How do I do a case-insensitive search in awk?

Use `tolower()` on the field before comparison: `awk 'tolower($0) ~ /error/'`. Alternatively, in gawk, set `IGNORECASE=1` in a BEGIN block. The `tolower()` approach is POSIX-portable and works in mawk and gawk. Avoid relying on `IGNORECASE` if your scripts need to run on systems where the default awk is mawk or nawk.

What is the difference between sed -i and sed -i.bak?

`sed -i` (GNU sed) overwrites the file with no backup. `sed -i.bak` creates a backup with the `.bak` extension before overwriting. On macOS BSD sed, the syntax is `sed -i '' 's/foo/bar/' file` for no backup - omitting the argument entirely is a syntax error. In production scripts, always use a backup suffix or copy the file before editing.

Can awk handle CSV files with quoted fields containing commas?

Standard awk with `-F,` breaks on quoted commas. For simple cases, use `FPAT` in gawk 4.0+: `gawk 'BEGIN{FPAT="([^,]+)|(\"[^\"]+\")"}' file.csv`. For anything more complex - nested quotes, escaped quotes - use a dedicated tool like `csvkit` or Python's `csv` module. Do not try to handle RFC 4180 edge cases in plain awk.

How do I print lines between two patterns in sed?

Use a range address: `sed -n '/START/,/END/p' file`. This prints from the first line matching START through the next line matching END, inclusive. If you want exclusive (without the delimiter lines), add a negation: `sed -n '/START/,/END/{/START/!{/END/!p}}' file`. The inclusive form is usually sufficient and cleaner.

How do I pass a shell variable into an awk script?

Use `-v varname="$SHELL_VAR"`. Do not embed shell variables directly inside awk single quotes - they are not expanded. Example: `awk -v threshold="$MAX_BYTES" '$10 > threshold {print}' access.log`. For strings with special characters, using `-v` handles most cases, but for values with backslashes, piping via `ENVIRON` or using `ARGV` is safer.

Linux sed and awk: Practical Examples for Sysadmins

sed Basics: Substitution, Deletion, and In-Place Editing

The substitute command is the core of sed usage. The basic form is `s/pattern/replacement/flags`. Without the `g` flag, only the first match per line is replaced. With `g`, all matches are replaced. The `/i` flag makes the match case-insensitive in GNU sed.

In-place editing with `-i` is where sed earns its keep in automation. On GNU sed, `-i` with no suffix overwrites the file directly. On macOS BSD sed, you must pass an empty string: `-i ''`. If you want a backup, pass the extension directly: `-i.bak`. In our experience, always using `-i.bak` in scripts that run in CI is worth the extra disk cost - recovering a mangled nginx.conf at 2am is not.

Deleting lines is done with the `d` command. Addresses can be line numbers, regex patterns, or ranges. To delete blank lines, use `/^$/d`. To delete lines 5 through 10, use `5,10d`. To delete from a matching line to end of file, use `/PATTERN/,$d`.

# Replace first occurrence per line
sed 's/error/ERROR/' app.log

# Replace all occurrences, case-insensitive
sed 's/timeout/TIMEOUT/gi' app.log

# In-place substitution with backup
sed -i.bak 's/127.0.0.1/0.0.0.0/g' /etc/redis/redis.conf

# Delete blank lines in place
sed -i '/^$/d' /etc/hosts

# Delete lines matching a pattern
sed -i '/^#/d' nginx.conf

# Delete from matching line to EOF
sed '/^\[legacy\]/,$d' config.ini

sed Address Ranges and Multi-Command Scripts

Addresses in sed can be combined with commas to define ranges. The range `/START/,/END/` applies the command to every line from the first match of START through the next match of END. This is useful for editing stanzas in config files.

Multiple commands are passed with `-e` or grouped inside `{}`. When you need more than three `-e` options, put the script in a file and use `-f script.sed`. On our test server, a sed script that reformatted 40MB of Apache access logs ran in 4.2 seconds versus 18 seconds for an equivalent Python script.

The `p` command prints the current line. Combined with `-n` (suppress default output), `sed -n '/PATTERN/p'` behaves like grep but with full sed addressing power. You can print a range of lines by number: `sed -n '100,200p' file.log`.

# Edit only lines inside an nginx server block
sed -i '/^server {/,/^}/{s/listen 80/listen 8080/}' nginx.conf

# Print lines 100-200 (like head/tail combo)
sed -n '100,200p' /var/log/syslog

# Extract lines between two markers
sed -n '/BEGIN CERTIFICATE/,/END CERTIFICATE/p' bundle.pem

# Multiple commands with -e
sed -e 's/foo/bar/g' -e '/^debug/d' -e 's/warn/WARN/g' app.log

# Append a line after a match
sed -i '/^\[mysqld\]/a bind-address = 127.0.0.1' /etc/mysql/mysql.conf.d/mysqld.cnf

awk Field Processing and Built-In Variables

awk splits each input line into fields on whitespace by default. `$1` is the first field, `$NF` is the last field, `$0` is the entire line. Change the field separator with `-F` or by setting `FS` in a `BEGIN` block. The output field separator is `OFS`, and setting it changes how `print $1, $2` joins fields.

Built-in variables you will use constantly: `NR` is the current record number (line number across all files), `FNR` is the record number within the current file, `NF` is the number of fields on the current line, `FILENAME` is the current input file name.

Pattern-action pairs are the structure of every awk program. A missing pattern matches all lines. A missing action defaults to `print`. The special patterns `BEGIN` and `END` run before the first line and after the last line respectively. Use `BEGIN` to initialize counters and `END` to print summaries.

# Print second and fifth fields from /etc/passwd
awk -F: '{print $2, $5}' /etc/passwd

# Sum the bytes column from an access log
awk '{sum += $10} END {print sum}' /var/log/nginx/access.log

# Print lines where field 3 is greater than 1000
awk '$3 > 1000' /var/log/app/metrics.log

# Count occurrences of unique values in field 1
awk '{count[$1]++} END {for (k in count) print count[k], k}' events.log | sort -rn

# Change OFS to comma for CSV output
awk 'BEGIN{OFS=","} {print $1,$3,$5}' data.tsv

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awk for Log Parsing and Reporting

Nginx and Apache access logs are the most common awk targets in production. The combined log format puts the HTTP status code in field 9 and response bytes in field 10 (when using default log format with quotes around the request). Because the request field is quoted, naive field splitting breaks. Use `awk -F'"'` to split on quotes first, or parse with a more targeted approach.

For structured log analysis at scale, we pipe awk output into sort and uniq. A full pipeline that counts the top 10 IPs hitting 5xx responses runs in under a second on a 500MB log file using GNU awk. If you are building this into a recurring job or scheduled report, tools like taskbotshub.ai can wrap these shell pipelines into triggered automations with alerting, which saves maintaining separate cron infrastructure.

Timestamp filtering in logs is a common requirement. If timestamps are ISO 8601 in field 1, you can compare them as strings directly in awk since lexicographic order matches chronological order for that format.

# Top 10 IPs with 5xx errors from nginx combined log
awk '$9 ~ /^5/ {print $1}' /var/log/nginx/access.log | \
  sort | uniq -c | sort -rn | head -10

# Total bytes served per HTTP status code
awk '{bytes[$9] += $10} END {for (s in bytes) print s, bytes[s]}' \
  /var/log/nginx/access.log | sort -k1

# Filter log entries between two timestamps (ISO 8601)
awk '$1 >= "2026-08-01T08:00" && $1 <= "2026-08-01T09:00"' app.log

# Count requests per minute from timestamps like [01/Aug/2026:14:32:10]
awk -F'[/: ]' '{print $5"-"$4"-"$3" "$6":"$7}' access.log | \
  uniq -c | sort -rn | head -20

awk Multi-File Processing and FNR vs NR

When awk processes multiple files, `NR` increments continuously while `FNR` resets to 1 for each new file. This distinction matters when you want to print headers from only the first file, or when joining two files by line number.

A classic two-file join in awk uses `NR==FNR` to load the first file into an array, then processes the second file using that array as a lookup. This replaces many use cases for `join` or database queries when the dataset fits in memory. On our test server, this pattern joined a 200,000-line user table with a 1.5 million-line event log in 3.8 seconds.

The `BEGINFILE` and `ENDFILE` patterns (GNU awk 4.1+) fire at the start and end of each input file. Use them to print per-file summaries or reset state without tracking `FNR==1`.

# Load file1 into array, use as lookup when processing file2
awk 'NR==FNR {lookup[$1]=$2; next} $1 in lookup {print $0, lookup[$1]}' \
  users.tsv events.log

# Print only the header line (line 1) from the first file
awk 'FNR==1 && NR==1 {print} FNR!=1' file1.csv file2.csv

# Per-file line count using BEGINFILE/ENDFILE (GNU awk 4.1+)
gawk 'BEGINFILE {count=0} {count++} ENDFILE {print FILENAME, count}' /var/log/*.log

# Merge two CSVs, skip header of second file
awk 'FNR==1 && NR!=1 {next} {print}' report1.csv report2.csv > merged.csv

Combining sed and awk in Pipelines

sed and awk complement each other in pipelines. A common pattern is using sed for structural cleanup (stripping comments, normalizing whitespace, removing blank lines) before passing to awk for field-level analysis. Reversing the order also works: awk to reformat fields, then sed to do final string substitutions.

When editing generated configuration files in deployment scripts, we typically use sed for targeted replacements and awk for anything requiring conditional logic or arithmetic. For example, dynamically setting `worker_processes` in nginx.conf based on the CPU count from `nproc` requires awk's ability to do arithmetic on shell-substituted values.

For teams building deployment or configuration automation, naming your automation scripts and projects clearly from the start saves significant refactoring. If you are registering a domain for an internal tooling project, nicename.me provides availability checking and registration in one place, which is useful when you want to quickly verify a project name is usable as both a hostname and a package name.

Avoid overly long pipelines. More than four stages of `sed | awk | sed | awk` usually means the logic should move into a single awk script or a Python script. The performance argument for awk over Python disappears when you add pipeline overhead from multiple forked processes.

# Strip comments and blank lines, then summarize with awk
sed -e '/^#/d' -e '/^$/d' /etc/nginx/nginx.conf | \
  awk '/worker_processes/ {print "Workers:", $2}'

# Set nginx worker_processes to CPU count
CPUS=$(nproc)
awk -v cpus="$CPUS" '/^worker_processes/ {$2=cpus";"} {print}' \
  nginx.conf > nginx.conf.new && mv nginx.conf.new nginx.conf

# Extract error messages, normalize whitespace, deduplicate
grep ERROR app.log | \
  sed 's/  */ /g' | \
  awk -F': ' '{print $2}' | \
  sort -u

# Parse systemd journal output: show service restarts in last hour
journalctl --since '1 hour ago' | \
  grep 'Started' | \
  awk '{print $5}' | \
  sed 's/\.service$//' | \
  sort | uniq -c | sort -rn

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Performance and GNU awk vs mawk vs gawk

On Ubuntu 24.04, the default `awk` symlink points to `mawk 1.3.4`. On RHEL 9, it points to `gawk 5.1.0`. mawk is faster for simple field processing - in our benchmark, processing a 1GB log file with a sum aggregation ran in 11.2 seconds with mawk versus 14.8 seconds with gawk. However, gawk supports `BEGINFILE`, `ENDFILE`, the `@include` directive, and the `gensub()` function, which mawk does not.

If you need `gensub()` for capture group references in substitutions, explicitly call `gawk`. If you are writing portable scripts that must run on both, avoid gawk-specific extensions or add a shebang that specifies the interpreter. For maximum portability across Linux and BSD (including macOS), restrict yourself to POSIX awk.

For sed, GNU sed 4.8 supports `\w`, `\b`, and other POSIX character class extensions that BSD sed does not. If your scripts run on both Linux and macOS, replace `\w` with `[[:alnum:]_]` and test on both platforms before deploying.

# Check which awk is running
awk --version 2>/dev/null || awk -W version

# gensub: replace with capture group reference (gawk only)
gawk '{print gensub(/([0-9]+)ms/, "[\\1ms]", "g")}' timing.log

# POSIX-portable equivalent using match() and substr()
awk '{
  while (match($0, /[0-9]+ms/)) {
    printf "%s[%s]", substr($0,1,RSTART-1), substr($0,RSTART,RLENGTH)
    $0 = substr($0, RSTART+RLENGTH)
  }
  print
}' timing.log

# Benchmark: mawk vs gawk on large file
time mawk '{sum+=$10} END{print sum}' /var/log/nginx/access.log
time gawk '{sum+=$10} END{print sum}' /var/log/nginx/access.log

Linux sed and awk: Practical Examples for Sysadmins and DevOps Engineers