Expansion and Quoting

The shell rewrites your line

When you press Enter, bash doesn’t hand your text straight to a program. It expands it first, rewriting the line in a fixed order ^{1 2}:

1. Brace expansion
2. Tilde expansion
3. Parameter expansion, arithmetic expansion, and command substitution (together, left to right)
4. Word splitting
5. Filename expansion (wildcards)
6. Quote removal

Only then does the program run, with whatever words are left as its arguments. The program never sees your *, ~, or $; it sees what they turned into. That’s why echo is the best way to preview what the shell will do: echo rm *.log shows exactly which files rm *.log would get ².

Brace expansion

Braces make several words from one pattern ^{2 1}:

me@linuxbox:~$ echo front-{A,B,C}-back
front-A-back front-B-back front-C-back
me@linuxbox:~$ echo {1..5} {a..e}
1 2 3 4 5 a b c d e
me@linuxbox:~$ echo {01..12}
01 02 03 04 05 06 07 08 09 10 11 12
me@linuxbox:~$ mkdir -p photos/{2025,2026}/{01..12}     # 24 folders in one go
me@linuxbox:~$ cp config.txt{,.bak}                      # cp config.txt config.txt.bak

Unlike wildcards, braces don’t look at what files exist; they just generate text.

Tilde, parameters, and arithmetic

Tilde: ~ becomes your home directory, ~bob becomes bob’s ².

Parameters: $NAME becomes the value of a variable ²:

me@linuxbox:~$ echo $USER $HOME
me /home/me
me@linuxbox:~$ echo $SUER

A misspelled variable doesn’t cause an error; it quietly expands to nothing ². Use braces when a name runs into other text: echo "${USER}_backup".

Arithmetic: $(( )) does whole-number math ²:

me@linuxbox:~$ echo $((2026 - 1905))
121
me@linuxbox:~$ echo $(( 7 / 2 )) $(( 7 % 2 ))
3 1

Command substitution

$( ) runs a command and puts its output in its place ²:

me@linuxbox:~$ echo "Today is $(date +%A)"
Today is Monday
me@linuxbox:~$ ls -l $(which cp)
-rwxr-xr-x 1 root root 141824 Apr  5  2024 /usr/bin/cp
me@linuxbox:~$ cp report.txt "report-$(date +%F).txt"     # report-2026-10-05.txt

You’ll see the older form with backquotes, `which cp`, in old scripts; $( ) does the same and nests cleanly ².

Word splitting and filename expansion

After those substitutions, bash splits any unquoted results into separate words at spaces, tabs, and newlines ^{2 1}. Then it expands wildcards (*, ?, [...]) in each word into matching file names, the step you met in module 2. Finally it removes the quote characters themselves ¹.

Splitting is why extra spaces vanish:

me@linuxbox:~$ echo this   is   a    test
this is a test

Quoting

Quoting switches expansions off ²:

• Double quotes "..." stop word splitting, wildcards, tildes, and braces, but keep $ expansions: variables, $(...), and $((...)). Inside them, only $, `, and \ stay special.
• Single quotes '...' stop everything. What you type is exactly what the program gets.
• A backslash \ protects the one character after it.

me@linuxbox:~$ echo text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER
text /home/me/notes.txt a b foo 4 me
me@linuxbox:~$ echo "text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER"
text ~/*.txt {a,b} foo 4 me
me@linuxbox:~$ echo 'text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER'
text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER
me@linuxbox:~$ echo "The balance for $USER is \$5.00"
The balance for me is $5.00

The classic surprise: echo The total is $100.00 prints The total is 00.00, because $1 is an (empty) variable ². Single quotes fix it.

A rule of thumb: single quotes for fixed text, like grep and find patterns; double quotes for text that should include variables.

Quote your variables

Double quotes also stop word splitting inside a variable’s value. That’s the source of a whole family of bugs ²:

me@linuxbox:~$ f="tax notes.txt"
me@linuxbox:~$ ls -l $f
ls: cannot access 'tax': No such file or directory
ls: cannot access 'notes.txt': No such file or directory
me@linuxbox:~$ ls -l "$f"
-rw-r--r-- 1 me me 812 Oct  5 09:10 tax notes.txt

The same goes for "$(...)": unquoted, a command’s multi-line output is flattened onto one line; quoted, the lines survive ². Compare echo $(df -h) with echo "$(df -h)".

Habit: write "$var" and "$(command)", always, unless you specifically want the result split. In scripts this one habit prevents most bugs with odd file names.

Watching the shell work

To see exactly what bash runs after expansion, turn on tracing ¹:

me@linuxbox:~$ set -x
me@linuxbox:~$ ls ~/*.txt
+ ls --color=auto /home/me/notes.txt /home/me/todo.txt
me@linuxbox:~$ set +x

Every line after + is the fully expanded command. It’s the first thing to reach for when a command doesn’t do what you meant.

Your turn

So

Bash expands every line before running it: braces, tilde, variables, arithmetic, and command substitution, then word splitting, wildcards, and quote removal. Double quotes keep the $ expansions but stop splitting and wildcards; single quotes stop everything. Preview with echo, trace with set -x, and quote your variables.