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Reading Lines from Files

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Reading Lines from Files

Example 1: Output specific lines by their line numbers using the sed command. The command allows viewing only the desired lines with explicitly specified line numbers. Syntax: $ sed -n -e Xp -e Yp FILENAME sed: command,…

Reading Lines from Files

Example 1: Printing specific lines by their numbers using the sed command

This command allows you to view only the lines with specific numbers that are explicitly specified.

Syntax:

  1. $ sed -n -e Xp -e Yp FILENAME

 

sed: the command that, by default, outputs all lines without exception -n: suppresses output -e CMD: executable command Xp: prints line number X Yp: prints line number Y FILENAME: name of the file being processed

In the example below, lines 120, 145, and 1050 of the syslog file are printed.

  1. $ sed -n -e 120p -e 145p -e 1050p /var/log/syslog

In the next example, you can view the contents of the /var/log/cron file, starting from line 101 and ending at line 110.

M – starting line N – ending line

Syntax:

  1. sed -n M,Np FILENAME

 

  1. $ sed -n 101,110p /var/log/cron

Example 2: Printing the first N lines of a file using the head command

In this example, the first 15 lines of the /var/log/maillog file are printed.

Syntax:

  1. head -n N FILENAME

 

  1. $ head -n 15 /var/log/maillog

To print the first 10 lines of the log, replace 15 with 10 in the example.

Example 3: Excluding the last N lines of a file using the head command

This example shows how to ignore the last N lines and display only the remaining lines from the beginning of the file.

Syntax:

  1. head -n -N FILENAME

The example shows how to display all lines of the /var/log/secure file except the last 250.

  1. $ head -n -250 /var/log/secure

Example 4: Display the last N lines of a file using the tail command

Syntax: tail -n N FILENAME

In this example, only the last 50 lines of the /var/log/messages file are displayed. To display the last 100 lines of the log, change 50 to 100.

  1. $ tail -n 50 /var/log/messages

Example 5: Skip the first N-1 lines of a file using the tail command

This command allows you to ignore the first N-1 lines and display only the remaining lines.

Syntax:

  1. tail -n +N FILENAME

This example shows how to skip the first 4 lines of the /etc/xinetd.conf file, which contain only comments.

  1. $ tail -n +5 /etc/xinetd.conf
  2. defaults
  3. {
  4. instances = 60
  5. log_type = SYSLOG authpriv
  6. log_on_success = HOST PID
  7. log_on_failure = HOST
  8. cps = 25 30
  9. }
  10. includedir /etc/xinetd.d

Example 6: View a growing log in real time using the tail command

This is probably one of the most frequently used commands by system administrators. To view a growing file and output only the updated content of the log, use the tail -f command as shown below.

In the following example, the content of the /var/log/syslog file is displayed in real time.

Syntax:

  1. tail -f FILENAME

 

  1. $ tail -f /var/log/syslog

Example 7: Output specific lines (by line number) using the head and tail commands

M – starting line N – ending line

Syntax:

  1. cat file | tail -n +N | head -n (M-N+1)

In the example below, lines 101–110 of the /var/log/anaconda.log file are displayed.

  1. $ cat /var/log/anaconda.log | tail -n +101 | head -n 10

 

cat: outputs the entire file to stdout. tail -n +101: ignores lines up to the specified number, then starts outputting lines beginning with that number. head -n 10: outputs the first 10 lines, i.e., from 101 to 110, ignoring the rest.

Example 8: Output lines matching a pattern, along with several lines immediately following the matches.

The following example shows how to output lines matching the pattern “Initializing CPU” from the /var/log/dmesg file, as well as 5 lines immediately following them.

  1. # grep "Initializing CPU#1" /var/log/dmesg
  2. Initializing CPU#1

[Note: this command outputs only lines matching the pattern]

  1. # grep -A 5 "Initializing CPU#1" dmesg
  2. Initializing CPU#1
  3. Calibrating delay using timer specific routine.. 3989.96 BogoMIPS (lpj=1994982)
  4. CPU: After generic identify, caps: bfebfbff 20100000 00000000 00000000
  5. CPU: After vendor identify, caps: bfebfbff 20100000 00000000 00000000
  6. monitor/mwait feature present.
  7. CPU: L1 I cache: 32K, L1 D cache: 32K

[Note: this command outputs the matching line and the 5 lines following it]

The following operations are also available with the grep command:

View lines matching a pattern – the basic function of the grep command. View only selected characters. View N lines after the match using the -A argument. View N lines before the match using the -B argument. View N lines before and after the match using the -C argument.

Example 9: Output specific bytes from a file.

The following example shows how to output either the first 40 or the last 30 bytes of a file.
Output the first 40 bytes of the syslog file.

  1. $ head -c40 /var/log/syslog

Output the last 30 bytes of the syslog file.

  1. $ tail -c30 /var/log/syslog

Example 10: Viewing compressed logs.

At regular intervals, all system logs are compressed. You can decompress them "on the fly" and process the output with another command. How to do this is shown below.

Output the first N lines of a compressed file.

  1. $ zcat file.gz | head -250

Output the last N lines of a compressed file.

  1. $ zcat file.gz | tail -250

Ignore the last N lines of a compressed file.

  1. $ zcat file.gz | head -n -250

Ignore the first N lines of a compressed file.

  1. $ zcat file.gz | tail -n +250

View lines matching a pattern

  1. $ zcat file.gz | grep -A2 'error'

View a set of lines by their specified line numbers.

  1. $ zcat file.gz | sed -n -e 45p -e 52p

Note: This post may be AI-assisted and then human-edited. Editorial responsibility: Dmytro Lomaka, Kaltenkirchen.

Categories: sed