M6 — Links, globs, find, and grep

Module brief#

Connect names to files with hard and symbolic links, select names with wildcards, locate files with find/locate, and search inside files with grep.

Learning objectives#

• Create hard links and symbolic links; remove them safely
• Explain when hard vs soft links matter
• Use wildcards *, ?, and []
• Find files by name and attributes with find
• Optionally use locate when an index exists
• Search file contents with grep and common flags

1. Hard links vs symbolic links#

A directory entry is a name pointing at file data. A hard link is another name for the same inode (same data). A symbolic link (symlink) is a small special file that stores a path to another name.

cd /tmp/linux-lab-pushpjeet
printf 'Line one\nLine two\nLine three\nLine four\nLine five\n' > notes.txt
ln notes.txt hard-notes          # hard link
ln -s notes.txt soft-notes       # symbolic link
ls -l notes.txt hard-notes soft-notes
ls -i notes.txt hard-notes soft-notes

-rw-r--r-- 2 box box 49 Sep 26 11:04 hard-notes
-rw-r--r-- 2 box box 49 Sep 26 11:04 notes.txt
lrwxrwxrwx 1 box box  9 Sep 26 11:04 soft-notes -> notes.txt

1084900 hard-notes
1084900 notes.txt
1084906 soft-notes

Observations:

• notes.txt and hard-notes share inode 1084900 and show link count 2
• soft-notes has a different inode and displays as type l with ->

stat -c '%n links=%h inode=%i' notes.txt hard-notes soft-notes

notes.txt links=2 inode=1084900
hard-notes links=2 inode=1084900
soft-notes links=1 inode=1084906

Remove a link name with rm or unlink (same idea for one name):

rm soft-notes      # removes the symlink, not the target content

Practical pattern: symlink a config into your home for convenience, for example ln -s /path/to/project/config ~/.myconfig — adjust paths to files you own.

2. Wildcards (globs)#

The shell expands globs before the command runs.

cd /tmp/linux-lab-pushpjeet
ls *.txt

a.txt  b.txt  notes-copy.txt  notes.txt  readme.txt

ls [ab].txt

a.txt  b.txt

Quote patterns when you want find or grep to see the wildcard characters themselves (otherwise the shell expands them too early). Example: find . -name '*.txt'.

3. find — walk the tree#

find . -name '*.txt'

./b.txt
./notes.txt
./a.txt
./readme.txt

Useful expressions:

find . -type f -name '*.log'
find . -type d -name 'src'
find . -mtime -1 -name '*.log'    # modified in the last 24 hours
find /tmp/linux-lab-pushpjeet -maxdepth 2 -type f

find is precise and always walks live directories. It can also run actions (-exec) — learn names and types first; add actions carefully later.

4. locate — fast index search (optional)#

locate queries a prebuilt database (often updated by updatedb on a schedule). It is fast but may be slightly stale, and some minimal systems omit it.

command -v locate
# locate notes.txt

If locate is missing, install a package such as plocate or mlocate on Ubuntu, or the RHEL-family equivalent — or simply rely on find for this course.

5. grep — search inside files#

grep -n Linux readme.txt

1:Hello Linux learners

grep -i linux readme.txt

Hello Linux learners

Common flags:

grep -rn 'Line' . --include='*.txt'

./notes-copy.txt:1:Line one
./notes-copy.txt:2:Line two
…
./notes.txt:1:Line one
…

Regex intro (enough to start):

• ^Hello — line starts with Hello
• learners$ — line ends with learners
• . — any single character
• * — previous item zero or more times (basic regex rules differ slightly from shell globs — keep them mentally separate)

grep -n '^Hello' readme.txt
grep -E 'learners|CLI' readme.txt

6. Putting it together#

A common admin loop:

1. find (or a glob) to locate candidate files
2. grep to inspect contents
3. Symlink or copy into place once you know the right file

Example lab sequence:

find . -name 'readme.txt'
grep -n CLI readme.txt
ln -sf "$(pwd)/readme.txt" /tmp/linux-lab-pushpjeet/readme-link
ls -l readme-link

Practice: MCQs#

Q1. After ln a.txt b.txt, both names share an inode. This is a:

• A) Symbolic link
• B) Hard link
• C) Broken mount
• D) Compressed archive

Q2. You delete the original name of a file that still has a hard link. What happens to the data?

• A) It vanishes immediately forever in all cases
• B) It remains available through the remaining hard link
• C) The disk reformats
• D) The symlink count increases automatically

Q3. Which pattern matches a.txt and b.txt but not notes.txt?

• A) *.txt
• B) ?.txt
• C) notes*
• D) *

Q4. Why quote '*.log' in find . -name '*.log'?

• A) Quotes delete log files
• B) So the shell does not expand * before find runs
• C) find cannot search without quotes ever
• D) It enables colour output

Q5. grep -n pattern file adds:

• A) Octal permissions
• B) Line numbers to matches
• C) A hard link
• D) Gzip compression

Q6. A symlink whose target was removed is often called:

• A) A hard link
• B) A dangling (broken) symlink
• C) An inode
• D) A daemon

Practice: Flashcards#

Practice: Match the columns#

Practice: Fill in the blank#

1. Symbolic link to notes.txt named shortcut: ln ___ notes.txt shortcut
2. Find all .log files under . : find . -name '___'
3. Case-insensitive grep for error in app.log: grep ___ error app.log
4. Grep with line numbers: grep ___ pattern file
5. Match any single-character .txt name: ls ___.txt

Practice: Mini terminal challenge#

Challenge: Symlink and search#

Goal: Create a symlink and use find/grep together.

Setup: cd /tmp/linux-lab-pushpjeet (recreate readme.txt if needed).

Tasks:

1. ln -s readme.txt home-readme and ls -l home-readme
2. find . -maxdepth 1 -type l and confirm your symlink appears
3. grep -n -i 'cli\|linux' readme.txt
4. Create three files report1.log report2.log notes.bak; use a glob to list only report*.log
5. rm home-readme and confirm readme.txt still exists

Verify: Symlink removal left the target intact; grep showed matching lines with numbers; glob listed only the report logs.

Stretch: Intentionally create a dangling symlink (ln -s missing-target broken-link) and observe ls -l and file broken-link.

Hands-on lab#

Goal: Links + discovery tools in one workflow.

Steps:

1. In your lab directory, create configs/app.conf with two or three settings lines.
2. Symlink it to ~/app.conf.link only if your home is writable and you are fine cleaning up — otherwise symlink inside the lab: ln -s configs/app.conf app.conf.link.
3. Use find . -name '*.conf' to locate it.
4. Use grep -n '=' (or a key name) to show setting lines.
5. Create a hard link to a data file and prove shared inodes with ls -i.

Verify: ls -l shows the symlink arrow; ls -i matches for the hard link pair; grep highlights your settings.

Stretch: Run man find and learn one predicate you have not used (-size, -user, or -mtime).

Common mistakes#

• Confusing shell globs with regular expressions in grep
• Forgetting to quote patterns for find -name
• Expecting hard links to work across different mount points
• Deleting a file and not noticing a dangling symlink still present
• Running recursive grep or find on huge trees (like /) without permission awareness — stick to lab paths

Summary#

You can add hard and soft links, select names with globs, locate files with find (and locate when available), and search contents with grep. Together with Modules 0–5, you now handle everyday filesystem literacy on Ubuntu and RHEL-family systems alike.

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