Linux Commands for Beginners

The shell takes user commands and passes them to the OS for execution.

Basic Navigation and Viewing

pwd

pwd displays the absolute path of the current directory.

---

ls

ls (list) lists the files in the current directory.

Options: -l lists each file's attributes; there is usually an alias ll for it.

drwxr-xr-x@ 2 fling staff  64  5 24 15:42 test
^ ^  ^  ^ ^ ^   ^     ^     ^   ^ ^  ^     ^
| |  |  | | |   |     |     |   | |  |     |
| |  |  | | |   |     |     |   | |  |     └─ 文件名/目录名 (Name)
| |  |  | | |   |     |     |   | |  └─────── 最后修改时间 (Modification Time)
| |  |  | | |   |     |     |   | └────────── 最后修改日期 (月份中的某一天) (Day of Month)
| |  |  | | |   |     |     |   └──────────── 最后修改月份 (Month)
| |  |  | | |   |     |     └──────────────── 文件大小 (字节) (Size)
| |  |  | | |   |     └────────────────────── 所属组 (Group)
| |  |  | | |   └──────────────────────────── 文件所有者 (Owner)
| |  |  | | └──────────────────────────────── 硬链接数量 (Number of Hard Links)
| |  |  | └────────────────────────────────── 扩展属性标志 (Extended Attributes Flag - @) (macOS specific)
| |  |  └──────────────────────────────────── 其他用户权限 (Others' Permissions)
| |  └─────────────────────────────────────── 组用户权限 (Group's Permissions)
| └────────────────────────────────────────── 文件所有者权限 (Owner's Permissions)
└──────────────────────────────────────────── 文件类型 (File Type)

--- Attribute details:

• File type:
  • -: regular file (e.g. 1.txt)
  • d: directory (e.g. test)
  • l: symbolic link
  • c: character device file
  • b: block device file
  • s: socket file
  • p: named pipe ---
• File permissions:
  • The permissions are divided into three groups of three characters, representing the file owner, the group, and other users.
  • Each group uses rwx for read, write, and execute permissions; missing permissions are indicated by -. For compact notation, r\w\x are assigned the numbers 4\2\1. ---
• Hard link count:
  • For a file, it indicates how many filenames point to that file's inode.
  • For a directory, it is usually 2 plus the number of subdirectories; every directory has . pointing to itself and .. pointing to the parent directory. --- -a lists all files, including hidden files. . .. .env 1.txt test --- ### cd cd enters a directory. As mentioned earlier, .. stands for the parent directory. cd - goes back to the last directory you were in. --- ### tree tree [] displays the structure below directory []. --- ## File and Directory Operations --- ### touch touch updates the timestamp, but it can also create files. --- ### mkdir mkdir creates a directory. --- ### cat cat outputs the contents of a file. --- ### less less displays a file; you can scroll up and down and press q to quit. It reads and displays only what the screen needs. g goes to the beginning, G to the end. % or p jumps by percentage. /pattern searches forward, ?pattern searches backward; n goes to the next match, N to the previous match. F keeps watching new content appended to the end of the file, like tail -f. --- ### more more displays a file but cannot scroll back up; press q to quit. It loads the entire file into memory at once and then displays it. --- ### head head outputs the beginning of a file. head --lines=n file outputs the first n lines of file. --- ### tail tail outputs the end of a file. tail --lines=n file outputs the last n lines of file. tail -f keeps viewing new content as it is appended. --- ### cp cp 源 目的 copies files or directories. --- ### mv mv 源 目的 moves, and can also be used to rename. --- ### rm rm deletes. rm -r deletes recursively; used for directories. --- ### ln ln links. ln -s 源 目的 creates a soft link: it does not store the file, only a shortcut file. ln 源 目的 creates a hard link: a pointer to the source file that shares the same inode; you can think of it as a reference, and it can only point to files. --- ## Permissions and Searching --- ### chmod Modify permissions. u\g\o represents the three permission groups. chmod u+x file gives the owner execute permission. You can also use numeric permissions, e.g., chmod 744 file. --- ### file file tells you the file type, e.g., 1.txt: ASCII text. --- ### where/which/whereis where finds where a file is. --- ## Output, Pipes, and Editing --- ### echo echo outputs to the command line; wrap special characters in " ". > redirects standard output to a file, e.g., echo "1" > 1.txt. >> appends to a file. < redirects standard input from a file. --- ### pipe command1 | command2 | command3 feeds one command's standard output to another command's standard input. ls -l | grep "*.txt" cat blog.log | grep "ERROR" | less --- ### nano Crtl+X exits, Crtl+O saves. --- ### vim In command mode, :wq saves and exits, :q! force quits; [n]yy copies n lines, [n]p pastes n times below the cursor, and dd cuts. In command mode, type i\a\o\I\A\O to enter insert mode; in insert mode, esc enters command mode. ---

    

In insert mode, ^ jumps to the beginning of the line and $ to the end of the line. --- In command mode, typing : enters command-line mode; in command-line mode, esc\enter returns to command mode. ---

Variables and Wildcards

---

Shell

Define a variable with variable=value and it's done. You can view it with echo ${variable}. The braces are used to prevent ambiguity. Trim from the front: for example, ff=week01, echo ${ff#week} outputs 01. Use % to trim from the end. ? is a placeholder for one character, * for any characters. --- for i in $(seq 1 10) iterates over the sequence 1 to 10. for ((i=0;i<10;i++)) is a C-style loop. Batch rename

for> do
for> mv ${d} chapter${d#week}
for> done