M10 — Variables, history, and line editing

Prerequisites#

• M8 (parameter and command expansion)
• M9 (quoting — you will need it around values with spaces)

Learning objectives#

By the end of this module you will be able to:

1. Set and print shell variables; tell local vs environment variables apart
2. Use export, inspect with env / set, and customise PATH carefully
3. Change PS1 for the current session to shorten or clarify your prompt
4. Reuse commands with history, !!, !<n>, !<string>, and Ctrl+R
5. Speed up typing with Tab completion

Variables: assign, expand, export#

Assign with no spaces around =:

COURSE_LAB=linux-from-scratch
echo "COURSE_LAB=$COURSE_LAB"

COURSE_LAB=linux-from-scratch

A plain assignment is visible to the current shell. Child processes (commands you launch) see it only after export:

export COURSE_LAB
env | grep '^COURSE_LAB='

COURSE_LAB=linux-from-scratch

Useful built-ins you already rely on:

echo "User=$USER Home=$HOME Shell=$SHELL Pwd=$PWD"
echo "PATH first three: $(echo "$PATH" | tr ':' '\n' | head -3 | paste -sd: -)"

Example from a lab session:

PATH first three: /usr/local/bin:/usr/bin:/bin

PATH — how the shell finds commands#

PATH is a colon-separated list of directories. When you type ls, bash searches those directories in order.

Temporary experiment (this shell only):

echo "$PATH"
# Prepend a personal bin directory IF you created one:
# export PATH="$HOME/bin:$PATH"

Do not wipe PATH with PATH= unless you like repairing the session. If you break it, start a new terminal tab.

Prompt string PS1#

PS1 controls your interactive prompt. Try a short temporary prompt:

OLD_PS1="$PS1"
PS1='lab> '
# … work …
PS1="$OLD_PS1"    # restore

You might see lab> instead of your usual long prompt. Closing the terminal discards the change — perfect for experiments. Permanent prompts belong in startup files (M11).

Common escapes inside PS1 (for later customising): \u user, \h host, \w working directory, \$ # for root else $.

History: stop retyping#

history | tail -n 10

Example workflow:

ls /tmp/linux-lab-pushpjeet
# … later …
!ls          # reruns the last command that began with ls

Safety: !! on a destructive command is still destructive. Glance before you press Enter — especially after sudo.

History is often stored in ~/.bash_history when the shell exits cleanly. Settings like HISTSIZE appear in M11.

Tab completion#

Type the start of a command or path, press Tab:

• One match → bash fills it in
• Several matches → Tab twice lists them

cd /tmp/linux-lab-pushpjeet/vi    # press Tab → vim-lab/ if unique

Completion works for many commands’ options too (distro-dependent). Make Tab a habit; your wrists will thank you.

Local vs environment — a quick picture#

┌─ your bash session ─────────────────────┐
│  MY_LOCAL=hello     (shell only)        │
│  export COURSE_LAB=…  (env + children)  │
│       │                                 │
│       ▼  child process (e.g. python)    │
│     sees COURSE_LAB, not MY_LOCAL       │
└─────────────────────────────────────────┘

Common mistakes#

1. VAR = value with spaces — bash tries to run a command named VAR. Use VAR=value.
2. Forgetting quotes on values with spaces — NAME=Ada Lovelace runs Lovelace as a command; use NAME='Ada Lovelace'.
3. Exporting secrets into the environment casually — anything exported can show up in process listings. Be thoughtful on shared machines.
4. Permanent PATH edits without a backup plan — experiment in the current shell first; persist in M11 only when happy.
5. Blind !! after a typo that did damage — check history first.

Hands-on lab#

mkdir -p /tmp/linux-lab-pushpjeet/m10-lab
cd /tmp/linux-lab-pushpjeet/m10-lab

1. Set CITY=Calcutta and echo it. Then export CITY and confirm with env | grep CITY.
2. Save your prompt: OLD="$PS1", set PS1='m10> ', run pwd, restore PS1="$OLD".
3. Run date, then re-run it with !! (or find it via Ctrl+R).
4. Type ec and Tab-complete toward echo, then print $COURSE_LAB if still set from earlier practice (or set it again).

Verify: env shows CITY; your prompt returned to normal after restore.

Practice: check your understanding#

Multiple choice#

1. Which assignment is syntactically correct in bash?

• A. COURSE = linux
• B. COURSE=linux
• C. export =linux
• D. set COURSE linux

2. What does export VAR=1 do beyond a plain assignment?

• A. Writes VAR to disk forever
• B. Makes VAR available to child processes
• C. Hides VAR from echo
• D. Deletes VAR after one use

3. What does !! do?

• A. Clear history
• B. Start vim
• C. Re-execute the previous command
• D. Toggle Tab completion

4. Which tool lists environment variables for child processes?

• A. cat
• B. env
• C. chmod
• D. tee

5. Changing PS1 in the current terminal without editing files is:

• A. Permanent for all users
• B. Permanent for your user only
• C. Temporary until the shell exits (unless saved in a startup file)
• D. Illegal in bash

Flashcards#

Fill in the blank#

1. Export a variable in one step: export COURSE=linux-from-scratch
2. Show history: history | tail
3. Re-run last command: !!
4. Temporary prompt: PS1='lab> '

Mini terminal challenge#

Goal: Prove you understand export vs local.

Setup

cd /tmp/linux-lab-pushpjeet

Tasks

1. LOCAL_ONLY=visible-in-shell (do not export).
2. export CHILD_SEES=visible-in-env
3. Run bash -c 'echo LOCAL=$LOCAL_ONLY CHILD=$CHILD_SEES'
4. Explain the printed line in one sentence (LOCAL empty or unset, CHILD set).

Verify: Output looks like LOCAL= CHILD=visible-in-env (LOCAL blank).

Stretch: Use Ctrl+R to find your export line and run a harmless echo of CHILD_SEES again.

Next: M11 — Shell startup files — make aliases and exports stick across sessions.

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