sed Command Reference

Displaying Specific Lines

To print the 10th line of /etc/passwd:

sed -n '10p' /etc/passwd

To print lines 8 through 15 of /etc/passwd:

sed -n '8,15p' /etc/passwd

To print line 8 and the next 5 lines of /etc/passwd:

sed -n '8,+5p' /etc/passwd

Pattern Matching

To print lines starting with 'nginx' in /etc/passwd:

sed -n '/^nginx/p' /etc/passwd

To print from a line starting with 'root' to a line starting with 'ftp' in /etc/passwd:

sed -n '/^root/,/^ftp/p' /etc/passwd

Combined Line Numbers and Patterns

To print from line 8 to the line containing '/sbin/nologin' in /etc/passwd:

sed -n '8,/\/sbin\/nologin/p' /etc/passwd

To print from the line containing '/bin/bash' to line 5 in /etc/passwd:

sed -n '/\/bin\/bash/,5p' /etc/passwd

Configuration File Processing Script

Objective

Develop a script to process a configuration file similar to my.cnf that:

• Counts the number of sections in the file
• For each section, counts the total number of configuration parameters

Expected Output

1: client 2
2: server 12
3: mysqld 12
4: mysqld_safe 7
5: embedded 8
6: mysqld-5.5 9

Implementation

Function to Extract All Section Names

To find all section names in the configuration file:

sed -n '/\[.*\]/p' config_file | sed -e 's/\[/\g' -e 's/\]//g'

The '.' matches any character, '*' represents zero or more characters, and '-e' allows multiple sed commands to be chained.

Configuration Processor Script

#!/bin/bash

CONFIG_FILE="/path/to/config.cnf"

# Function to retrieve all section names
get_all_sections() {
    echo $(sed -n '/\[.*\]/p' "$CONFIG_FILE" | sed -e 's/\[/\g' -e 's/\]//g')
}

# Function to count items in a specific section
count_items_in_section() {
    # Find content between [section] and the next [.*]
    # Filter out comment lines and empty lines
    items=$(sed -n '/\['"$1"'\],/\[.*\]/p' "$CONFIG_FILE" | \
            grep -v "^$" | grep -v "^#" | grep -v "\[.*\]" | wc -l)
    echo $items
}

section_number=0
for section in $(get_all_sections)
do
    section_number=$((section_number + 1))
    item_count=$(count_items_in_section "$section")
    echo "$section_number: $section $item_count"
done

Function Details

get_all_sections Function

This function identifies all section headers in the configuration file by:

1. Using sed to find all lines containing section headers (enclosed in [ ])
2. Removing the brackets using sed substitution commands
3. Returning the section names

count_items_in_section Function

This function counts the configuration items in a specified section by:

1. Extracting the content between the specified section header and the next section header
2. Filtering out empty lines, comment lines (starting with #), and section headers
3. Counting the remaining lines using word count utility