Exercise

Shell Expansions Exercise

Overview

Bash inspects your command line before the command runs, and it replaces pattern characters such as *, ?, and [...] with matching file names.

In this hands-on exercise you will navigate to /etc/ssh on servera, match real configuration files and host keys using each glob pattern, and observe pathname expansion directly with echo.

Target host: servera

Match file names with an asterisk (*)

Connect to servera, change to the /etc/ssh directory, and list all files. Then list all public host key files ending in .pub, followed by all configuration files ending with config.
```
ssh student@servera
cd /etc/ssh
ls
ls *.pub
ls *config
```
What matched? /etc/ssh contains configuration files and host key pairs. *.pub matched all three public keys (ssh_host_ecdsa_key.pub, ssh_host_ed25519_key.pub, and ssh_host_rsa_key.pub). *config matched both ssh_config (client configuration) and sshd_config (SSH daemon configuration).
Match single characters with a question mark (?)

The question mark (?) matches exactly one character. Run ls to review the directory, then use ? to match files with specific character lengths.
```
ls
ls ssh?_config
ls ssh_host_???_key.pub
```
Single character matching ssh?_config matched sshd_config because of the single d, but did not match ssh_config (which has no character between ssh and _config). ssh_host_???_key.pub matched only ssh_host_rsa_key.pub because rsa is exactly three characters, while ecdsa (5) and ed25519 (7) do not match three question marks.
Match character sets with brackets ([...])

Brackets ([...]) match any single character from the specified set or range. Compare matching specific algorithm prefixes with matching numeric digits.
```
ls ssh_host_[e]*.pub
ls ssh_host_[r]*.pub
ls ssh_host_[re]*.pub
ls *[0-9]*
```
Character sets and ranges ssh_host_[e]*.pub matches keys starting with e (ECDSA and ED25519). ssh_host_[r]*.pub matches RSA. ssh_host_[re]*.pub matches keys starting with either r or e. Finally, *[0-9]* uses the range 0-9 to match any file containing digits, selecting only the ed25519 keys.
Observe pathname expansion directly with echo

Run echo with unquoted patterns to see how Bash expands globs into arguments before the command runs.
```
echo *
echo *.pub
```
What happened? echo never received an asterisk. Bash expanded the pattern into the list of matching file names and passed those file names as arguments to echo.

Overview

Match file names with an asterisk (*)

Match single characters with a question mark (?)

Match character sets with brackets ([...])

Observe pathname expansion directly with echo