Master Essential Linux Commands: From pwd to du with Real Examples

Foreword

Previously we covered how to install a Linux operating system and its boot process.

Today we will study common Linux commands with practical usage examples.

Basic Operations on Directories and Files

1. pwd

Description: pwd displays the current working directory.
Usage: pwd [options]...
Options:
 -P  Show the physical (real) path of a symbolic link.

[root@qll ln-test]# pwd
/root/ln-test
[root@qll ln-test]# pwd -P
/root/test
# /root/ln-test is a symbolic link to /root/test
[root@qll ln-test]# ll -h /root/ln-test
lrwxrwxrwx. 1 root root 4 Feb 8 17:55 /root/ln-test -> test

2. cd

Description: cd changes the current working directory.
Usage:
 cd ..   Move to the parent directory.
 cd -    Return to the previous directory.
 cd      Change to the current user's home directory.

[root@qll opt]# cd /root/test   # switch to /root/test
[root@qll test]# cd ..          # go up one level
[root@qll ~]# cd -               # return to previous directory (/root/test)
[root@qll test]# cd               # go to home directory

3. ls

Description: ls lists directory and file information.
Usage: ls [options]... [file/dir]
Options:
 -a  Show all files, including hidden ones.
 -d  Show information about the directory itself, not its contents.
 -h  Human‑readable sizes.
 -l  Long format with detailed info.
 -i  Show inode numbers.
 -S  Sort by size.
 -u  Show last access time.

[root@qll ~]# ls               # list names in current directory
[root@qll ~]# ls /usr          # list /usr contents
[root@qll ~]# ls -a            # include hidden files
[root@qll ~]# ls -l            # detailed list
[root@qll ~]# ls -lh           # human‑readable sizes
[root@qll ~]# ls -lu /etc/passwd   # show last access time of /etc/passwd

4. touch

Description: Create a new file or update the timestamps of an existing file.
Example:
 [root@qll ~]# touch test.txt   # creates test.txt if it does not exist; otherwise updates its timestamps.

5. mkdir

Description: Create directories.
Usage: mkdir [options]... directory...
Options:
 -p  Create parent directories as needed (multi‑level).

[root@qll ~]# mkdir data
[root@qll ~]# mkdir -p /home/123/456

6. cp

Description: Copy files and directories.
Usage: cp [options] source destination
Options:
 -r  Recursively copy directories.
 -a  Preserve all attributes (permissions, timestamps, etc.).

[root@qll ~]# cp /etc/passwd /opt               # copy passwd to /opt
[root@qll ~]# cp /etc/passwd /opt/passwd.bat   # copy and rename
[root@qll ~]# cp -r /var/log /tmp/            # recursive copy of /var/log

7. rm

Description: Delete files or directories.
Usage: rm [options]... file...
Options:
 -f  Force deletion without prompts.
 -r  Recursively delete directories and their contents.

[root@qll ~]# rm test.txt          # delete a file
[root@qll ~]# rm -rf /tmp/data    # delete a directory tree without confirmation

8. mv

Description: Move or rename files and directories.

[root@qll ~]# mv red.txt red2.txt          # rename file
[root@qll ~]# mv red2.txt /tmp/           # move file to /tmp

9. find

Description: Search for files or directories.
Usage: find [options] [path] [expression]
Common options:
 -name   Search by exact name.
 -iname  Search by name, case‑insensitive.
 -mtime  Search by modification time.
 -group  Search by group.
 -user   Search by owner.
 -size   Search by size.
 -type   Search by type (f=file, d=directory, l=link, b=block, c=character).
 -exec   Execute a command on each matched file.
 -a      Logical AND.
 -o      Logical OR.

[root@qll ~]# find -name "pa.txt"                     # exact name in current dir
[root@qll ~]# find -iname "Pa.txt"                    # case‑insensitive
[root@qll ~]# find /etc -name "*.deny"                # files ending with .deny in /etc
[root@qll ~]# find / -mtime -1                        # modified within last day
[root@qll ~]# find / -mtime +3                        # modified more than 3 days ago
[root@qll ~]# find / -group qll                       # files owned by group qll
[root@qll ~]# find / -user qll                        # files owned by user qll
[root@qll ~]# find /opt -size +50M                    # files larger than 50 MiB
[root@qll ~]# find / -size +500M -exec ls -lh {} \;   # list details of files >500 MiB
[root@qll ~]# find / -size +1M -a -type f             # files >1 MiB that are regular files

10. du

Description: Estimate file and directory space usage.
Usage: du [options]... [file/dir]...
Options:
 -h  Human‑readable output.
 -s  Show only the total size.

[root@qll ~]# du -h /etc          # human‑readable sizes for /etc and sub‑dirs
[root@qll ~]# du -sh /etc         # total size of /etc only