A log file can hold more lines than you can scan by eye when you need to find an error message. grep is a command-line tool that searches text and prints the lines that match.
I’ll show you how to search a file with grep and read its matches and exit status.
TL;DR: Search for Text with grep
grep prints each input line that matches a search expression, so you can search a file or filter output from another command.
grep -n -i 'error' service.log
- Use -F for literal text and -E for extended regular expressions.
- Add -i to ignore letter case and -n to show line numbers.
- Status 0 means a match, 1 means no match, and 2 means an error.
What Is the grep Command?
grep is a command-line search tool that selects whole lines containing a search expression. With a filename, it scans that file.
The basic form is grep [options] expression [file…]. It prints complete matching lines. Regular expressions are the default, so a period can have special meaning.
Use -F for literal text or -E for operators such as the vertical bar in grep -E ‘error|failed’ app.log.
How to Use grep in Linux Step by Step
Start with a readable file and a short search term, then add only the options needed. I use a small log file here to make case-insensitive matching and line numbers visible.
Step 1: Search one file
Give grep a quoted search term followed by a filename.
grep 'error' service.log
This prints lines containing lowercase error. grep writes selected lines to standard output, leaving the original file unchanged.
To restrict a search to a whole word, use -w. That keeps a search for cat from selecting a line containing catalog.
Step 2: Add line numbers and ignore case
Use -n to prefix each line with its 1-based number and -i to ignore case. I ran this exact command against service.log:
grep -n -i 'error' service.log
The exact output was:
1:error: disk full
4:ERROR: connection refused
The line numbers identify each match, and -i includes both uppercase and lowercase forms.
Step 3: Search several files or a directory tree
List filenames after the search term to scan several files.
A filename wildcard such as *.log is expanded by the shell before grep runs. This is useful for a group of logs, but a very broad wildcard can include files you did not intend to search. Narrow the file expression or name the paths directly when accuracy matters.
grep -n 'timeout' app.log worker.log
grep -R -n 'timeout' logs/
Recursive mode descends into directories, so choose a specific starting location. Use the Linux find command to locate paths by name before searching their contents.
Step 4: Filter command output
A pipe sends one command’s standard output to grep as standard input. The example selects listing lines that begin with d, which marks directory entries in long listing format.
ls -l | grep '^d'
This searches text produced by ls, not file contents. See the ls command guide for listing files.
Step 5: Choose the output you need
Choose an option based on what you need to inspect.
| Option | Effect |
|---|---|
| -w | Match a whole word |
| -v | Select lines that do not match |
| -c | Count matching lines per file |
| -l | Print names of files with a match |
-c counts matching lines rather than every occurrence. Use -o when you need only the matched text.
The context options help when a matching line needs surrounding information. -A shows following lines, -B shows preceding lines, and -C shows context on both sides. If nearby matches overlap, grep avoids printing a line twice.
grep Results, Errors, and Useful Next Steps
grep’s exit status distinguishes a match, no match, and an error. Status 1 can be a normal result when a search finds nothing.
| Status | Meaning | What to check |
|---|---|---|
| 0 | At least one line matched | Review the selected lines |
| 1 | No line matched | Check spelling, case, path, and expression type |
| 2 | An error occurred | Check the filename, permissions, and diagnostic message |
If a pipeline is still running, Ctrl+C interrupts it. grep exits on its own when the search completes.
A missing or unreadable file produces an error diagnostic. GNU-specific options may not exist in every implementation, so check the local manual with man grep.
Conclusion: Read the Match and the Exit Status
Begin with a quoted search term and a file, then add options such as -n or -i when needed.
For the full option list and regular-expression details, consult the GNU grep manual or run man grep on your system.
FAQ
These answers cover two common questions about using grep in a shell.
What does grep do in Linux?
grep searches text input for a pattern and prints the lines that match it.
How do I search without matching letter case?
Add the -i option, as in grep -i ‘error’ file.log.
How do I stop a grep command that is still running?
Press Ctrl+C to interrupt the foreground command. A grep search that has completed exits on its own.