Day 5 - Week 5 Comprehensive Review: The Shell on One Page

📌 Key Points

• There are only two families of shells: the Bourne family (sh, ksh, bash, zsh, dash) and the C family (csh, tcsh). Only the two with a c in their name are C family.
• A shell variable reaches only the current shell; an environment variable reaches children. The promotion command is export.
• A login shell reads .bash_profile, and a non-login shell reads .bashrc.
• > overwrites and >> appends, and errors use channel 2.
• To apply settings immediately use source, and to remove an alias use unalias.

The Shell on One Page

   Shell families
     Bourne family   sh(Bourne) · ksh(Korn) · bash(Fox) · zsh · dash
     C family        csh(Joy) · tcsh                    ← only these two!
 
   Variables
     NAME=value          shell variable        → check with set   (up to the current shell)
     export NAME=value   environment variable  → check with env   (down to children)
 
   Configuration files
     Login       /etc/profile  →  ~/.bash_profile  →  (calls ~/.bashrc)
     Non-login                    ~/.bashrc
     Logout                       ~/.bash_logout
     Apply now   source file   or   . file
 
   Input/output
     0 stdin      <
     1 stdout     >   >>          &>  both
     2 stderr     2>  2>>         2> /dev/null  discard
     Pipe         command | command
 
   Convenience
     alias / unalias       aliases
     history · !! · !number  history

From Login to Logout — The Week as One Flow

What we learned separately over four days is really the order of what the shell does in a day.

   ① Connect        Looks at the 7th field of /etc/passwd and launches /bin/bash as the login shell
   ② Read settings  /etc/profile → ~/.bash_profile → ~/.bashrc → /etc/bashrc
   ③ Prepare vars   Variables like PATH, HOME, PS1 are filled in
   ④ Prompt         Shows [user@server1 ~]$ in the PS1 format and waits for input
   ⑤ Enter command  ll *.log > out.txt 2> err.txt
   ⑥ Interpret      ll → ls -l (alias) · *.log → file names (wildcard) · connect > and 2>
   ⑦ Execute        Finds ls in PATH, runs it as a child process, and waits for it to finish
   ⑧ Result         Stores the exit status in $? and shows the prompt again
   ⑨ Logout         Runs ~/.bash_logout and writes history to ~/.bash_history

• In ⑥, the shell is what resolves aliases, wildcards, and variables. ls receives only the fully resolved result.
• In ②, .bashrc and /etc/bashrc are not read by bash automatically; one line inside the preceding file calls them (on the Red Hat family).

Finding the Cause from the Symptom

Reading Redirection Practice

ls /etc /nodir is a command that produces both results and errors. Follow what goes where for each notation.

Symbols at a Glance

Contrasting Easily Confused Pairs

Commonly Missed Points

1. zsh is C family → No. It is Bourne family. The C family is only csh and tcsh.
2. NAME = linux → You must not put spaces around the equals sign. NAME=linux.
3. You add $ when creating a variable too → No. Only when retrieving it.
4. env shows everything → No. set shows everything; env shows only environment variables.
5. The PATH separator is a semicolon → It is the colon (:). The semicolon is Windows.
6. export PATH=/opt/bin → The existing paths are lost. Always include $PATH:.
7. $? being 1 means success → 0 is success.
8. A new terminal window reads /etc/profile → It does not. A non-login shell reads only .bashrc.
9. Edited settings apply immediately → No. You must re-read them with source or log in again.
10. > saves errors too → No. Errors are redirected separately with 2>.
11. You remove an alias with unset → It is unalias. unset is for variables.
12. $SHELL is the shell currently running → It is the login shell. For the current one, look at echo $0 or ps -p $$.
13. The first shell to add history and aliases was bash → It was csh. bash inherited them.
14. > a 2>&1 and 2>&1 > a are the same → They differ. With the latter, the error stays on the screen.
15. .bash_logout runs every time you close a terminal window → Only when leaving a login shell.
16. . file and ./file are the same → The former reads in the current shell (source), the latter executes.
17. /sbin/nologin means the account was deleted → The account stays, and only login is blocked.

Values That Appear on the Exam as They Are

Files at a Glance

Commands at a Glance

Next week is editors and processes. We cover vi's three modes and process commands such as ps and kill.

📖 Glossary

• Bourne family / C family : The two branches that divide shells. The C family is only csh and tcsh.
• export : The command that promotes a shell variable to an environment variable.
• Login shell / non-login shell : The configuration files read differ depending on whether the login process was gone through.
• source : The command that re-reads a configuration file in the current shell. Same as the dot (.).
• File descriptor : The number attached to an input/output channel. 0, 1, 2 are standard input, output, and error.
• /dev/null : A special file that discards any data sent to it.