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Variables & Expansion

Master variables, quoting, parameter expansion, command and arithmetic substitution, and globbing — and the exact order the shell applies them.

Before bash runs a command, it expands the line you typed: it substitutes variables, runs sub-commands, does arithmetic, splits the result into words, and matches filename patterns. These steps happen in a fixed order, and getting the order wrong is the source of countless shell bugs.

The stepper below rewrites one command line stage by stage, ending in the final argument list (argv) the program actually receives. Step through each example.

env:USER=learnerHOME=/home/learnerfile=report.tar.gzgreeting= (empty)files: a.txt b.txt notes.md report.tar.gz
bracetildeparametercommandarithmeticword-splitglob
source line
echo {a,b}.txt
The unexpanded command. Press play to apply each expansion in order.
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Variables and quoting

Assign with no spaces around =; read back with $:

name="Ada Lovelace"
echo $name      # Ada Lovelace  -> but TWO arguments after word-splitting
echo "$name"    # Ada Lovelace  -> ONE argument; quotes preserve the space

The difference between quotes is the single most important rule:

  • Single quotes '...' are literal: nothing inside is expanded.
  • Double quotes "..." expand $var, $(...), and $(( )), but suppress word-splitting and globbing.
echo '$name costs $5'   # $name costs $5
echo "$name is here"    # Ada Lovelace is here

Rule of thumb: always double-quote your variable expansions unless you specifically want splitting or globbing.

Parameter expansion

${...} does more than fetch a value — it can supply defaults and trim text:

echo "${greeting:-hello}"   # use "hello" if greeting is unset/empty
echo "${count:=0}"          # also ASSIGN the default back to count
file=report.tar.gz
echo "${file%.gz}"          # report.tar   -> %  strips shortest suffix
echo "${file%%.*}"          # report       -> %% strips longest suffix
echo "${file#*.}"           # tar.gz       -> #  strips shortest prefix
echo "${#file}"             # 14           -> length

Command and arithmetic substitution

$(...) runs a command and pastes its standard output into the line. $(( )) evaluates integer math:

today=$(date +%F)            # capture command output
files=$(ls | wc -l)          # count entries in the current directory
echo "$(( (3 + 4) * 2 ))"    # 14  -> normal arithmetic precedence
i=0; echo "$(( i + 1 ))"     # 1

Word splitting and globbing — last

After substitution, unquoted results are split on $IFS (space, tab, newline), then patterns are matched against filenames:

  • * matches any run of characters, ? matches one, [abc] a character set.
  • A pattern that matches nothing is left unchanged (by default).
for f in *.txt; do echo "$f"; done   # loops over matching files

This is exactly why rm "$file" is safe but rm $file can detonate when the name has spaces or globs.

Takeaways

  • Expansion order is fixed: brace, tilde, parameter/command/arithmetic, word splitting, then globbing.
  • Single quotes are literal; double quotes expand but block splitting and globs.
  • Double-quote variable expansions by default to avoid word-splitting surprises.
  • ${var:-default}, ${var%pat}, $(...), and $(( )) are everyday tools.

References