LPIC-1

Manipulating Text File Output II

4 min read

1. Change a Letter or Number in a Text File to a Different One (tr)

The tr (translate) command is used to replace characters in a text file or stream. It can perform a simple character-by-character replacement or delete characters.

Basic Syntax:

tr [SET1] [SET2]

Where:

  • SET1 is the set of characters to be replaced.
  • SET2 is the set of characters to replace them with.

Example 1: Change a Letter

If you want to replace the letter a with z in a text file, you can use:

tr 'a' 'z' < input.txt

This will replace all occurrences of a with z in input.txt.

Example 2: Change Multiple Letters

If you want to replace multiple letters at once, you can specify multiple characters in each set. For example, replacing a with x and b with y:

tr 'ab' 'xy' < input.txt

This will replace:

  • a → x
  • b → y

Example 3: Replace Numbers

You can replace digits too. For example, to replace the number 1 with 9:

tr '1' '9' < input.txt

If you wanted to replace all digits (0-9) with a different set, like turning them into letters (e.g., 1 → a, 2 → b), use:

tr '0123456789' 'abcdefghij' < input.txt

2. Translate a Text File into Octal and Hexadecimal Formats (od)

The od (octal dump) command is used to display the contents of a file in various formats, including octal, hexadecimal, ASCII, and others.

Basic Syntax:

od -[format] [file]

Where:

  • [format] specifies the format in which to display the content, such as -x for hexadecimal, -o for octal, etc.
  • [file] is the file to be displayed.

Example 1: Hexadecimal Format

To display the contents of file.txt in hexadecimal format, use:

od -x file.txt

This will show the file’s contents in hexadecimal pairs (byte-by-byte).

Example 2: Octal Format

To display the contents in octal format, use:

od -o file.txt

This will show the file’s contents in octal (base 8).

Example 3: ASCII with Hexadecimal

If you want to see both ASCII characters and their corresponding hexadecimal values:

od -c -x file.txt

Here:

  • -c shows the ASCII characters.
  • -x shows the hexadecimal representation.

Example output:

0000000    T   h   i   s       i   s       a       t   e   s   t   .
0000014

3. Stream Editor for Filtering and Transforming Text (sed)

The sed (stream editor) command is one of the most powerful tools in Linux for text manipulation. It processes a file line by line, and you can use it to perform a wide range of transformations like replacing text, deleting lines, inserting text, etc.

Basic Syntax:

sed 's/pattern/replacement/' file

Where:

  • s stands for substitution.
  • pattern is the text you want to replace.
  • replacement is the text to replace the pattern with.

Example 1: Replace Text in a File

To replace apple with orange in a file:

sed 's/apple/orange/' input.txt

This replaces the first occurrence of apple with orange on each line.

Example 2: Replace All Occurrences

To replace all occurrences of apple with orange in the file:

sed 's/apple/orange/g' input.txt

The g after the replacement means global replacement (replace all occurrences on a line).

Example 3: Delete a Line Containing a Pattern

To delete lines that contain the word apple:

sed '/apple/d' input.txt

This will delete all lines that contain the word apple.

Example 4: Delete Blank Lines

To delete all blank lines from a file:

sed '/^$/d' input.txt

This matches empty lines (^$ stands for lines that start and end with nothing) and deletes them.

Example 5: In-place Editing

To edit the file directly (i.e., change the original file rather than outputting to the screen), use the -i option:

sed -i 's/apple/orange/g' input.txt

This will replace all instances of apple with orange in input.txt, modifying the file in place.

Example 6: Insert Text Before or After a Line

To insert a line of text before or after a line containing a pattern:

  • Insert before a line:
sed '/pattern/i\This is the inserted line' input.txt
  • Insert after a line:
sed '/pattern/a\This is the inserted line' input.txt

Example 7: Replace the N-th Occurrence

You can replace only the N-th occurrence of a pattern on a line. For example, replacing only the second occurrence of apple with orange:

sed 's/apple/orange/2' input.txt

Summary of Commands:

  • tr: Used for character replacement in text files (e.g., tr 'a' 'z').
  • od: Converts text files into different formats like octal, hexadecimal, ASCII, etc. (e.g., od -x for hexadecimal).
  • sed: Stream editor for transforming text in files (e.g., sed 's/old/new/' for substitution).

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