System Info Commands — du / df / date, and more

Checking Disk Usage — du and df

This article covers four kinds of check commands: ① disk usage (du / df), ② date and time (date), ③ system and user (uname / whoami / id), and ④ sequences and arithmetic (seq / expr).

du reports how much space a directory or file uses, while df reports the free space and usage of the whole disk.

Reach for du when you want "how much space is this directory taking up?" and df for "how much free space is left on the whole disk?".

When you're running low on space, a common flow is to check the whole disk with `df -h` first, then hunt down large directories with `du -sh`.

mkdir -p data            # create something to measure
echo 'sample' > data/a.txt   # put a little content inside
du -sh data              # total size of data (e.g. 4.0K data)
df -h                    # the whole disk's free-space list

① Run mkdir -p data to create a directory to measure.

② Run echo 'sample' > data/a.txt to put a file inside it.

③ Add the two options for "summary only" and "human-readable units" to du and show the size of data.

④ Add the human-readable option to df and show the whole disk's free space. (Run it correctly and the explanation will appear.)

Checking the Date and Time — date

date shows the current date and time.

Run it with no arguments and it prints the default format, including the weekday, month, day, and time.

Pass a format after +, like date +%Y-%m-%d, and you get just the date in a fixed shape such as `2026-05-18`. `%Y` is the 4-digit year, `%m` the 2-digit month, and `%d` the 2-digit day.

It's handy for things like putting a date in a log file name.

date             # e.g. Mon May 18 14:30:05 UTC 2026 (default full output)
date +%Y-%m-%d   # e.g. 2026-05-18 (just year-month-day)

① Run date with no arguments to show the current date and time in the default format.

② Add + to date and pass the year-month-day format to pull out just the date, in a shape like 2026-05-18. (Run it correctly and the explanation will appear.)

Checking the System and User — uname / whoami / id

uname -a prints system info about the running OS all at once — the kernel name, version, architecture, and more.

whoami shows the name of the user currently running the command.

id shows that user's UID (user number) and the GID (group number) of their group, so you can check what privileges you're running with.

It's also used inside scripts to decide "who am I running as?"

uname -a   # kernel name, version, architecture, and more
whoami     # the current user name (e.g. root)
id         # e.g. uid=0(root) gid=0(root)

① Add the option that prints all info to uname and show the system information.

② Use whoami to check the current user name.

③ Use id to check the UID and GID.

Sequences and Arithmetic — seq and expr

seq generates a sequence of numbers. seq 1 5 prints 1 through 5, one per line, which is handy for setting a loop count.

Pass three numbers, like seq 2 2 10, and they're read as start, step, end — so you get 2 to 10 in steps of 2.

expr does simple arithmetic on the command line. expr 3 + 4 prints 7, and each number and operator must be separated by spaces.

It's used inside scripts to count or add numbers.

seq 1 5      # 1 2 3 4 5, one per line
seq 2 2 10   # 2 to 10 in steps of 2
expr 3 + 4   # 7 (space-separated)
expr 10 - 6  # 4

① Use seq to print 1 through 5, one number per line.

② Pass three numbers to seq to print 2 to 10 in steps of 2.

③ Compute 3 + 4 with expr and check the result.

④ Compute 10 - 6 with expr and check the result.