Chapter 4: Essential Linux Commands
__Linux 101 Hacks
Hack 17. Grep Command
grep command is used to search files for a specific text.
Syntax: grep [options] pattern [files]
Option -v, will display all the lines except the match.
Option -c, displays the total number of lines that contains the text.
get the total number of lines that did not match the specificpattern by passing option -cv.
option -i (ignore case), which will ignore the case while searching.
option -r (recursive) for searching all subdirectories for a text matching a specific pattern.
This will display the output in the format of "filename: line that matching the pattern".
option -l, which will display only the name of the file that matches the pattern.
grep
# grep John /etc/passwd
jsmith:x:1082:1082:John Smith:/home/jsmith:/bin/bash
jdoe:x:1083:1083:John Doe:/home/jdoe:/bin/bash
# grep -v John /etc/passwd
jbourne:x:1084:1084:Jason Bourne:/home/jbourne:/bin/bash
# grep -c John /etc/passwd
2
# grep -cv John /etc/passwd
39
# grep -i john /etc/passwd
jsmith:x:1082:1082:John Smith:/home/jsmith:/bin/bash
jdoe:x:1083:1083:John Doe:/home/jdoe:/bin/bash
# grep -ri john /home/users
/home/users/subdir1/letter.txt:John, Thanks for your contribution.
/home/users/name_list.txt:John Smith
/home/users/name_list.txt:John Doe
# grep -ril john /root
/home/users/subdir1/letter.txt
/home/users/name_list.txt
Hack 18. Find Command
find is frequently used command to find files in the UNIX filesystem based on numerous conditions.
Syntax: find [pathnames] [conditions]
How to find files containing a specific word in its name?
The following command looks for all the files under /etc directory with mail in the filename.
# find /etc -name "*mail*"
How to find all the files greater than certain size?
The following command will list all the files in the system greater than 100MB.
# find / -type f -size +100M
How to find files that are not modified in the last x number of days?
The following command will list all the files that were modified more than 60 days ago under the current directory.
# find . -mtime +60
How to find files that are modified in the last x number of days?
The following command will list all the files that were modified in the last two days under the current directory.
# find . –mtime -2
How to delete all the archive files with extension *.tar.gz and greater than 100MB?
The best practice is to execute the same command with ls –l to make sure you know which files will get deleted when you execute the command with rm.
# find / -type f -name *.tar.gz -size +100M -exec ls -l {} \;
# find / -type f -name *.tar.gz -size +100M -exec rm -f {} \;
How to archive all the files that are not modified in the last x number of days?
The following command finds all the files not modified in the last 60 days under /home/jsmith directory and creates an archive files under /tmp in the format of ddmmyyyy_archive.tar.
# find /home/jsmith -type f -mtime +60 | xargs tar -cvf /tmp/`date +%d%m%Y'_archive.tar`
Hack 19. Suppress Standard Output and Error Message
Sometime while debugging a shell script, you may not want to see either the standard output or standard error message. Use /dev/null as shown below for suppressing the output.
Suppress standard output using > /dev/null
This will be very helpful when you are debugging shell scripts, where you don’t want to display the echo statement and interested in only looking at the error messages.
# cat file.txt > /dev/null
# ./shell-script.sh > /dev/null
Suppress standard error using 2> /dev/null
This is also helpful when you are interested in viewing only the standard output and don’t want to view the error messages.
# cat invalid-file-name.txt 2> /dev/null
# ./shell-script.sh 2> /dev/null
Note: One of the most effective ways to use this is in the crontab, where you can suppress the output and error message of a cron task as shown below.
30 1 * * * command > /dev/null 2>&1
Hack 20. Join Command
Join command combines lines from two files based on a common field.
In the example below, we have two files – employee.txt and salary.txt. Both have employee-id as common field. So, we can use join command to combine the data from these two files using employee-id as shown below.
join
$ cat employee.txt
100 Jason Smith
200 John Doe
300 Sanjay Gupta
400 Ashok Sharma
$ cat bonus.txt
100 $5,000
200 $500
300 $3,000
400 $1,250
$ join employee.txt bonus.txt
100 Jason Smith $5,000
200 John Doe $500
300 Sanjay Gupta $3,000
400 Ashok Sharma $1,250
Hack 21. Change the Case
Convert a file to all upper-case
# cat employee.txt
100 Jason Smith
200 John Doe
300 Sanjay Gupta
400 Ashok Sharma
# tr a-z A-Z < employee.txt
100 JASON SMITH
200 JOHN DOE
300 SANJAY GUPTA
400 ASHOK SHARMA
Convert a file to all lower-case
$ cat department.txt
100 FINANCE
200 MARKETING
300 PRODUCT DEVELOPMENT
400 SALES
$ tr A-Z a-z < department.txt
100 finance
200 marketing
300 product development
400 sales
Hack 22. Xargs Command
xargs is a very powerful command that takes output of a command and pass it as argument of another command.
1. When you are trying to delete too many files using rm, you may get error message: /bin/rm Argument list too long – Linux. Use xargs to avoid this problem.
# find ~ -name ‘*.log’ -print0 | xargs -0 rm -f
2. Get a list of all the *.conf file under /etc/.
# find /etc -name "*.conf" | xargs ls –l
3. If you have a file with list of URLs that you would like to download, you can use xargs as shown below.
# cat url-list.txt | xargs wget –c
4. Find out all the jpg images and archive it.
# find / -name *.jpg -type f -print | xargs tar -cvzf images.tar.gz
5. Copy all the images to an external hard-drive.
# ls *.jpg | xargs -n1 -i cp {} /external-hard-drive/directory
Chapter 4: Essential Linux Commands (part 2) __Linux 101 Hacks
__To be continued.
