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grep reads standard input or named files and prints the lines that contain a match for your pattern. A practical starting point is grep -E 'error|failed' app.log: the shell passes the single-quoted regular expression unchanged, and GNU grep prints every line containing either word. Add anchors, character classes, or options such as -i, -w, and -x when a broad substring search is not precise enough.
This guide explains GNU grep’s regular-expression dialects, reliable shell quoting, pattern construction, multiple-pattern searches, file traversal, and the locale and binary-data cases that can make an apparently correct command behave unexpectedly.
What grep actually searches
GNU’s definition is precise: grep searches named input files—or standard input when no file is named, or when a single hyphen-minus is used as a file name—for lines containing a match to the supplied pattern. The match is normally any part of a line; grep then writes the complete matching line, not just the matched characters.
For example:
grep 'timeout' server.logThis searches server.log with grep’s default regular-expression syntax and prints every line containing a sequence that matches timeout. A command can also receive data through a pipe:
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journalctl -u my-service | grep -i 'failed'The pattern is interpreted by grep after the shell has processed the command line. That distinction explains why quoting is a core part of writing safe regular-expression commands.
Choose the matching mode before writing the pattern
GNU grep provides four matcher families. Select the simplest one that expresses the search you need.
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| Mode | Option | What it interprets | When to choose it |
|---|---|---|---|
| Basic regular expressions | -G (default) |
POSIX BRE syntax | Simple patterns, or scripts that already use BRE quoting. |
| Extended regular expressions | -E |
POSIX ERE syntax | Grouping, alternation, and repetition are easier to read. |
| Fixed strings | -F |
Literal text, not regex metacharacters | Exact phrases or large lists of literal terms; avoids regex interpretation. |
| Perl-compatible regular expressions | -P |
PCRE features in GNU grep builds that provide them | Only when a PCRE-only feature is required and portability is acceptable. |
The default is BRE, so grep 'timeout' file and grep -G 'timeout' file select the same family. ERE is usually the clearest choice when a pattern has alternatives or groups:
grep -E 'timeout|connection refused' app.log-P is not a universal portability switch. Support and diagnostics vary between builds, so do not use PCRE syntax merely out of habit. If a BRE or ERE can express the condition, it is generally the more portable option.
Quote patterns so the shell cannot change them
Regular expressions contain characters that also have meaning to a shell, including $, *, parentheses, brackets, and the vertical bar. Single quotes are the safest default because the shell passes every character between them literally:
grep -E '^(ERROR|WARN):' app.logWithout quotes, the shell may expand a wildcard, treat a dollar sign as a variable, or interpret other syntax before grep sees it. Use double quotes only when you intentionally need shell expansion, and then escape any regular-expression characters that the shell should not process.
A pattern that starts with a hyphen can be mistaken for an option. Give it explicitly with -e, or terminate options with --:
grep -e '-pattern' file.txt
grep -- '-pattern' file.txtThe -e form is especially useful in scripts because it unambiguously marks the following argument as a pattern.
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Build a regex from anchors, literals, and classes
By default, grep looks for a match anywhere in each input line. Anchors constrain where that match may occur.
| Expression | Meaning | Example use |
|---|---|---|
^ |
Beginning of the line | ^ERROR finds lines whose first characters are ERROR. |
$ |
End of the line | ready$ finds lines ending in ready. |
. |
One character (other than the line terminator) | v.. finds v followed by two characters. |
[0-9] |
One character from the listed range | [0-9][0-9] finds two digits. |
[[:digit:]] |
POSIX digit character class | Use when a named class is clearer or locale-aware behavior matters. |
To match a complete status line rather than a substring, combine both anchors with -x or write the anchors yourself. These are equivalent for the shown literal:
grep -x 'READY' status.txt
grep '^READY$' status.txtBracket expressions can list characters, ranges, or POSIX classes. POSIX classes such as [[:lower:]] and [[:digit:]] communicate intent better than ad hoc ranges when a script may run under different locale settings. A range such as [a-z] can be interpreted according to the active locale, so its set is not necessarily identical in every environment.
The dot is deliberately broad: it can match punctuation, digits, and letters. Replace it with a bracket expression when the allowed character set matters. For example, file[0-9] requires one digit after file, whereas file. accepts any single character.
BRE and ERE: the syntax difference that causes most surprises
Both BRE and ERE search for a match within a line, but several operators have different spellings. In BRE, parentheses, braces, and alternation commonly need backslashes to become special. ERE makes the same constructs readable without those backslashes.
| Intent | BRE (default) | ERE (-E) |
|---|---|---|
| Group two alternatives | (cat|dog) |
(cat|dog) |
| Repeat a preceding item two to four times | ab{2,4} |
ab{2,4} |
| Match either word at the start of a line | ^(ERROR|WARN): |
^(ERROR|WARN): |
For a simple literal, the modes are interchangeable:
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grep -E 'timeout' server.logUse ERE when alternatives, groups, or intervals are central to the condition. It reduces visual escaping and makes review easier. Keep BRE when maintaining an existing command or when a minimal pattern does not benefit from ERE.
POSIX defines an ERE match as matching some part of an input line, excluding the terminating newline. That is why an unanchored expression such as error also matches database error: retrying.
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Use options to control precision and case
Ignore case with -i
-i makes alphabetic matching case-insensitive:
grep -i 'warning' app.logCombine it with -w when the term should stand alone:
grep -iw 'warning' app.logMatch a whole word with -w
-w restricts a match to a word boundary as grep defines it. It prevents a search for warning from selecting a longer word such as warningly, while still matching a line such as warning: disk nearly full. It is a word test, not a whole-line test; surrounding punctuation and other words may remain.
Match the entire line with -x
-x selects only lines whose complete content matches the pattern:
grep -x 'READY' status.txtThis is useful for allowlists, state files, and one-value-per-line inputs. A line containing READY now will not match.
Invert selection with -v
-v prints lines that do not match:
grep -v '^#' config.confThat command removes lines beginning with #, but it does not remove blank lines. If both comments and blanks must be excluded, express both cases in one pattern and invert it, for example with ERE:
grep -Ev '^(#|$)' config.confOptions can be combined when their meanings do not conflict. For example, grep -ivw 'warning' file performs a case-insensitive whole-word search and prints matching lines.
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Put alternatives in one ERE
Use the ERE alternation operator when a short list belongs to one logical condition:
grep -E 'timeout|connection refused' app.logGrouping lets you attach an anchor or suffix to the alternatives as a unit:
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grep -E '^(ERROR|WARN):' app.logThe anchor applies to the group, so only lines beginning with either label qualify.
Use repeated -e options
Separate -e options are clearer when patterns are independent or contain different syntax:
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Read many patterns from a file with -f
Store one pattern per line in a file and pass it with -f:
grep -f patterns.txt access.logEach line in patterns.txt is interpreted as a pattern in the selected matcher family. This keeps a growing list out of shell quoting and makes it easy to review or version-control the list. If the entries are literal phrases rather than regexes, add -F so punctuation is not treated as metacharacters:
grep -F -f literal-terms.txt access.logChoose literal matching when regex is unnecessary
Regex syntax is useful only when you need pattern behavior. For an exact phrase containing characters such as ., *, or ?, -F avoids escaping and prevents accidental broad matches:
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grep -F 'version 1.2.3' release-notes.txtUse -F for large lists of literal identifiers as well. Use the default BRE or -E when the expression must vary by character, position, repetition, or alternative.
Search one file, standard input, or a directory tree
Named files and standard input
A filename after the pattern restricts the search to that file. Omitting filenames makes grep read standard input, which is why it works naturally in pipelines:
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cat access.log | grep -E ' 5[0-9][0-9] '
grep -E ' 5[0-9][0-9] ' access.logThe second form avoids an unnecessary cat process and is easier to extend with additional filenames.
Recursive traversal
When the text may be spread across a directory tree, use grep’s recursive options rather than invoking it separately for every file:
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grep -RInE 'TODO|FIXME' src/Here -R traverses directories, -I skips binary files, and -n includes line numbers; -E enables alternation. Traversal can include generated files, dependency trees, or very large data, so narrow the starting directory and pattern when possible. If you need behavior that follows or avoids symbolic links, check the distinction between your platform’s recursive options before using the command in a script.
Locale, encoding, and binary-data caveats
Character ranges and POSIX classes are affected by the current locale. A script that must behave consistently across machines should choose an intentional locale and prefer named POSIX classes where they express the requirement. Do not assume that a range such as [a-z] has exactly the same ordering or membership under every locale.
GNU grep may classify input containing null bytes or encoding errors as binary. In that situation it can suppress the normal stream of matching lines and instead report that a binary file matches. This is a property of the input classification, not proof that the regex is wrong. If the file is known to be text despite the bytes it contains, use the text-handling option appropriate to your GNU grep version, or preprocess the data into a text encoding you control. For portable scripts, document the expected encoding rather than silently relying on a terminal’s locale.
PCRE mode has an additional portability caveat: -P may be unavailable or may emit different warnings on different GNU grep builds. Treat it as an environment-dependent feature, not a baseline requirement for Linux shell scripts.
A repeatable workflow for writing a grep regex
- Decide whether the input is literal or patterned. Start with
-Ffor a phrase or identifier that should be taken literally. - Define the match boundary. Choose an unanchored substring, a whole word with
-w, or a whole line with-x. Use^and$when the boundary is part of a larger expression. - Select the dialect. Keep BRE for simple or existing expressions; use
-Efor readable groups, alternation, and intervals; reserve-Pfor required PCRE features. - Add character constraints. Replace broad dots with bracket expressions or POSIX classes when the allowed characters matter.
- Quote the pattern. Single-quote it unless intentional shell expansion is required.
- Test against positive and negative examples. Include lines that should match, near misses such as longer words, and empty or comment lines when those are relevant.
- Scale the input only after the pattern is correct. Move from one file to a pipeline or recursive directory search, then add line numbers or binary-file handling if needed.
Common failure modes and fixes
The pipe character is printed literally or behaves strangely
If you run grep 'error|failed' file without ERE mode, the expression is interpreted using BRE rules, where alternation is not written the same way. Use grep -E 'error|failed' file, or write the BRE form with the required escaping.
Parentheses or braces do not group or repeat
Check the dialect first. In BRE, grouping and interval expressions commonly require backslashes; in ERE, write (...) and {m,n} directly after adding -E.
A pattern matches too many lines
An unanchored regex searches any part of a line, and a dot matches almost any character. Add ^ or $, use -w or -x, and replace . with a narrower bracket expression.
A pattern beginning with a dash is treated as an option
Pass it with -e or place -- before the pattern. Quoting alone does not stop option parsing.
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Check case with -i, verify that -v was not accidentally used, and inspect whether -x or -w made the condition stricter than intended. If grep reports a binary match instead of lines, review the file’s bytes and encoding.
Quick Recap
Quick decision table
| Need | Command shape | Reason |
|---|---|---|
| Literal text | grep -F 'text' file |
Metacharacters remain literal. |
| Either of two terms | grep -E 'one|two' file |
ERE alternation is readable. |
| Case-insensitive whole word | grep -iw 'warning' file |
Combines case folding and word boundaries. |
| Exact line | grep -x 'READY' file |
Rejects surrounding characters. |
| Exclude comments | grep -v '^#' file |
Inverts a beginning-of-line test. |
| Many patterns | grep -f patterns.txt file |
Keeps one pattern per file line. |
Pattern starts with - |
grep -e '-pattern' file |
Marks the argument as a pattern. |
| Directory tree | grep -R 'pattern' directory/ |
Traverses files below the chosen directory. |
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