Remove Lines from Middle of File in Linux Terminal

Extracting Data from File Midsections

Managing your own servers frequently necessitates the retrieval of specific data located within a file. This task arises in various scenarios, such as analyzing log files or isolating a particular table from a MySQL backup file, as was the case in a recent instance.

Identifying Line Numbers with Grep

Determining the relevant line numbers is a crucial first step. A straightforward approach involves utilizing the grep -n command. The `-n` argument ensures that line numbers are included in the output, facilitating easy identification of the desired data range.

For example, the following command can be used:

grep -n wp_posts howtogeekdb010114.bak | more

This command searches for the string "wp_posts" within the file "howtogeekdb010114.bak" and displays the matching lines along with their corresponding line numbers. Piping the output to "more" prevents rapid scrolling, allowing for convenient review of the initial results.

The output will resemble this, clearly indicating line numbers on the left:

4160:-- Table structure for table `wp_posts`
4163:DROP TABLE IF EXISTS `wp_posts`;
4166:CREATE TABLE `wp_posts` (
4203:-- Dumping data for table `wp_posts`
4206:LOCK TABLES `wp_posts` WRITE;
4207:/*!40000 ALTER TABLE `wp_posts` DISABLE KEYS */;
4208:INSERT INTO `wp_posts` VALUES (1,2,'2006-09-11 05:07:23','2006-09-11

With the starting and ending line numbers identified, the extraction process can proceed.

Utilizing Sed for Line Extraction

While redirecting grep output to a file is a possibility:

grep keyword filename.txt > outputfile

This method may encounter issues, such as import failures, as experienced in a specific situation. Consequently, an alternative approach using sed proved successful.

The following sed command extracts lines within a specified range:

sed -n '4160,4209p' howtogeekdb0101140201.bak > outputfile

The syntax requires the `-n` argument and the inclusion of "p" following the last line number. This effectively isolates the desired lines and saves them to a new file.

The general format for this sed command is:

sed -n 'FIRSTLINENUMBER, LASTLINENUMBERp' filename > outputfilename

Alternative Methods for Line Extraction

Other techniques for extracting specific lines from a file exist. Combining head (with the `+number` argument) and tail can achieve this, but it's often less efficient due to increased overhead.

Another option involves using the split command to divide the file into smaller segments at the desired line number. Subsequently, head or tail can be employed to extract the necessary lines from these segments.

However, for simplicity and effectiveness, sed remains a preferred solution for extracting data from the middle of a file.