Master Essential Linux Commands: From uname to updatedb

1. uname – display system information

uname -a

– shows all system information.

uname -m

– displays the hardware architecture.

uname -n

– shows the host name.

uname -r

– displays the kernel release version.

uname -s

– shows the kernel name.

uname -p

– displays the processor type.

uname -o

– shows the operating system name.

uname -i

– displays the hardware platform.

2. hostname – display or set the system host name

hostname

– shows the current host name.

hostname -A

– temporarily changes the host name (lost after reboot).

vi /etc/hostname

– permanently modify the host name (effective after reboot).

vi /etc/hosts

– add a name resolution entry for a host (e.g., Mr.white).

hostname -I

– retrieves all IP addresses of the system (useful for multi‑NIC hosts).

3. dmesg – kernel ring buffer diagnostics

dmesg | less

– view kernel messages, useful for hardware fault diagnosis.

4. stat – display file or filesystem status

stat /etc/hosts

– shows detailed information about the file.

stat -f /etc/hosts

– displays filesystem attributes.

5. du – estimate disk space usage

du -a

– shows space used by all files and directories.

du -s

– displays the total size of a directory.

du -h

– presents sizes in human‑readable units (K/M/G).

du -sh

– shows the total size of the current directory in a readable format.

du -h --max-depth=1 /usr/local/

– lists sizes of the first‑level subdirectories.

du -h --max-depth=2 /usr/local/

– includes second‑level directories.

du -h --max-depth=2 /usr/local/ --exclude=/usr/local/share

– same as above but excludes a specific path.

6. date – display or set system time

date +%y

– short year (e.g., 23).

date +%Y

– full year (e.g., 2023).

date +%m

– month number.

date +%d

– day of month.

date +%H

– hour (24‑hour format).

date +%M

– minutes.

date +%S

– seconds.

date +%F

– formatted date (YYYY‑MM‑DD).

date +%T

– formatted time (HH:MM:SS).

date +%F -d '-1day'

– yesterday’s date.

date +%F -d 'yesterday'

– another way to show yesterday.

date +%F -d '-1440min'

– yesterday using minutes offset.

date +%F -d '+1day'

– tomorrow’s date.

date +%F -d '+tomorrow'

– alternative tomorrow command.

date +%F -d '1month'

– date one month from now.

date +%F -d '1year'

– date one year from now.

date -d 'Thu Jul 6 21:41:16 CST 2017' '+%Y-%m-%d %H:%M:%S'

– convert a custom date string to a standard format.

date -s 20201220

– set system date (YYYYMMDD).

date -s '18:24:30 20201220'

– set both time and date.

date +%D%n%T

– display date, newline, then time.

7. echo – output a line of text

echo Hello world!

– prints the string.

echo 'Hello world!'

– prints the string with single quotes.

echo "hello world!"

– double quotes cause the exclamation mark to be interpreted specially.

Without the exclamation mark, the text prints normally.

echo "hello world"！

– the exclamation can be placed after the string.

echo "hello world\!"

– escape the exclamation to print it.

echo -e "hello\tworld"

– prints a tab between words.

echo -e "hello
world"

– prints a newline.

echo hello world >> hello.txt

– appends output to a file.

echo hello; echo world

– use a semicolon to run two commands sequentially.

echo -n hello; echo world

– -n suppresses the newline after the first echo.

echo -e '\033[31m red text mr.white training \033[0m'

– prints colored text (red).

echo -e '\033[41;37m red background white text mr.white training \033[0m'

– prints text with red background and white foreground.

echo $PATH

– displays the current PATH environment variable.

8. watch – monitor command execution

watch -n 1 -d netstat -ant

– refresh every second, highlight changes in network connections.

watch cat test1220.txt

– monitor changes to a specific file.

watch -t cat test1220.txt

– same as above but without the header.

9. which – locate the full path of a command

which date

– shows the full path of the date command.

which which

– reveals the path (and any alias) of the which command itself.

which java

– displays the location of the Java executable.

which -a java

– lists all occurrences of java in the PATH.

10. whereis – locate command binaries, sources, and manuals

whereis java

– shows binary, source, and manual locations for Java.

whereis -b java

– searches only for executable binaries.

whereis -m java

– searches only for manual pages.

whereis -s java

– searches only for source files (none found in this example).

11. locate – fast file path lookup

Install the utility: yum install mlocate.

locate pwd

– finds paths containing “pwd”.

locate -c pwd

– counts matching entries.

locate /etc/sh

– returns any path containing the substring.

locate /etc/sh*

– uses a wildcard to match patterns.

locate /etc/*sh*

– another wildcard example.

12. updatedb – update the mlocate database

updatedb

– creates or refreshes the database used by locate. A daily cron job typically runs this.

To update a specific directory: updatedb -vU /root/mytest20201219/ – verbose output, updating only the given path.

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