Quoting, expansion, and globbing · Linux

The single most common source of bugs in shell scripts is incorrect quoting. The shell does more to what you type than any other piece of software on the system, and almost none of it is visible.

Three kinds of quotes

Quote	Meaning
`'single quotes'`	Literal. No expansion happens inside. Every character is taken as-is.
`"double quotes"`	Most expansions happen, but word splitting and pathname expansion are suppressed.
`backticks` and `$( … )`	Command substitution. The output of the inner command becomes part of the outer command.
`\`	Escape. Removes the special meaning from the next character.

Read-only / Safequote comparison— Single quotes: no expansion. Double quotes: variable expanded. Backslash escapes a single character even inside double quotes.

$ echo literal-USER; echo "double: $USER"; echo "escaped: $USER"

literal $USER
double: root
escaped: $USER

Illustrative output

The expansion order

Before bash runs a command, it expands parts of the line. The order is fixed and matters:

Brace expansion — a{1,2,3}b becomes a1b a2b a3b. This happens first, before any other expansion
Tilde expansion — ~user becomes that user's home directory
Parameter expansion — $var and ${var} are replaced by the variable's value
Command substitution — $(cmd) and ` cmd ` are replaced by the command's output
Arithmetic expansion — $((expr)) is replaced by the result
Word splitting: the result of unquoted expansions is split on IFS (default: space, tab, newline). Suppressed inside double quotes.
Pathname expansion (globbing): *, ?, and [] match filenames. Suppressed inside double quotes.
Quote removal. The special meaning of the quotes themselves is stripped

A subtle point: word splitting and pathname expansion are the two things that double quotes suppress. Everything else (parameter, command, arithmetic) still happens inside double quotes.

Globbing

Glob patterns match filenames against the current directory:

Pattern	Matches
`*`	Any string, including empty (not crossing `/`)
`?`	Any single character
`[abc]`	Any character in the set
`[!abc]` or `[^abc]`	Any character not in the set
`[a-z]`	Any character in the range
`{a,b,c}`	Brace expansion: literal alternatives

Read-only / Safeglob examples— *.conf matches files ending in .conf in /etc. host* matches anything starting with host.

$ ls /etc/*.conf 2>/dev/null | head -5; echo ---; ls /etc/host* 2>/dev/null

/etc/adduser.conf
/etc/ca-certificates.conf
/etc/debian_version.conf
/etc/deluser.conf
/etc/environment.conf
---
/etc/host.conf
/etc/hostname
/etc/hosts
/etc/hosts.allow
/etc/hosts.deny

Illustrative output

Word splitting in scripts

Word splitting is the silent force that breaks unquoted variable references:

Read-only / Safeword splitting on spaces— Both forms print 'Hello Alice Smith' here because echo concatenates its arguments. But try `cp $name /tmp/` — the unquoted form becomes `cp Alice Smith /tmp/`, three arguments; the quoted form is `cp "Alice Smith" /tmp/`, two arguments.

$ name="Alice Smith"; echo Hello $name; echo "Hello $name"

Hello Alice Smith
Hello Alice Smith

Illustrative output

A safer pattern: always quote. The next lesson covers command composition, which is where word splitting becomes most dangerous.

Special variables worth knowing

Variable	Meaning
`$0`	The shell’s own name (or the script’s name when sourced)
`$1` … `$9`	Positional parameters
`$@`	All positional parameters, each as a separate word
`$*`	All positional parameters joined by the first character of IFS
`$#`	Number of positional parameters
`$?`	Exit code of the last command
`$$`	PID of the current shell
`$!`	PID of the last backgrounded process
`$-`	Current shell option flags

Read-only / Safespecial variables— $$ is your shell's PID; $# is the count of positional parameters; $@ is all of them. The output shows zero args because no arguments were passed.

$ echo "I am $$, with $# args: $@"

I am 12345, with 0 args:

Illustrative output

Three kinds of quotes

The expansion order

Globbing

Word splitting in scripts

Special variables worth knowing

Knowledge check