Syntax
grep [OPTIONS] PATTERN [FILE...]
Description
The grep command searches for lines containing a specified pattern. It reads from files or standard input and outputs only matching lines. This is one of the most powerful text filtering tools in Unix-like systems.
Nash’s grep performs substring matching (not regex) and operates entirely within the virtual filesystem.
Options
The text pattern to search for. Performs literal substring matching.
Invert match. Show lines that do NOT contain the pattern.Alias: --invert-match
Ignore case distinctions. Makes the search case-insensitive.Alias: --ignore-case
Prefix each output line with its line number.Alias: --line-number
Examples
Basic pattern search
echo -e "apple\nbanana\napricot\ncherry" | grep ap
Case-insensitive search
echo -e "Apple\nBANANA\napricot" | grep -i apple
Invert match
echo -e "keep\nremove\nkeep" | grep -v remove
Show line numbers
echo -e "line one\nline two\nline three" | grep -n line
1:line one
2:line two
3:line three
Search in files
echo -e "error: failed\nwarning: slow\ninfo: done" > log.txt
grep error log.txt
Combine flags
echo -e "ERROR\nWarning\nerror" | grep -in error
Pipeline Examples
Filter process output
PATH=/usr/local/bin:/usr/bin:/bin
Multi-stage filtering
ls -la | grep txt | grep -v test
.txt files but excludes those with “test” in the name.
Count matches
cat access.log | grep "404" | wc -l
cat users.csv | grep "@gmail.com"
Practical Use Cases
Log analysis
cat server.log | grep -i error | grep -n "database"
Configuration validation
grep "DEBUG=true" .env && echo "Debug mode enabled"
Filter file listings
ls | grep "\.js$" | grep -v test
Quick content check
grep -i "todo" *.txt || echo "No TODOs found"
cat data.csv | grep "active" | cut -d, -f1,3
Exit Status
- 0 - At least one match found
- 1 - No matches found
Nash’s grep uses substring matching, not regular expressions. The pattern "a.b" matches the literal string “a.b”, not “a” followed by any character and “b”.
Tips
Multiple patterns
Use multiple grep commands in a pipeline:
cat file.txt | grep pattern1 | grep pattern2
OR logic with invert
# Show lines that don't contain "skip" or "ignore"
cat file.txt | grep -v skip | grep -v ignore
Debugging pipelines
cat data.txt | grep -n error
sed - Stream editor for text transformationcut - Extract fields from linessort - Sort lines of textuniq - Filter duplicate lines
