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The command sed -i 's/port/listen_port/g' app.yml replaces every occurrence of the letters port on each line, including inside longer names and paths. To target a standalone word with GNU sed, use a word boundary such as b—but first preview the result and inspect a diff before changing the file.
Why did sed change four lines when I wanted one?
The pattern port has no boundaries, so sed treats it as a substring to find, not as a complete word or YAML key. The g flag asks sed to replace every match on each line. In Anguishe’s October 1, 2026 example, the command sed -i 's/port/listen_port/g' app.yml changed all four lines in this file:
port: 8080
support_email: ops@example.com
transport: tcp
report_dir: /var/reportsThe unwanted matches occur inside support_email, transport, and report_dir. The command reportedly exited with status 0, which indicates it ran successfully—not that the result matched the author’s intention. This is the author’s reported example, not an independently reproduced test.
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How do I replace only the exact word with sed?
With GNU sed, put the pattern between word-boundary markers:
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sed 's/bportb/listen_port/g' app.ymlGNU sed 4.10 defines b as a word boundary. In GNU sed, word characters are letters, digits, and underscores, so this matches standalone port but not the substring in db_port. See the GNU sed 4.10 manual.
This syntax is not portable POSIX sed syntax. Boundary behavior and in-place options vary across implementations, including GNU and BSD sed. If a script must run on multiple platforms, check the manual for each target sed rather than assuming GNU syntax will work everywhere. The POSIX.1-2024 sed specification describes the portable interface.
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Decide whether underscores belong to the name
Because GNU sed treats underscore as a word character, bportb will not match db_port. That is often desirable for identifiers, but the intended definition of “whole word” depends on the data. If underscores should separate words, GNU’s b does not express that rule; define a pattern suited to the actual naming convention and test it against representative input.
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A safer workflow is to generate the proposed output, compare it with the original, and only then decide whether to write the change. Anguishe’s example uses:
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sed 's/port/listen_port/g' app.yml | diff app.yml -This sends the transformed text to diff without writing it to app.yml. Review every changed line. If the diff shows matches in names or paths you did not intend to alter, refine the pattern and preview again. Apply an in-place edit only after the preview contains the expected changes and no surprises.
| Approach | Match scope | Portability | Can inspect before writing? |
|---|---|---|---|
s/port/listen_port/g |
Every occurrence of the substring port on each line |
Basic substitution syntax is widely supported | Yes, if run without an in-place option and reviewed with a diff |
s/bportb/listen_port/g |
Standalone GNU sed word, with underscore counted as a word character | b is GNU-specific; check other implementations |
Yes, if previewed and diffed before in-place editing |
| Structure-aware parser | A field or value selected according to the format’s structure | Depends on the parser and its availability | Yes, when its output is rendered and reviewed before saving |
What else can change the meaning of a substitution?
Replacement text can expand ampersands
The replacement side of a sed substitution has special syntax too. An unescaped & inserts the complete text matched by the pattern. For example, s/port/&_name/ turns a match into the matched text followed by _name. To put a literal ampersand in the replacement, escape it as &. GNU sed documents this behavior, which POSIX sed also specifies: GNU sed manual and POSIX.1-2024 sed specification.
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Choose a different delimiter when slashes get in the way
The slash is conventional, not mandatory. When a pattern or replacement contains paths with slashes, another delimiter can be easier to read. For example, sed 's|/var/reports|/srv/reports|g' app.yml uses a vertical bar as the delimiter.
Use a parser when the edit depends on structure
Sed normally processes input line by line, but GNU sed can put multiple lines into its pattern space with commands such as N. That does not make a text substitution a reliable substitute for understanding a file’s data model. When an edit depends on which YAML key or JSON field a value belongs to, use a structure-aware tool such as jq for JSON, or an appropriate YAML parser. The general rule is to match the tool to the edit: text tools for text patterns, parsers for structure-dependent changes.
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