Essential Linux Commands: uname, hostname, dmesg, du, date, echo and More

uname – display system information

Common options: uname -a – show all related system information. uname -m – display hardware architecture. uname -n – show the host name. uname -r – display kernel release number. uname -s – show kernel name. uname -p – display processor type. uname -o – show operating system name. uname -i – display hardware platform.

hostname – view or set the system host name

hostname

– print the current host name. hostname A – temporarily change the host name (lost after reboot). vi /etc/hostname – permanently modify the host name. vi /etc/hosts – add a custom name resolution entry (e.g., for Mr.white). hostname -I – list all IP addresses assigned to the machine.

dmesg – diagnose system boot and kernel messages

Typical usage:

dmesg | less    # view kernel buffer, useful for hardware fault investigation

stat – display file or filesystem status

stat /etc/hosts

– show file metadata. stat -f /etc/hosts – display filesystem attributes.

du – summarize disk space usage

du -a

– list space used by all files and directories. du -s – show total size of a directory. du -h – present sizes in human‑readable units (K/M/G). du -sh – combined total size in a readable format. du -h --max-depth=1 /usr/local/ – show first‑level directory sizes. du -h --max-depth=2 /usr/local/ – show first two levels. du -h --max-depth=2 /usr/local/ --exclude=/usr/local/share – same as above but exclude a specific path.

date – display and set system time

date +%y

– short year (YY). date +%Y – full year (YYYY). date +%m – month. date +%d – day of month. date +%H – hour (24‑hour). date +%M – minute. date +%S – second. date +%F – ISO date (YYYY‑MM‑DD). date +%T – time (HH:MM:SS).

Relative date examples: date -d '-1day' +%F (yesterday), date -d '+1day' +%F (tomorrow), date -d '1month' +%F (one month later), date -d '1year' +%F (one year later).

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

Set date: date -s 20201220 (date only) or date -s '18:24:30 20201220' (full timestamp).

Combined display: date +%D%n%T – date, newline, then time.

echo – output text to the terminal

echo Hello world!

– simple output. echo 'Hello world!' – output with single quotes. echo "hello world!" – double quotes (note special handling of ! in some shells).

Escape characters: echo "hello\!" prints hello!.

Tab and newline: echo -e "hello\tworld", echo -e "hello\nworld".

Redirect to file: echo hello world >> hello.txt.

Command chaining: echo hello; echo world or echo -n hello; echo world (no newline after first).

Colored output: echo -e '\033[31m red text \033[0m' (red foreground), echo -e '\033[41;37m red background white text \033[0m'.

Print environment variable: echo $PATH.

watch – monitor command execution periodically

watch -n 1 -d netstat -ant

– run netstat -ant every second, highlighting changes. watch cat test1220.txt – watch a file for changes. watch -t cat test1220.txt – same without the header.

which – locate the full path of a command

which date

– show path of date. which which – reveal alias information if any. which java – path of java. which -a java – all locations containing java.

whereis – find command binaries, sources and manuals

whereis java

– display binary, source and man page locations. whereis -b java – only binaries. whereis -m java – only manual pages. whereis -s java – only source files (often none).

locate – quickly find files by name

First install the database utility:

yum install mlocate

locate pwd

– list paths containing pwd. locate -c pwd – count matching entries. locate /etc/sh – fuzzy match. locate /etc/sh* – wildcard search. locate /etc/*sh* – broader wildcard pattern.

updatedb – update the mlocate database

The updatedb command builds or refreshes the index used by locate. It is typically scheduled via a daily cron job. updatedb – initialize the database.

Check the cron schedule for automatic updates. updatedb -vU /root/mytest20201219/ – verbose update limited to a specific directory.