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cd, pwd and ls are the most-typed commands on any server. The details worth knowing are the flags for sorting and sizes, and how symlinked directories change what pwd reports.

Track: Core · Interview weight: Low

Must-Know Facts

Fact Value Verify with
Absolute path Starts at / cd /etc/ssh
Relative path Starts at the current directory; . current, .. parent cd ../log
Home cd, cd ~; another user's home is ~user echo ~root
Previous directory cd - (prints it); $OLDPWD holds it echo $OLDPWD
Logical vs physical path pwd keeps symlink names; pwd -P resolves them pwd -P
Hidden files Names starting with .; shown by ls -a ls -a ~
Newest last ls -ltr ls -ltr /var/log
Largest first ls -lS ls -lhS
Directory itself, not contents ls -ld <dir> ls -ld /tmp

Moving Around

cd /etc/ssh
pwd
cd -
cd ~-
cd ..
pwd
cd
pwd

Output:

/etc/ssh
/home/laborant
/etc
/home/laborant

cd - returns to the previous directory and prints it; ~- expands to $OLDPWD without printing. With a symlinked directory, pwd keeps the link name, pwd -P and cd -P resolve it.

Listing

In a directory with three sample files, one of them dated January:

ls -lh
ls -ltr

Output:

total 12K
-rw-rw-r-- 1 laborant laborant 4.9K Sep 16 13:53 big.log
-rw-rw-r-- 1 laborant laborant    0 Jan 15  2026 old.conf
-rw-rw-r-- 1 laborant laborant    6 Sep 16 13:53 small.txt
total 12
-rw-rw-r-- 1 laborant laborant    0 Jan 15  2026 old.conf
-rw-rw-r-- 1 laborant laborant    6 Sep 16 13:53 small.txt
-rw-rw-r-- 1 laborant laborant 5000 Sep 16 13:53 big.log
Flag Effect
-l Long format: type and mode, links, owner, group, size, mtime, name
-a / -A Include hidden entries / same without . and ..
-h Human-readable sizes (with -l)
-t, -r Sort by modification time, reverse the order
-S Sort by size
-d List the directory, not its contents
--time-style=long-iso Full date, including the year for recent files
-u, -c Show access time or change time instead of mtime

ls changes its date format for older files

ls prints the year instead of the time for files older than six months, so --time-style=long-iso keeps output consistent for scripts. tree -L 2 /etc/ssh shows a directory as a tree (package tree).

Do not parse ls in scripts

Column widths, date formats and quoting of odd names change between versions and locales. Use find -printf, stat -c or a glob instead.

Interview Checkpoints

L1: What is the difference between an absolute and a relative path?

Say first: an absolute path starts at / and means the same thing everywhere; a relative path starts from the current directory.

Proof: cd /var/log works from anywhere; cd log works only from /var.

Follow-up: Why should scripts and cron jobs use absolute paths?

L2: Show the five most recently modified files in /var/log.

Say first: sort by time.

Proof: ls -lt /var/log | head -6

Follow-up: How would you do it recursively? (find /var/log -type f -printf '%T@ %p\n' | sort -n | tail -5.)

L2: Show the permissions of a directory, not its contents.

Say first: add -d.

Proof: ls -ld /tmp

Follow-up: What does the t at the end of the mode mean?

L2: Return to the directory you were in before the last cd.

Say first: cd -.

Proof: cd /etc; cd /tmp; cd - prints /etc.

Follow-up: How do you keep several directories on a stack? (pushd, popd.)

L3: pwd shows one path, but a script using the same directory writes elsewhere.

Say first: the directory is reached through a symlink; compare the logical and physical paths.

Proof: pwd versus pwd -P; readlink -f .

Follow-up: Which path does a program's getcwd() return? (The physical one.)

L3: ls hangs when listing a directory.

Say first: check whether the directory is a network mount or holds millions of entries.

Proof: findmnt -T <dir> shows nfs; ls -f (no sorting) or ls -1U | head returns faster on huge directories.

Follow-up: Why does ls -l make a slow NFS mount worse? (One stat per entry.)

Captured on Rocky Linux 10.2 and Ubuntu 24.04.4 LTS (iximiuz Labs microVMs, kernel 6.1.167), 2026-09.