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Stream Editor(sed commands).md

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sed (Stream Editor)

The sed command (short for Stream Editor) is a powerful text manipulation tool in Linux/Unix. It is used to perform basic text transformations on an input stream (a file or input from a pipeline). sed is commonly used for tasks such as search-and-replace, text insertion, deletion, and more.

Basic Syntax of sed:

sed 'command' filename
  • command: The operation sed should perform.
  • filename: The file on which the sed operation is to be applied.

Common Uses of sed

1. Substitute (Search and Replace):

The most common use of sed is for search and replace, using the s command. The basic syntax is:

sed 's/old_text/new_text/' filename
  • This will replace the first occurrence of old_text with new_text in each line.

Example:

sed 's/hello/world/' file.txt
  • This replaces the first occurrence of "hello" with "world" in each line of file.txt.

2. Global Replacement:

To replace all occurrences of a pattern in a line, use the g flag:

sed 's/old_text/new_text/g' filename

Example:

sed 's/hello/world/g' file.txt
  • This replaces every occurrence of "hello" with "world" in each line of file.txt.

3. In-place Editing:

To modify the file directly without needing to redirect the output to a new file, use the -i option:

sed -i 's/old_text/new_text/' filename
  • This changes the file filename in place.

Example:

sed -i 's/hello/world/g' file.txt
  • This will replace every occurrence of "hello" with "world" directly in file.txt.

4. Delete Lines:

You can delete lines that match a specific pattern using the d command.

sed '/pattern/d' filename
  • This will delete any line containing the word pattern.

Example:

sed '/error/d' file.txt
  • This deletes all lines that contain the word "error" in file.txt.

5. Print Specific Lines:

To print only specific lines (using line numbers), use the -n option in conjunction with the p command:

sed -n '5p' filename
  • This will print the 5th line of the file filename.

Example:

sed -n '3,6p' file.txt
  • This will print lines 3 through 6 from file.txt.

6. Replace Text on Specific Line Number:

You can also replace text only on a specific line or range of lines by specifying the line number:

sed '3s/old_text/new_text/' filename
  • This will replace old_text with new_text only on the 3rd line.

Example:

sed '2,4s/hello/world/' file.txt
  • This will replace "hello" with "world" in lines 2 through 4 of file.txt.

7. Insert Text:

You can insert a line before or after a matching line with the i (insert) or a (append) commands.

  • Insert before a line:

    sed '/pattern/i\new_line' filename
    • This inserts new_line before every line containing pattern.

Example:

sed '/error/i\This is an error message' file.txt

  • This inserts "This is an error message" before every line containing "error".

  • Append after a line:

    sed '/pattern/a\new_line' filename
    • This appends new_line after every line containing pattern.

Example:

sed '/error/a\Please check this error' file.txt
  • This appends "Please check this error" after every line containing "error".

8. Change Text:

To change a line, you can use the c command:

sed '3c\new_line' filename
  • This changes the 3rd line of the file to new_line.

Example:

sed '3c\This is the new line' file.txt
  • This changes the 3rd line to "This is the new line."

9. Using Regular Expressions:

sed supports regular expressions (regex) for more advanced pattern matching.

  • Match any character:

    sed 's/./X/' file.txt
    • This replaces the first character of each line with "X".

  • Match one or more digits:

    sed 's/[0-9]\+/NUMBER/' file.txt
    • This replaces one or more digits with the word "NUMBER".

10. Multiple Commands:

You can combine multiple sed commands using the -e option or by separating commands with semicolons.

sed -e 's/old_text/new_text/' -e 's/foo/bar/' filename
  • This applies two commands in sequence: replacing old_text with new_text and foo with bar.

Example:

sed 's/hello/world/; s/foo/bar/' file.txt
  • This replaces "hello" with "world" and "foo" with "bar" in the same pass.

Examples of sed Commands

  1. Replace "cat" with "dog" in a file:

    sed 's/cat/dog/' file.txt

  2. Replace all occurrences of "cat" with "dog":

    sed 's/cat/dog/g' file.txt

  3. Delete lines that contain "error":

    sed '/error/d' file.txt

  4. Print lines 1 to 5:

    sed -n '1,5p' file.txt

  5. Insert "This is a new line" before every line containing "pattern":

    sed '/pattern/i\This is a new line' file.txt

  6. Change line 2 to "This is a changed line":

    sed '2c\This is a changed line' file.txt

  7. Replace digits with the word "number":

    sed 's/[0-9]\+/number/' file.txt

  8. Replace only on the 3rd line:

    sed '3s/old_text/new_text/' file.txt

  9. In-place editing:

    sed -i 's/old_text/new_text/' file.txt

Summary of Common sed Commands

Command Description Example
s/old_text/new_text/ Substitute old_text with new_text sed 's/cat/dog/' file.txt
d Delete lines matching a pattern sed '/error/d' file.txt
i\new_line Insert new_line before lines matching a pattern sed '/error/i\This is an error' file.txt
a\new_line Append new_line after lines matching a pattern sed '/error/a\Please check this error' file.txt
c\new_line Change lines to new_line sed '3c\New line' file.txt
-i Edit the file in place (without creating a new file) sed -i 's/cat/dog/' file.txt
-n Suppress automatic output, useful for selective printing sed -n '3p' file.txt
g Global replacement (replace all occurrences in a line) sed 's/cat/dog/g' file.txt

sed is a powerful and efficient tool for text manipulation in Unix-like systems. It is particularly useful for batch processing, scripting, and automation tasks involving text files.