Introduction to sed -i

The sed (stream editor) command is a powerful text manipulation tool in Unix-like systems. While the standard sed outputs modified content to the terminal, the -i flag enables in-place editing, allowing direct modification of file contents without redirection.

Basic Syntax

sed [-nefr] [address]command [file]

Common Options

• -n: Suppress automatic output. Only lines processed by explicit print commands are displayed.
• -e: Execute multiple editing commands in sequence.
• -f: Read editing commands from a file instead of the commmand line.
• -r: Enable extended regular expression support.
• -i: Edit files in place, modifying the original file directly.

Address Specification

The optional address restricts commands to specific lines or ranges:

[start_line[,end_line]]command

Editing Commands

• a\: Append text after the current line.
• c\: Replace lines in the specified range with new text.
• d: Delete lines matching the address.
• i\: Insert text before the current line.
• p: Print the current line (often used with -n).
• s/pattern/replacement/flags: Substitute matching pattern with replacement text.

Practical Examples

String Replacement

The most common use of sed -i involves find-and-replace operations:

sed -i 's/old_text/new_text/' filename.txt
sed -i 's/old_text/new_text/g' filename.txt

Without the g flag, only the first occurrence per line is replaced. With g, all occurrence are substituted.

Example file contents:

d
ddd
#ff

Applying the commands:

sed -i 's/d/7523/' sample.txt
# Result:
7523
7523dd
#ff

sed -i 's/d/7523/g' sample.txt
# Result:
7523
752375237523
#ff

Removing Leading Characters

To remove a specific character from the beginning of each line, use the caret anchor in regex:

sed -i 's/^@//' targetfile

This removes the @ symbol from the start of every line.

Inserting Lines Before or After Matching Patterns

Add content relative to lines containing specific text:

# Insert a new line before lines containing "pattern"
sed -i '/pattern/i\New line content' datafile

# Append a new line after lines containing "pattern"
sed -i '/pattern/a\New line content' datafile

Deleting Lines

Remove lines matching a pattern:

sed -i '/unwanted_text/d' filename

This deletes all lines containing unwanted_text from the file.

Escaping Special Characters

When dealing with special shell characters in replacements, proper escaping is essential:

sed -i "s/\\\"/\\\\\\\\/g" temp.txt

This example escapes backslashes and quotes for replacement operations involving literal special characters.

Important Considerations

• Always create a backup before using sed -i on important files, or use sed -i.bak to automatically generate backup files with the .bak extnesion.
• Test your patterns with sed without the -i flag first to verify the results.
• The -i flag behavior varies slightly across different sed implementations (GNU vs. BSD).