Friends,
Last weekend my FireNookz’ (desktop) HDD crashed and I had a chance to install RHEL 5 in my system. After a long time I was trying to mess with some of the UNIX commands. I am not that expert in SED and AWK however I can survive and I am still learning though… I shall also share a “Practically Useful GREP Commands” as soon as possible but for now… Here are few commands which every UNIX professional may find useful.
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1. Display specific lines (based on line number) of a file using sed command
Syntax: $ sed -n -e Xp -e Yp FILENAME
sed : sed command, which will print all the lines by default.
-n : Suppresses output.
-e CMD : Command to be executed
Xp: Print line number X
Yp: Print line number Y
FILENAME : name of the file to be processed.
Example: $ sed -n -e 120p -e 145p -e 1050p /var/log/syslog
2. Viewing the content of var/log/cron from line number 101 to 110.
Syntax: sed -n M,Np FILENAME
M – Starting line number
N – Ending line number
Example: $ sed -n 101,110p /var/log/cron
3. Display first N lines of a file using head command
Syntax: head -n N FILENAME
Example: $ head -n 15 /var/log/maillog
4. Ignore last N lines of a file using head command and show only the remaining lines from the top of file.
Syntax: head -n -N FILENAME
Example: $ head -n -250 /var/log/secure
5. Display last N lines of the file using tail command
Syntax: tail -n N FILENAME
Example: $ tail -n 50 /var/log/messages
6. Ignore first N-1 lines of the file using tail command and show only the remaining of the lines.
Syntax: tail -n +N FILENAME
Example: $ tail -n +5 /etc/xinetd.conf
7. View growing log file in real time using tail command
Syntax: tail -f FILENAME
Example: $ tail -f /var/log/syslog
8. Display specific lines (based on line number) of a file using head and tail command
The example below will display line numbers 101 – 110 of /var/log/anaconda.log file
M – Starting line number
N – Ending line number
Syntax: cat file | tail -n +N | head -n (M-N+1)
Example: $ cat /var/log/anaconda.log | tail -n +101 | head -n 10
cat : prints the whole file to the stdout.
tail -n +101 : ignores lines upto the given line number, and then start printing lines after the given number.
head -n 10 : prints the first 10 line, that is 101 to 110 and ignores the remaining lines.
9. Display lines matching a pattern, and few lines following the match.
The following example displays the line that matches “Initializing CPU” from the /var/log/dmesg and 5 lines immediately after this match.
# grep "Initializing CPU#1" /var/log/dmesg
Initializing CPU#1
[Note: The above shows only the line matching the pattern]
# grep -A 5 "Initializing CPU#1" dmesg
Initializing CPU#1
Calibrating delay using timer specific routine.. 3989.96 BogoMIPS (lpj=1994982)
CPU: After generic identify, caps: bfebfbff 20100000 00000000 00000000
CPU: After vendor identify, caps: bfebfbff 20100000 00000000 00000000
monitor/mwait feature present.
CPU: L1 I cache: 32K, L1 D cache: 32K
[Note: The above shows the line and 5 lines after the pattern matching]
PS:
Viewing N lines after the match with -A option.
Viewing N lines before the match with -B option.
Viewing N lines around the match with -C option.
10. Displaying specific bytes from a file.
Display first 40 bytes from syslog.
$ head -c40 /var/log/syslog
Display last 30 bytes from syslog.
$ tail -c30 /var/log/syslog
11. Viewing compressed log files
The following example explains how to display either the top 40 or the last 30 bytes of a file.
# Display the first N lines of a compressed file.
$ zcat file.gz | head -250
# Display the last N lines of a compressed file.
$ zcat file.gz | tail -250
# Ignoring the last N lines of a compressed file.
$ zcat file.gz | head -n -250
# Ignoring the first N lines of a compressed file.
$ zcat file.gz | tail -n +250
# Viewing the lines matching the pattern
$ zcat file.gz | grep -A2 'error'
# Viewing particular range of lines identified by line number.
$ zcat file.gz | sed -n -e 45p -e 52p
Happy Learning!
