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Syntax

sed [OPTIONS] SCRIPT [FILE...]

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Description

The sed (stream editor) command performs text transformations on input streams. It’s particularly powerful for search-and-replace operations, line deletion, and pattern-based text manipulation. Nash implements a subset of sed’s functionality, focusing on the most common operations while maintaining full sandbox isolation.

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Options

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SCRIPT
string
required
The sed script expression to execute. See supported operations below.
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-n
flag
Silent mode. Suppress automatic printing of pattern space. Only print lines explicitly selected with the p command.
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-e
flag
Specify the script expression explicitly. Useful when the script starts with a dash.

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Supported Operations

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Substitution: s/old/new/[flags]

Replace text matching a pattern. Flags:
  • g - Global: replace all occurrences in each line (not just the first)
  • p - Print: output lines where substitution occurred (use with -n)

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Line deletion: Nd

Delete line number N. Print only line number N.

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Delete all: d

Delete all lines (results in empty output).

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Examples

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Basic substitution

echo "hello world" | sed 's/world/universe/'

hello universe

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Global substitution

echo "foo bar foo baz" | sed 's/foo/XXX/g'

XXX bar XXX baz
Without the g flag, only the first occurrence is replaced: 
echo "foo bar foo baz" | sed 's/foo/XXX/'

XXX bar foo baz

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Delete specific line

echo -e "line 1\nline 2\nline 3" | sed '2d'

line 1
line 3

echo -e "line 1\nline 2\nline 3" | sed -n '2p'

line 2

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Silent mode with substitution

echo -e "apple\nbanana\napricot" | sed -n 's/a/A/p'

Apple
bAnana
Apricot
Only lines where substitution occurred are printed.

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Using different delimiters

echo "/usr/local/bin" | sed 's|/usr/local|/opt|'

/opt/bin
Useful when the pattern contains slashes.

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Pipeline Examples

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Transform log levels

cat app.log | sed 's/ERROR/[ERROR]/g' | sed 's/WARN/[WARN]/g'
Wraps log levels in brackets.

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Clean up whitespace

echo "hello    world" | sed 's/  */ /g'

hello world

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Replace file extensions

ls *.txt | sed 's/\.txt$/.md/'
Shows how filenames would look with .md extension.

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Chain transformations

echo "Hello World" | sed 's/Hello/Hi/' | sed 's/World/Universe/'

Hi Universe

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Practical Use Cases

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Configuration file updates

cat config.ini | sed 's/debug=false/debug=true/' > config-dev.ini

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Remove sensitive data

cat users.csv | sed 's/@.*\.com/@REDACTED.com/g'
Redacts email domains.

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Format conversion

cat data.csv | sed 's/,/\t/g'
Converts CSV to TSV (comma to tab).

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Version string updates

cat package.json | sed 's/"version": "1.0.0"/"version": "1.1.0"/'

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Extract and transform

grep "ERROR" app.log | sed 's/.*ERROR: //' | sort | uniq
Extracts error messages, cleans them up, and shows unique errors.

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Line numbering cleanup

cat -n file.txt | sed 's/^[ ]*[0-9]*[ ]//'
Removes line numbers added by cat -n.

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Advanced Examples

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Multiple substitutions in order

echo "abc" | sed 's/a/1/g' | sed 's/b/2/g' | sed 's/c/3/g'

123

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Conditional replacement with print

echo -e "keep\nchange\nkeep" | sed -n 's/change/modified/p'

modified

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Remove empty lines

cat file.txt | sed '/^$/d'

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Extract specific range

echo -e "a\nb\nc\nd\ne" | sed -n '2p' && echo -e "a\nb\nc\nd\ne" | sed -n '4p'

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Tips

Substitution is literal: Nash’s sed performs literal string replacement, not regular expressions. s/a.b/xyz/ replaces the literal text “a.b”, not “a” followed by any character and “b”.

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Delimiter flexibility

You can use any character as the delimiter: 
sed 's/old/new/'    # Forward slash
sed 's|old|new|'    # Pipe
sed 's:old:new:'    # Colon
sed 's#old#new#'    # Hash

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Global flag matters

echo "aaa" | sed 's/a/X/'    # Xaa (first only)
echo "aaa" | sed 's/a/X/g'   # XXX (all)

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Silent mode usage

Use -n with p flag to see only modified lines: 
cat large.txt | sed -n 's/error/ERROR/p'
Only shows lines containing “error”.
  • grep - Search for patterns
  • cut - Extract fields from lines
  • sort - Sort lines of text

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