String-Processing.md

String Processing in Linux

Why is string processing used? String processing is commonly used to filter and extract specific parts of output from commands such as ifconfig, ip a, etc.

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Displaying File Content

1. head Command
   The head command is used to display the first few lines of a file. By default, it shows the first 10 lines.

   head 1.txt   # Displays the first 10 lines of 1.txt
   head -n 5 1.txt   # Displays the first 5 lines of 1.txt
   head -n 3 1.txt   # Displays the first 3 lines of 1.txt
   head -c 50 1.txt   # Displays the first 50 bytes of 1.txt

   You can also display the first few lines from multiple files:

   head 1.txt messages /var/log/centos.rep   # Displays first 10 lines from all three files

2. tail Command
   The tail command is used to display the last few lines of a file. It shows the last 10 lines by default.

   tail 1.txt messages passwd centos.rep   # Displays the last 10 lines of each file
   tail -f /var/log   # Displays the last 10 lines of log files and updates when new entries appear

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Counting Lines, Words, and Characters: wc Command

1. wc Command
   The wc command counts the lines, words, and bytes in a file.

   wc 1.txt   # Output: lines words bytes
   wc -l 1.txt   # Counts the number of lines
   wc -w 1.txt   # Counts the number of words
   wc -c 1.txt   # Counts the number of bytes

2. Counting for Multiple Files

   wc *   # Counts lines, words, and bytes for all files
   ls -1   # Lists files in the directory
   wc -l   # Counts the number of files

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Sorting Data: sort Command

1. Basic Sorting

   sort test1.txt   # Sorts lines alphabetically

2. Additional Sort Options

   • Sort in numeric order (for numbers)

     sort -h test1.txt   # Sorts numbers in ascending order

   • Sort in reverse order

     sort -r test1.txt   # Sorts lines in reverse order

   • Sort randomly

     sort -R test1.txt   # Sorts lines randomly

   • Remove duplicates

     sort test1.txt | uniq -c   # Removes duplicates and shows their count

3. Sorting by Specific Columns

   sort -k 2 test1.txt   # Sorts by the second column

4. Handling Delimited Files

   sort -t ',' -k 2 test1.txt   # Sorts by the second column of a comma-separated file

5. Sorting by Month

   sort -M test1.txt   # Sorts by month names

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Watching File Changes: watch Command

• Monitor the output of a command periodically

  watch -n 5 date   # Displays the current date every 5 seconds
  watch -n 1 date   # Displays the current date every second

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Filtering Text: grep Command

1. Basic Search

   grep searchword filename   # Finds occurrences of 'searchword' in the file
   grep "root" /etc/passwd   # Finds 'root' in the /etc/passwd file

2. Search in Multiple Files

   grep root /etc/passwd /etc/shadow   # Finds 'root' in both files

3. Ignore Case (Case-insensitive search)

   grep -i root messages   # Finds both 'root' and 'Root'

4. Search for Multiple Words

   grep -E "(session|root|mounting)" var/log/messages   # Finds 'session', 'root', or 'mounting'

5. Search for Words in the Same Line

   grep "session" var/log/messages | grep root   # Finds lines containing both 'session' and 'root'

6. Exclude a Word (Using -v)

   grep "session" var/log/messages | grep -v root   # Finds lines with 'session' but not 'root'

7. Search for a Word at the Beginning or End of a Line

   • Word at the beginning of a line:

     grep "^root" var/log/messages   # Finds lines where 'root' appears at the start

   • Word at the end of a line:

     grep "root.$" var/log/messages   # Finds lines where 'root' appears at the end

8. Search by Date

   grep "^2sep" var/log/messages | grep root   # Finds logs for 'root' on 2nd September

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Advanced grep Usage

1. Find Single Character After a Word

   grep 'roo.' filename   # Finds 'roo' followed by any single character

2. Find Multiple Characters After a Word

   grep 'roo..' filename   # Finds 'roo' followed by two characters

3. Find Empty Lines

   grep '^$' filename   # Finds empty lines

4. Exclude Lines Starting with # (e.g., Comments)

   grep -v '^#' ssd_config   # Excludes lines starting with #

5. Find Alphanumeric Characters

   grep "[[:alnum:]]" filename   # Finds all alphanumeric characters

6. Search for a Pattern from Another File

   grep -f domain.txt url.txt   # Searches for domains from domain.txt in url.txt

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Cutting Columns: cut Command

1. Extract Columns from CSV File

   cut -d ',' -f 2 my-csv.csv   # Extracts the second column (e.g., names)
   cut -d ',' -f 1,3 my-csv.csv   # Extracts columns 1 and 3
   cut -d ',' -f 1-3 my-csv.csv   # Extracts columns 1 to 3

2. Replace Commas with Spaces

   cut my-csv.csv | tr ',' ' '   # Replaces commas with spaces

3. Find Specific IP Address

   ifconfig | grep 'inet' | cut -d ' ' -f 9   # Extracts the IPv4 address

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Joining Files: paste Command

1. Combine Files Column-wise

   paste name.txt surname.txt   # Joins columns from two files
   paste -d ' ' name.txt surname.txt   # Joins with space delimiter

2. Save Output to a New File

   paste -d ' ' name.txt surname.txt > fullname.txt   # Saves the output in fullname.txt

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