Day 2 - Shell Variables and Environment Variables

📌 Key Points

• You create a variable with name=value. There must be no spaces on either side of the equals sign.
• A shell variable lives only in the current shell, while an environment variable is passed down to child processes. That difference is everything.
• The command that promotes a shell variable to an environment variable is export.
• To get a value out, use $name, as in echo $PATH.
• PATH is the list of paths where commands are searched for, and the separator is the colon (:).

Creating Variables

NAME=linux           # create — no spaces around the equals sign!
echo $NAME           # linux
echo ${NAME}         # wrapping in braces also works
NAME="hello world"   # use quotes when the value contains spaces
unset NAME           # delete

Spaces are an easy source of mistakes. If you write NAME = linux, the shell treats NAME as a command and tries to run it. Write = with no spaces on either side.

• By convention, variable names use uppercase. It is not required, but it makes them easy to tell apart from system variables.
• Add $ only when retrieving the value. Do not add it when creating the variable.

Naming Rules and Braces

FILE=report
echo $FILE_backup      # (blank line)        ← looks for a variable named FILE_backup
echo ${FILE}_backup    # report_backup

• Braces mark where the variable name ends. They are needed when other characters follow immediately.
• Reading an undefined variable gives an empty value, not an error. A mistyped name passes silently, so typos are hard to notice.

Values That Change Depending on Quotes

NAME=linux
echo "I use $NAME"            # I use linux           double quotes — variable is replaced by its value
echo 'I use $NAME'            # I use $NAME           single quotes — literal text
echo "Today is `date +%F`"    # Today is 2026-10-14   backticks — result of running a command
echo "Today is $(date +%F)"   # same as above

Double quotes let it through; single quotes block it. Both group whitespace into a single value, but they differ in whether $ is interpreted. Backticks look like quotes, but they do something completely different: run a command.

Shell Variables vs Environment Variables

   Current shell
   ├── NAME=linux        (shell variable)        ─┐
   └── export AGE=30     (environment variable)  ─┼─ what gets passed to the child?
        │                                         │
        ▼ run a child process                     │
      Child shell                                 │
        NAME → empty  ◄───────────────────────────┘  not passed
        AGE  → 30                                    passed

NAME=linux              # shell variable
export NAME             # promote to environment variable
export AGE=30           # create and make it an environment variable at once

set shows everything, env shows only environment variables. This pair is within the exam scope. In terms of output range, set is the wider one.

Check It Yourself: Is It Visible in the Child Shell?

NAME=linux             # shell variable
export AGE=30          # environment variable
bash                   # enter a child shell
echo "[$NAME] [$AGE]"
# [] [30]              ← NAME is empty and only AGE came through
AGE=99                 # try changing the value in the child
exit                   # return to the parent shell
echo $AGE
# 30                   ← the parent's value is unchanged

• Passing goes one way, parent → child. The child receives a copy of the parent's environment, so whatever the child changes never reaches the parent.
• A script run with bash script.sh is also a child shell. A variable you export inside a script disappears when the script ends.
• Once a variable has been exported, it stays an environment variable even if you change its value again. You do not need to export again each time you change it.

More Commands for Working with Variables

export -p | head -3
# declare -x HOME="/home/user"
# declare -x LANG="ko_KR.UTF-8"
# declare -x LOGNAME="user"
printenv HOME          # /home/user        ← giving a name prints only that value
env | grep LANG        # LANG=ko_KR.UTF-8
export -n AGE          # turn an environment variable back into a shell variable (value remains)
unset AGE              # delete the variable itself
env LANG=C date        # run this one command with LANG changed
# Wed Oct 14 09:30:00 KST 2026

• In the declare -x from export -p output, the x is the export attribute. So declare -x NAME is the same as export NAME, but a plain declare NAME without the option does not create an environment variable.
• To delete, use unset NAME. If you write unset $NAME, it tries to delete the variable whose name is the value contained in NAME.

Major Environment Variables

echo $HOME       # /home/user
echo $USER       # user
echo $PWD        # current directory — same value as the pwd command
echo $LANG       # ko_KR.UTF-8

Variables Often Asked Together

• HISTSIZE is the number to remember in memory, and HISTFILESIZE is the number to keep in the file. The names are similar, so questions swap them.
• USER and LOGNAME usually hold the same value.

PS1 — Prompt Appearance

echo $PS1
# [\u@\h \W]\$
# On screen it looks like this →  [user@server1 ~]$

• \w and \W are distinguished by case. When you are in /usr/local/bin, \w shows /usr/local/bin and \W shows bin.
• A prompt ending in # means you are working as root. \$ is what creates this distinction.

PATH — Where Are Commands Looked Up?

echo $PATH
# /usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin
#                └ separated by colons (:)

• When you type ls, the shell searches the directories listed in PATH in order from the front to find the executable.
• The separator is the colon (:). This differs from Windows, which uses a semicolon.
• Paths earlier in the list are searched first. If commands with the same name exist in several places, the earlier one wins.

export PATH=$PATH:/opt/myapp/bin      # append at the end (recommended)
export PATH=/opt/myapp/bin:$PATH      # prepend (to take priority)
which ls                              # /usr/bin/ls — the location actually found

⚠️ If you leave out $PATH, as in export PATH=/opt/bin, all the existing paths vanish and even ls will not run. Always include $PATH:.

• As a rule, the current directory (.) should not be put in PATH. If a malicious file with the same name is planted, that is what would run. That is why scripts in the current location are run with a path, like ./script.sh.

What Happens When It Is Not in PATH

ls -l backup.sh
# -rwxr-xr-x 1 user user 120 Oct 14 09:00 backup.sh
backup.sh
# bash: backup.sh: command not found
./backup.sh
# backup start...

• The file is right in front of you and has execute permission, yet it is not found. This is because the shell searches only the places listed in PATH, not the current directory.

• which tells you the location of the executable found in PATH, and type also tells you whether the name is an alias or a builtin.
• Builtin commands such as cd, echo, and export run regardless of PATH. So even if you have broken PATH, you can revive it by typing export PATH=/usr/bin:/bin.

Special Variables

ls /nonexistent
echo $?          # 2  ← non-zero means failure
ls /etc
echo $?          # 0  ← success

• If $? is 0, it succeeded. A non-zero value means an error. This is the opposite of ordinary true/false, so it is easy to get confused.

grep root /etc/passwd > /dev/null
echo $?        # 0
echo $?        # 0     ← this is the result of the echo just run. The grep value has already been overwritten
echo $$        # 2314  ← PID of the current shell
sleep 100 &
echo $!        # 2377  ← PID of the sleep just sent to the background

• There is only one thing to memorize — only 0 is success; everything else is failure. The value range is 0 to 255, and the detailed meaning of non-zero values varies by command.
• $? remembers only the one previous command. To check it, read it right away without putting another command in between.

Variables Disappear When You Leave the Shell

export PATH=$PATH:/opt/bin     # valid only in this shell right now

• A variable created with a command disappears when you close the terminal.
• To keep using it, you must write it in a configuration file — ~/.bashrc or ~/.bash_profile. That is tomorrow's topic.

Tomorrow we cover when and in what order these variables are read: the shell environment configuration files.

📖 Glossary

• Shell variable : A variable valid only inside the current shell. It is not passed to child processes.
• Environment variable : A variable passed down to child processes. Created with export.
• export : The command that promotes a shell variable to an environment variable.
• set : The command that shows both shell variables and environment variables.
• env : The command that shows only environment variables.
• PATH : The list of directories to search for executables. Separated by colons.
• PS1 : The environment variable that defines the appearance of the prompt.
• $? : A special variable holding the exit status of the previous command. 0 means success.