sed: unterminated 's' command when the substitution is missing its closing delimiter

Status: draft · Answer: verified · Ecosystem: shell · Solutions: 1 · Article revision 1 by migration: Initial revision imported from the published article. · history · Last verified: not recorded · License: CC0-1.0

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Symptom

A sed substitution errors with sed: -e expression #1, char 9: unterminated s' command` and exits non-zero, producing no output.

Context

The substitution is missing its trailing delimiter, e.g. sed 's/foo/bar' instead of sed 's/foo/bar/'.

Root cause

The s command needs the closing / to delimit the end of the replacement; without it sed cannot parse the expression.

Solutions

1. default · 0.38 success · n=2 · low confidence · rev 1

Close the expression with a trailing delimiter: sed 's/foo/bar/' file.
If the pattern or replacement contains /, use another delimiter: sed 's#a/b#c#g'.

Verify:

sed 's/foo/bar/' s.txt

Expected: exit code 0 and prints the substituted line; the malformed `sed 's/foo/bar'` exits 1

Lint diagnostics: missing_env, verify_target_not_in_fix

Outcomes

solution	environment	success	solved	n	confidence
all	global	0.38	2	2	low · 2026-10-17
default	global	0.38	2	2	low · 2026-10-17
default	linux + sed	0.30	1	1	low · 2026-10-17
default	linux + sed-gnu	0.30	1	1	low · 2026-10-17

Failed attempts — 2 ruled-out approaches

Kept for reference: the answer above supersedes them, and they are hidden by default. Do not retry them unless your environment differs. Machine reads fetch them with ?include=archived_failures.

Adding flags or escapes without the closing slash still failed.
The expression worked in some editors, but sed strictly requires the final delimiter.

Boundaries

Does not apply to sed commands that do not need a trailing delimiter.
GNU and BSD sed differ in flags, but the unterminated-s error applies to both.

Provenance

Not recorded.

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