Linux study notes - commands related to system management [Repost]

Command Wall Commands with System Management The function of this command is to send information to all logged in users. Users can write the information you want to send in a file, then enter: # Wall

Example 1: The system is turned off after ten minutes and will be restarted immediately. # Shutdown - R 10 Case 2: The system is turned off immediately and does not restart. # Shutdown - h Nowfree Command Free Command is to view the current system memory usage, which displays the remaining and used physical memory and exchange memory in the system, and shared memory and buffers used by the core. The general format of this command is: The meaning of each option in the free [-b | -k | -m] command is shown below: -b Display in bytes. -k is displayed in k bytes. -m is displayed in megabytes. Example: $ FREETOTAL Used Free Shared Buffers Cachedmem: 63076 32020 31056 8204 16360 6048 - / Buffers / Cache: 9612 53464SWAP: 64476 2240 62236UPTIME command uPtime command display system has run how long, it sequentially display the following information: Today, How long is the system has run, how much login users are available, and the system has an average load in the past 1 minute, 5 minutes and 15 minutes. The general format of this command is: uptime example: # uptime4: 43pm Up 1 day, 5:51, 2 user, load average: 0.01, 0.01, 0.00 related command passwd command for system security, Linux system In addition to its username, each user has its corresponding user password. Therefore, when using the Useradd command to increase, you also need to use the passwd command to set your password for each newly added user; users can change their passwords with the passwd command. The general format of this command is: Passwd [User Name] where the username is the username that needs to modify the password. Only super users can use the "PasswD User Name" to modify the passwords of other users, and ordinary users can only modify their own passwords with the Passwd command without parameters. The method of this command is as follows: Enter passwd ; in (CURRENT) UNIX Passwd: Enter the current password in New Password: Enter the new password under prompt (see this password on the screen): System prompt to enter again This new password. After entering the correct, this new password is encrypted and put in / etc / shdow file. It is important to select a password that is not easy to decipher. Selecting the password should follow the following rules: The password should have at least six (preferably eight) characters; the password should be case the case, the punctuation, and the numbers are mixed. Super user modifies the password of other users (xxq), # passwd xxqnew unix password: retype new unix password: passwd: all authentication tokens Updated successful autonomics us It allows an ordinary user with a superuser or other user permissions, or makes superusers do something in a normal user. Ordinary users must have passwords for superusers or other users when using this command. To leave the current user's identity, you can play exit. The general form of this command is: SU [option] [?] [User account] Description: If you do not specify the user account, the system preset is a superuser root. The meanings of each option in this command are:? C to end after execution of a command. • The purpose of adding this minus is to make the environment variables and users who want to convert. ? m keeps environment variables unchanged.

Example 1: Change the root user $ su? Password: [Enter the password of the superuser] Example 2: Turn into an XU user, and execute a command to end. $ su-xu? c "rmdir cat1" other command Echo command echo command function is displayed on the display, which generally play a prompt. The general format of this command is: echo [-n] string which option n means that the output text is not changed; the string can be quoted, or the quotation marks may not be quoted. When using the echo command to output the string of the quotation number, the string is output; when the echo command outputs the string of the non-quotation number, the various words in the string are output as a string, and each string is divided by a space. . Example 1: $ echo 'Thank you!' Thank you! Example 2: $ Echo Thank you! Thank you! The function of the CAL command CAL command is the calendar that shows a certain month. The general format of this command is: CAL [Option] [Month [Year]] command The meaning of each option is: - j Shows every day in the scheduled month is the first few days in the year (from January 1 Mandarin). - Y shows the calendar of the whole year. Example 1 shows the calendar in November 1999. $ CAL 11 1999november 1999su Mo Tu We TH fr Sa1 2 3 4 5 67 8 9 10 11 12 1314 15 16 17 28 26 2728 29 30 Example 2 shows every day in November 1999 is a year The first few days. $ CAL - J 11 1999November 1999Sun Mon Tue Wed Thu fri sat305 306 317 313 319 315 321 322 323 324 330 326 327 328 329 330 33332 333 334date command Date command function is displayed and set system date and time.

The general format of this command is: DATE [Option] Display Time Format (with start, rear connection format) Date Set the meaning of each option in the time format command is: -d datestr, --Date dateStr display by DateSTR description -s datestr, - set dateStr Set DateSTR Description Date-U, --Universal Displays or sets a generic time period% H / hour (00..12)% i hour (01..12)% k hours (0. .23)% l Het (1..12)% M points (00..59)% P showed AM or PM% R Time (HH: mm: SS AM or PM), 12 hours% s from 1970 1 On 1 day 00:00:00 to the current experience of the second few% s second (000)% T time (24 hours) (HH: mm: SS)% x display time format (% h:% M :% S)% z Time zone Date Date% A Type of Types of Heaven (Sun..SAT)% a Sunday..saturday% B (JAN..DEC)% B month's full name ( January..DECEMBER)% C Date and time (MON NOV 8 14:12:46 CST 1999)% D for the first few days (01..31)% D Date (mm / dd / yy)% h and% B option The same% J one year (001..366)% M month (01..12)% W One week (0 represents Sunday)% W Year's Day (00 ..53, Monday, the first day)% x Display date format (mm / dd / yy)% Y year's last two numbers (1999 is 99)% Y year (eg: 1970, 1996, etc.) Special description is that only super users can set the time with the Date command, and the general user can only display time with the date command. Example 1: Display time with the specified format. $ DATE ' THIS DATE NOW IS =>% X, Time Is Now =>% X, Thank you!' this date now => 11/12/99, time is now => 17:53:01, Thank you ! Example 2: Displays the current time with a predetermined format. # Datefri Nov 26 15:20:18 CST 1999 Example 3: Setting time is 14:36 ​​pm. # Date -s 14: 36: 00fri Nov 26 14:15:00 CST 1999 Example 4: Setting time is November 28, 1999.