DEV Community: Kurumi

Terminal Commands that Every Developers Should Know

Kurumi — Wed, 19 Apr 2023 12:12:19 +0000

Today, I'm going to introduce some useful commands that every developer should know.
I will update this article as I learn more commands, so please bookmark it and check it back later!

Table of Contents:

grep
sed
awk
find
rm
lsof -i
kill
ps
history

grep

grep is a command that searches for a pattern in a file and prints the lines that contain that pattern.

grep "pattern" file

Example

This command searches for the string "walnut07", which is my username,
in the README.md file under my current directory, and prints the lines that contain the string "walnut07".

grep "walnut07" ./README.md

> # Kurumi's Blog walnut07.com

Some Points

Strongly recommend using git grep instead of grep if you are in a git repository.
- git grep is a command that searches for a pattern in the git repository that the current directory is in.
Use -i option to ignore case. E.g.: git grep -i "pattern"
Use -n option to print the line number. E.g.: git grep -n "pattern"

  git grep -n "walnut07" ./README.md
  > README.md:1:# Kurumi's Blog walnut07.com

Use -v option to print the lines that do not contain the pattern. git grep -v "pattern"

  git grep -v "walnut07" ./README.md
  > README.md:# Kurumi's Blog walnut07.com

sed

sed is a command that filters and transforms text.

sed "command" file

Example

This command replaces the string "walnut07" with "foobar07" in the README.md file under my current directory, and outputs the result.

sed "s/walnut07/foobar07/g" ./README.md
> # Kurumi's Blog foobar07.com

The s command is used to replace the string. The g option is used to replace all occurrences of the string.

Some Points

Note that sed does not edit the file in place.
- If you want to edit the file in place, use -i and -e option. E.g.: sed -i -e "s/walnut07/foobar07/g" ./README.md
Use /number to specify the Nth occurrence of the string. E.g.: sed -i -e "s/walnut07/foobar07/2" ./README.md
- This command replaces the 2nd occurrence of the string "walnut07" with "foobar07" in the README.md file under my current directory, and edit the file.
Use & to refer to the string that is being replaced. E.g.: sed -i -e "s/walnut07/&07/g" ./README.md
- This command replaces the string "walnut07" with "walnut0707" in the README.md file under my current directory, and edit the file.

awk

awk is a command that filters and transforms text.

awk '{action}' file

Example

This command prints the 2nd column of the README.md file under my current directory.

awk '{print $2}' ./README.md

Result:

Kurumi's

tech


Built
Next.js
TypeScript
CI/CD

Some Points

Use BEGIN and END to specify the action that is executed at the beginning and the end of the file.

  awk 'BEGIN {print "Start"} {print $2} END {print "End"}' ./README.md
  > Start
  > Kurumi's
  > tech
  > Built
  > Next.js
  > TypeScript
  > CI/CD
  > End

find

find is a command that finds files in a directory hierarchy.

find directory -name "pattern"

Example

This command finds all files that have the string "README.md" in their name under the current directory.

find . -name "*README.md*"

Result:

./blog/README.md
./README.md
./README.md-e

Some Points

Use -type option to specify the type of the file. E.g.: find . -type f
- f is used to specify a regular file.
- d is used to specify a directory.
- find . -type f -name "*README.md*" finds all files that have the string "README.md" in their name under the current directory.
- find . -type d -name "*blog*" finds all directories that have the string "blog" in their name under the current directory.

rm

rm is a command that removes files.

rm file

Example

This command removes the README.md file under the current directory.

rm ./README.md

Some Points

Use -r option if the file is a directory. E.g.: rm -r ./blog

lsof -i

lsof -i is a command that lists open files.

lsof -i

Result:

COMMAND   PID USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
node    12345 user    3u  IPv4 0x1234567890abcdef      0t0  TCP *:3000 (LISTEN)
loginwindow  123 user   11u  IPv4 0x1234567890abcdef      0t0  TCP *:60000 (LISTEN)

Some Points

Add a port number to the command to specify the port. E.g.: lsof -i :3000
- This command lists open files that are using port 3000.
- Result:
```
  COMMAND   PID USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
  node    12345 user    3u  IPv4 0x1234567890abcdef      0t0  TCP *:3000 (LISTEN)
```
- Use -iTCP or -iUDP option to specify the protocol. E.g.: lsof -iTCP
- You can also run -iTCP:PORT_NUMBER or -iUDP:PORT_NUMBER option to specify the protocol and the port. E.g.: lsof -iTCP:3000

kill

kill is a command that sends a signal to a process.

kill -signal PID

Example

This command sends a signal to terminate the process whose process id is 12345.

kill -9 12345

Some Points

Use -9 option to send a signal to the process. E.g.: kill -9 12345
- -9 is used to send a signal to the process to terminate it.
Remember that we can get a process id by using lsof -i command? Go get a process id and kill it by using kill -9 command!

cut

cut is a command that removes sections from each line of files.

cut -d "delimiter" -f "field" file

Example

This command prints the 2nd column of the README.md file under my current directory.

cut -d " " -f 2 ./README.md

ps

ps is a command that reports a snapshot of the current processes.

ps

Example

ps

Result:

  PID TTY           TIME CMD
12345 ttys000    0:00.00 -bash
12346 ttys000    0:00.00 ps

Some Points

Why is this command useful?
- This command is useful when you want to know the process id of a process.
- For example, if you want to kill a process, you can use kill -9 command with the process id.
- Also, you can use lsof -i command to get the process id of a process that is using a specific port.
- Then, you can kill the process by using kill -9 command with the process id.
Use -a option to show processes for all users. E.g.: ps -a
Use -u option to show processes for a specific user. E.g.: ps -u user

history

history is a command that lists the history of commands.

history

Example

This command lists the history of commands in the terminal you are working with.

history

Result:
This is a history of my terminal commands.

 1024  lsof -i:3000
 1025  kill -9 70894
 1026  lsof -i:3000

What is $PATH and how to use it?

Kurumi — Wed, 19 Apr 2023 12:10:42 +0000

In this article, we will learn about what PATH is and why we need it.
We'll cover PATH first, and then dive into .zshrc/bash_profile to deepen our understanding.

What is PATH?

In short, PATH is a list of directories that your shell searches for executable files.
The variable PATH can look like this: /usr/local/bin:/usr/bin:/bin, which represents a list of directories.

For example, if you type ls in your shell, your shell will search for the executable file ls in the directories listed in PATH.
In other words, an executable file for the command ls is somewhere in the directories listed in PATH.

I'll guide you through the process of finding the executable file for ls in PATH.
First of all, let's use the command which to find out where the executable file for ls is located.
which is a command that prints the location of a file.

which ls
> /bin/ls

(1) Now we know that the executable file for ls is located under /bin.

Next, let's check if PATH has /bin in it. echo is a command that prints the value of a variable.

echo $PATH
> /usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

(2) Now we know that /bin is in PATH in the third position.
Note that the directories listed in PATH are separated by :.

As you can see, (1) the command ls is implemented by a file located under /bin
and (2) /bin is in PATH. Since shell searches for executable files in the directories listed in PATH, it can find the executable file for ls in /bin.

Now you know about PATH and why we need it. Let's move on to the next topic.

.zshrc/bash_profile

First things first, .zshrc and .bash_profile are files that contain commands that will be executed when you open a new shell.
Let's open up .zshrc or .bash_profile and see what's inside.

nano ~/.zshrc
# or
nano ~/.bash_profile

You will see something like this:

export PATH=/bin:/usr/bin:/usr/local/bin

This means that a variable PATH, a string that has the three directories /bin, /usr/bin and /usr/local/bin, is exported when you open up a new shell.
If you have other directories that you already added to PATH, they will be listed as well. In my case, I have export PATH=$PATH:$HOME/go/bin underneath.

export here is a command that ensures that environment variables to be passed to child processes.
Therefore, any child process of the shell that you just opened will have the PATH variable set to the value of the PATH variable declared in .zshrc or .bash_profile.

This is why your shell knows what's in the variable PATH whenever you use shell.

Fun fact

So, .zshrc and .bash_profile are just files that will be executed when you open a new shell.
You can encourage yourself by displaying a message when you open a new shell like this :)

How did I do this? I just used the echo that we learned earlier and added it to my .zshrc!

A little more about PATH

So far, we have learned that PATH is a list of directories that your shell searches for executable files.
But what if there are same executable files in different directories? For example, what if there is a file named java in usr/bin and usr/local/bin?
Which file will be executed?

The answer is that the file in the directory that comes first in PATH will be executed.
Let's say you have PATH like this

echo $PATH
> /usr/local/bin:/usr/bin:/bin

In this case, the file in /usr/local/bin will be executed when you type java in your shell.
This can be problematic when you have an older java version in the first file but want to use a newer version which is in the second file.

Conclusion

In conclusion, we have learned about the importance of the PATH variable and how it is used to search for executable files in directories listed in the PATH.
Additionally, we have discussed that a shell read the first command in PATH when there are multiple executable files with the same name.
Remember that this can cause problems especially when you want to use an updated version of a command. Overall, understanding the concept of PATH is crucial for anyone working with the command line in a Unix-like environment.