w) the data directory—for example, by adding or deleting files within it. To enter a directory, a user must have “search” permission, represented by the “execute” permission (x), for both that directory and its parent directories. To list its contents, the user must have read permission (r) for the directory. Permissions have similar meanings for files. Note that programs in the /bin directory include the x permission in the final group, which represents everyone else, meaning that 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

Connecting Programs#

In the shell, programs have two primary “streams”: an input stream and an output stream. When a program attempts to read information, it reads from its input stream; when it prints information, it writes to its output stream. Normally, a program’s input and output streams are connected to your terminal—your keyboard provides input, and your display shows output. However, we can also redirect these streams! The simplest forms of redirection are < file and > file. These commands redirect a program’s input and output streams, respectively, to files:

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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

The Root User#

Access denied (permission denied) sudo