🚨 Bash Error Guide: '[: missing ]' — Fix Unterminated Test Brackets

Overview

Bash prints [: missing ']' when a [ test command starts but never receives its closing ] as a separate final argument:

./run.sh: line 5: [: missing `]'

[ is literally a command whose name is [, and it requires its last argument to be ]. That means ] must be surrounded by spaces so the shell passes it as its own word. Forgetting the bracket, jamming it against a value, or letting an unquoted operand consume it all produce this error.

Symptoms

• A conditional fails at parse-of-arguments time with missing ']'.
• The test looks like it has a ], but it’s glued to a variable ("$x"]) or missing a space.
• It appears after refactoring a long [ ... ] line or converting from [[ ]].
• Only some inputs trigger it — a hint that an empty operand is eating the bracket.

Common Root Causes

• No closing bracket at all — [ -f "$f" with the ] forgotten.
• No space before ] — [ -f "$f"] passes the single word "$f"], so [ never sees a bare ].
• No space after [ — [-f "$f" ] runs a command literally named [-f.
• Unquoted empty operand — [ $x ] where $x is empty can leave the argument list malformed.
• A ] that got quoted or escaped — [ -f "$f" "]" ] or \] isn’t recognized as the terminator.
• Line continuation gone wrong — a \ newline that dropped the bracket.

Diagnostic Workflow

Show exactly what words [ receives, including whether ] is its own word:

f=/etc/hosts
set -- -f "$f"]      # simulate the glued bracket
printf '[%s]\n' "$@" # each argument on its own line
# [-f]
# [/etc/hosts]       <-- no standalone ] argument

Syntax-check and trace:

bash -n ./run.sh
bash -x ./run.sh 2>&1 | grep "missing\|'\['"

Prefer [[ ]], whose brackets are shell keywords and don’t need surrounding spaces the same way — a quick way to confirm the problem is [ ] argument tokenization.

Example Root Cause Analysis

A cleanup script checked for a lockfile:

if [ -f "$LOCKDIR/$SERVICE.lock"]; then
  echo "already running"; exit 0
fi

It failed with:

cleanup.sh: line 2: [: missing `]'

There was a ] on the line — but no space before it. The shell tokenized "$LOCKDIR/$SERVICE.lock"] as a single argument, so [ received -f and /var/lock/api.lock] and never got a standalone ]. Adding the space fixes it:

if [ -f "$LOCKDIR/$SERVICE.lock" ]; then
  echo "already running"; exit 0
fi

Better still, use [[ ]], which is more forgiving and supports richer tests:

if [[ -f "$LOCKDIR/$SERVICE.lock" ]]; then
  echo "already running"; exit 0
fi

Prevention Best Practices

• Always put spaces around [ and ] — [ ... ], never [... ] or [ ...]. The brackets are arguments, not punctuation.
• Prefer [[ ... ]] in Bash scripts — clearer, no word-splitting, and less bracket-spacing fragility.
• Quote operands — [ -f "$f" ] prevents an empty value from disturbing the argument list.
• Syntax-check in CI with bash -n and ShellCheck (SC1009/SC1073 flag the malformed test).
• Format tests on one clear line or use proper \ continuation so the closing bracket is never dropped.

Quick Command Reference

[ -f "$f" ]                 # spaces around [ and ] are mandatory
[[ -f "$f" ]]              # keyword form: no bracket-spacing traps
bash -n ./script.sh         # catch the malformed test pre-run
shellcheck ./script.sh      # flags missing/glued brackets
set -- -f "$f" ]; printf '[%s]\n' "$@"   # inspect the argument words

Related Guides

• Bash Error Guide: ‘unary operator expected’ — another [ ] failure from a missing operand.
• [Bash Error Guide: ’: too many arguments’ — the opposite: word-splitting adds extra arguments.
• Bash Error Guide: ‘syntax error near unexpected token’ — related parse-time failures in conditionals.

Conclusion

[: missing ']' is [ telling you its closing bracket never arrived as its own word — usually a forgotten bracket or a missing space that glued ] onto a value. Keep spaces around [ and ], quote your operands, and prefer [[ ... ]] in Bash. A bash -n and ShellCheck pass in CI catches the malformed test before it ever runs.