Variables and the Environment

Strings by default

Assign a variable with name=value, with no spaces around =: count = 3 tries to run a command named count. Read it with $name or ${name}. Bash variables are strings by default; arithmetic happens only in arithmetic contexts. Names are case-sensitive; by convention, use lowercase for script-local variables and UPPERCASE for environment variables and constants, to avoid clobbering system variables such as PATH, HOME or USER. A plain variable is visible only in the current shell; export name puts it in the environment so child processes (commands you run) inherit it. Setting a variable just for one command is possible with a prefix: LANG=C sort file. readonly makes a variable constant; local (inside functions) limits its scope; unset removes it. Special variables include $0 (script name), $1…$9 (arguments), $# (argument count), $? (last status), $$ (shell PID) and $! (last background PID).

Shell variables and the environment

Exported variables are copied into each child process's environment; plain ones stay in the shell.

Assignment, export and per-command variables

Only exported variables reach child processes.

greeting="hello"            # no spaces around =
readonly MAX_RETRIES=3

bash -c 'echo "child sees: [$greeting]"'   # empty: not exported

export API_URL="https://api.example.com"
bash -c 'echo "child sees: $API_URL"'     # inherited

TZ=Asia/Kolkata date          # variable set only for this one command

echo "script: $0, args: $#, pid: $$"

Never name a variable PATH

A loop such as for PATH in ... silently overwrites the command search path, after which every external command fails with "command not found". Lowercase names for your own variables avoid this class of bug.

Quick check: Which assignment is valid Bash?

• count = 3
• $count=3
• count=3
• let count := 3