RTFM · Tools

sed and awk: the classic pair

Two complementary stream editors: sed rewires text line-wise by substitution; awk treats fields as structured data. Together they replace most ad-hoc scripting temptations.

sed: substitute, delete, print selectively

Saphira ships sed supporting extended regular expressions via -E, plus the classic substitution and filtering verbs.

sed repertoire

sed 's/old/new/' file           # first occurrence per line
sed 's/old/new/g' file          # every occurrence
sed -E 's/(\w+)@(\w+)/\2@\1/g'  # extended regex + capture swap
sed -n '/ERROR/p' app.log       # silent mode + explicit print
sed '/^#/d;/^$/d' config        # strip comments and blanks
sed -i.bak 's/a/b/g' file       # in-place with backup safety

-i semantics vary between sed flavours regarding backup suffix handling; pass .bak explicitly or verify behaviour on a scratch copy first.

awk: fields, patterns, arithmetic

awk splits each line on whitespace into $1..$NF; the entire line is $0. That model alone covers most log-reduction tasks.

awk patterns

awk '{print $1}' access.log                    # first column
awk '$9 == 500 {print $7}' access.log          # conditionally filter
awk -F: '{print $1, $3}' /etc/passwd           # colon-delimited
awk '/error/ {count++} END {print count}' app.log  # tally by pattern
awk '{sum+=$2} END {printf "%.1f MB\n", sum/1024}' sizes.txt

The END block runs after input ends — perfect place for totals. BEGIN handles pre-input setup. Between them lies the per-record body, conditionally gated by pattern expressions.

Choosing between them

sed vs awk selection heuristics
Task shape	Reach for
Search-replace textual pattern	sed
Extract/filter columns numerically	awk
Multi-step conditional transformations	awk
Simple global character cleanup	sed
Aggregations over delimited data	awk

Learning both pays compound interest: outputs from one feed inputs of the other, chained through pipelines endlessly.

Prove it works: stream editor confidence

a one-liner written from scratch replaces a manual spreadsheet edit on a real log within five minutes.

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