In this article we are going to learn some more advanced commands in linux OS. Please check Basic Linux Commands for newbies. & Basic Commands in Linux Operating System if you’re interested in learning a few basic commands.
We will look at the following commands in this article:
- grep
- sort
- wc
- pipe
- awk
- find
- ps
- nohup
- head
- tail
- jobs
- kill
1.grep
- grep is abbreviated as Global Regular Expression Print.
- The grep filter searches a file for a particular pattern of characters or word and displays all lines that match the pattern.
- It is generally used to search files or STDOUT (output of a command).
- It can also be used to search a pattern in multiple files at once.
Syntax: grep <search word> <filename>
$ grep "rose" rose.txt
Variations in grep command:
a) grep -c
The flag -c is used to display the number of times(count) the searched word is present.
Syntax: grep –c <search word> <filename>
$ grep -c "rose" rose.txt
$ grep -c "Rose" rose.txt
b) grep -v
The flag -v ignores the lines that match with the searched word and prints the rest of the lines.
Syntax: grep –v <search word> <filename>
$ grep -v Rose rose.txt
c) grep -n
This variation is used to display the line number and the line where the search word is present in the file.
Syntax: grep –n <search word> <filename>
$ grep -n rose rose.txt
d) grep -l
This variation is used to list all the files containing the searched word.
Syntax: grep –l <search word> <*.txt>
$ grep -l "rose" *.txt
e) grep -o
This command is used to print the matched words in separate lines.
Syntax: grep –o <search word> <filename>
$ grep -o flowers flowers.txt
f) grep -i
This command is used to print the lines that match with the searched word. The search is not case-sensitive.
Syntax: grep –i <search word> <filename>
$ grep -i FLOWERS flowers.txt
2.sort
- Sort command is used to order the records.
- We can arrange the data alphabetically or numerically in ascending or descending order.
Syntax: sort <filename> (to print data in ascending order).
$ sort <name.txt>
Syntax: sort -r <filename> (to print data in reverse or descending order).
$ sort -r <name.txt>
3.wc
- wc stands for word count.
- As the name implies this command is mainly used to count the number of lines, number of words, number of bytes and characters in the file.
Syntax: wc <file name>
$ wc rose.txt
Variations in wc command:
- wc –l prints the number of lines in a file.
- wc –w prints the number of words in a file.
- wc –c prints the total number of bytes in a file.
- wc –m prints the number of characters in a file.
- wc –L prints only the length of the longest line in a file.
4.pipe
- This command is used to combine two or more commands.
- The standard output of one command acts as standard input to the next command.
- The symbol (|) denotes pipe command.
Syntax : cat <filename> | grep <search word> <filename>
$ cat rose.txt | grep rose rose.txt
5.awk
- awk command stands for the first letters of three authors’ names (aho,weinberger,kernighan).
- This command is widely used for text processing.
- awk command searching for a specific word or pattern in a certain line or a certain column in a file you provide.
Syntax: awk ‘{action}’ <file name> (condition to be given in curly braces and its enclosed with single quotes)
For example if you want to print the 1st column of the file. Use awk ‘{print $1}’.
$0 – represents the entire column of the file.
$1 – represents the first column of the file.
$2 – represents the second column of the file.
.
.
$NF – represents the last column of the file.
$ awk '{print $1}' rose.txt 
To print more than one column use awk command as awk ‘{print $1,$3}’ here $1 is the first column and $3 is the third column.
$ awk '{print $1,$3}' rose.txt 
Here (,) or (“ ”) is used to provide space between two columns and output is clearly readable.
6.find
- find command is the most important and frequently used command in linux os.
- It is used to search and locate the list of files and directories based on conditions you specify for files that match the arguments.
Syntax: find [options] [path…] [expression]
$ find /home -name sample.txt
- To find the files names using the extension of the file.
$ find /home -name "*.txt"
- To find the files using type variation:
- The -type f variation is used to find and list the file’s name only.
- The -type d variation is used to find and list the directories name only.
$ find /home -type f
$ find /home -type d
- To find and list the empty files and directories.
$ find /home -empty
7.ps
- ps stands for process status.
- This command is used to display or to view information related to the process running in the system.
$ ps
- PID – Every process is assigned a PID (Process IDentifier) which is a unique identifier associated with the running process.
- TTD-Controlling terminal associated with the process.
- TIME-Total time of CPU Usage,shows in minutes and seconds.
- CMD-The command that is executed by the process.
ps -ef
The -e option instructs ps to display all processes. The -f stands for full-format listing, which provides detailed information about the running processes.
$ ps -ef
Before we see about nohup command we should know about how to create a shell script(sh) file and how to execute the sh file.
- Create a new sh file using touch command.Write the script you are going to run in sh file using vi command.
- kindly check whether the sh file is having executable permission, if it is not there, please assign execute permission using chmod command.
- To run the script file use ./filename command and the standard output will display in the terminal and the process continues and the output will display continously.To stop the process give ctrl+c.
8.nohup
- nohup, no hang up command keeps the process running even after exiting the terminal or shell.
- It prevents the process or jobs from receiving the signal SIGHUP.
- SIGHUP (SIGnal Hang Up) is a signal used to close and exit the terminal.
$ nohup ./multiplication.sh > multiply.log
In the above process once ctrl+c given the shell script is stopped and the saved output can be viewed in the log file using cat command.
This nohup command with (&) symbol in the end is used to run the process in the background.
Syntax: nohup (./sh file) > (file name) &
$ nohup ./multiplication.sh > multiply.log &
9.head
This command print the first 10 lines of the specified file. If more file name is provided then data from each file is preceded by its file name.
Syntax: head <file name>
$ head multiply.log
10.tail
This command by default prints the last 10 lines of the specified file and exit.
Syntax: tail <file name>
$ tail multiply.log
tail -f
This command will show the last ten lines of a file. As the file gets updated, the tail command will show those lines in the output.
Syntax: tail -f <file name>
$ tail -f multiply.log
11.jobs
This command is used to list the processes running in the background.
$ jobs
12.kill
It is a built in command used to terminate the process manually.
Syntax: kill <PID/process name>.
kill -9 command sends a kill signal to terminate a process immediately or forcefully.
$ kill -9 80154
kill -15 command sends a kill signal to terminate the process gracefully
$ kill -15 81387
Thanks for reading this article. I hope it is useful for you.
