Course Content#
uptime System Uptime and Average Load#
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Used to monitor how long the system has been running, how many users are logged in, and the system load.
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Learn to use --help to view command help.
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w User List and Running Tasks#
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When the system is slow, you can check the commands being executed by the current processes.
who Display Current Login User Information#
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Similar to a simplified version of w.
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When there is too much information in --help, learn to use tldr.
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-q View all logged-in users and the number of users.
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-H Display headers.
whoami, who am i Current Effective/Real Username#
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sudo -i Start the default shell with superuser privileges.
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who i am = who am i
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who am i actually belongs to the who command, where am i is a parameter.
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last Recently Logged-in Users#
Application: Find the three most frequently logged-in users.
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Clean empty lines [ "^$" ], split by spaces and take the first column, sort in dictionary order [ASCII code], count, and sort in reverse numerical order.
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cut -c 10-15: Cut by character, take a column corresponding to characters 10-15.
[Extension] Word Frequency Statistics (a.log contains the contents of the ls, man, stdio, scanf sections of the man page)
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tr command for character replacement
- -c Reverse replacement
- -s Compress consecutive duplicate characters
uname System Information#
- uname -a Complete system information [Use this command when first starting on a machine]
date Display or Set System Time and Date#
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Custom string formats can be used, try different cases to see the results.
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[PS] NTP - Network Time Protocol, synchronizes time through time zones.
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- From Wikipedia, a distributed structure similar to DNS, but it is unidirectional.
- Applications: banking systems, online shopping.
- Time server configuration can be found in /etc/ntp.conf.
- Time zones can be viewed in /usr/share/zoneinfo.
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- The time zone can be modified using the ln -sf [force overwrite symbolic link] command to link /etc/localtime to the time zone.
- Example: sudo ln -sf /usr/share/zoneinfo/Asia/City /etc/localtime
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Additional Knowledge#
- cal Display a calendar.
- echo -e enables the use of backslashes .
- For example, echo -e "\033[32m123\033[0m456" outputs colored text, similar to printf.
- $[] is used for integer calculations only.
Points to Consider#
Tips#
- Abbreviations may represent:
- -h: help, human, header (w -h)
- -p: pretty
- Spaces are sensitive in shell scripts.
- Variables defined in the shell are only available within the process. They disappear when the shell is exited [or the SSH connection is disconnected].
- wget can be used to download document data.