Frequently used commands

Some of the most frequently used UNIX / Linux commands.

User

Add user with custom shell and comment

• useradd <USER_NAME>

• -s=User login shell

• -c=Comment

$ useradd testuser -s /bin/sh -c "Some Comment"

Change user shell

$ chsh -s /bin/bash <USERNAME>

List user

$ cat /etc/passwd

Add user to wheel group

$ sudo usermod -G wheel <USER_NAME>

# OR modify the following file

$ sudo vim /etc/group
## File: /etc/group
...
wheel:*:0:root,<USER_NAME>
...

Permission/Access Control

Modify directory owner

• Use -R(recursive) to include files in directory

• chown <USER_NAME>:<GROUP_NAME> <DIRECTORY>

$ chown -R testuser:testgroup dirname

Modify access permissions

• u=user, g=group, o=others

• r=read, w=write, x=execute

• +=add permission, -=remove permission

• Use comma to separate the multiple permission

• chmod <OPTIONS> <MODE> <FILE>

$ chmod u+r,g+x filename

File

Cleanup file

$ cat /dev/null > <LOG_FILE>

Finding top N largest file

$ du -Sh | sort -rh | head -n <N>

List open file for specific port

$ lsof -p <PORT> | wc -l

Split file by line

• split <OPTIONS> <LINE_NUMBER> <FILE_NAME> <SPLITTED_FILE_NAME>

• -l=linebumber, -b=bytes

• splitted file name will be <SPLITTED_FILE_NAME>aa <SPLITTED_FILE_NAME>ab in the following example

$ split -l 200 filename splittedfilename

Sort output and print sizes in human readable form

$ ls -lSh

SSH

Generate ssh key

$ ssh-keygen -t rsa -C <YOUR_NAME>

Untagged

Allow sudo command

• modify sshd_config and reload sshd service

$ sudo vim /etc/ssh/sshd_config
## File: /etc/ssh/sshd_config
...
PasswrodAuthentication yes
...

# Reload sshd service
$ sudo service sshd reload

Finding pid with specific port

$ sudo lsof -i :80 | grep LISTEN

Free memory

$ free && sync && echo 3 > /proc/sys/vm/drop_caches && free

Merge config file

$ cat /<DIR>/* > /tmp/all.conf

Find out linux distribution name and vesion

$ cat /etc/*-release

Generate random string with length

$ openssl rand -hex <LENGTH>

Retrieve filename from file/directory

$ for i in $(ls file/*.mp4); do echo $(basename $i .mp4); done
$ for i in $(cat filename); do echo $(basename $i .mp4); done