Last updated: Thursday 25 September 2025 @ 18:06:06

Shell Commands

As said in the lecture shell commands....

To successfully make use of a terminal and to become competent, knowing the commonly used posix and gnu-core-utils packages are paramount. It is a tall order to learn every package, out there, for instance the nixos package manager has over 80,000 packages!

So below is a sample of useful packages to become familar with.

1. Navigating the File System

1.1 `pwd` (Print Working Directory)

Description: Displays the current directory.

pwd

Example:

Open your terminal.
Type pwd and press Enter.
You will see the full path of the directory you’re currently in, like /home/user.

1.2 `cd` (Change Directory)

Description: Changes the working directory.

cd [directory]

Examples:

To move to the /home directory:
```
cd /home
```
To go back to the previous directory:
```
cd -
```

1.3 `ls` (List Directory Contents)

Description: Lists files and directories.

ls [options] [directory]

Examples:

List all files in the current directory:
```
ls
```
List hidden files:
```
ls -a
```
Show detailed information (file size, permissions, etc.):
```
ls -l
```

2. File Operations

2.1 `cp` (Copy Files)

Description: Copies files or directories.

cp [source] [destination]

Examples:

Copy a file:
```
cp file.txt /backup/file.txt
```
Copy a directory:
```
cp -r /source/dir /backup/dir
```

2.2 `mv` (Move/Rename Files)

Description: Moves or renames files.

mv [source] [destination]

Examples:

Move a file:
```
mv file.txt /backup/file.txt
```
Rename a file:
```
mv oldname.txt newname.txt
```

2.3 `rm` (Remove Files)

Description: Deletes files or directories.

rm [options] [file/directory]

Examples:

Remove a file:
```
rm file.txt
```
Remove a directory:
```
rm -r /path/to/directory
```

3. File Permissions

3.1 `chmod` (Change File Permissions)

Description: Changes permissions of a file or directory.

chmod [options] [permissions] [file/directory]

Examples:

Make a file executable:
```
chmod +x script.sh
```
Set specific permissions (e.g., read-write for owner, read for others):
```
chmod 644 file.txt
```

3.2 `chown` (Change File Ownership)

Description: Changes the owner of a file or directory.

chown [owner][:group] [file/directory]

Examples:

Change owner:
```
chown newuser file.txt
```
Change owner and group:
```
chown newuser:newgroup file.txt
```

4. Searching and Finding Files

4.1 `find` (Search for Files)

Description: Searches for files in a directory hierarchy.

find [path] [options]

Examples:

Find all .txt files in the current directory:
```
find . -name "*.txt"
```
Find files modified in the last 7 days:
```
find /path -mtime -7
```

4.2 `grep` (Search Text in Files)

Description: Searches for a pattern in files.

grep [options] [pattern] [file]

Examples:

Search for "hello" in a file:
```
grep "hello" file.txt
```
Search recursively in a directory:
```
grep -r "hello" /path/to/directory
```

5. Disk Usage

5.1 `df` (Disk Free)

Description: Displays disk space usage.

df [options]

Example:

Check disk usage in human-readable format:
```
df -h
```

5.2 `du` (Disk Usage)

Description: Shows the size of files and directories.

du [options] [file/directory]

Example:

Check the size of a directory:
```
du -sh /path/to/directory
```

6. Process Management

6.1 `ps` (Process Status)

Description: Shows information about active processes.

ps [options]

Example:

Show all running processes:
```
ps aux
```

6.2 `kill` (Terminate Processes)

Description: Sends a signal to a process (usually to terminate it).

kill [options] [process_id]

Example:

Kill a process with PID 1234:
```
kill 1234
```

7. Networking

7.1 `ping` (Check Network Connectivity)

Description: Sends ICMP echo requests to test network connectivity.

ping [host]

Example:

Ping Google to check internet connectivity:
```
ping google.com
```

7.2 `ifconfig` / `ip` (Network Interface Configuration)

Description: Displays or configures network interfaces.

ifconfig [interface]

ip a

Example:

Check the network interface configuration:
```
ifconfig
```

ip a

8. Archiving and Compression

8.1 `tar` (Create and Extract Archives)

Description: Creates or extracts archives.

tar [options] [archive] [file/directory]

Examples:

Create a .tar.gz archive:

tar -czvf archive.tar.gz /path/to/directory

Extract a .tar.gz archive:
```
tar -xzvf archive.tar.gz
```

8.2 `gzip` / `gunzip` (Compress/Decompress Files)

Description: Compresses or decompresses files using gzip.

gzip [file]

gunzip [file.gz]

Examples:

Compress a file:
```
gzip file.txt
```
Decompress a file:
```
gunzip file.txt.gz
```

9. Text Processing

9.1 `cat` (Concatenate and Display Files)

Description: Displays file contents.

cat [file]

Example:

Display the contents of a file:
```
cat file.txt
```

9.2 `head` / `tail` (View Beginning or End of Files)

Description: Displays the first or last lines of a file.

head [file]

tail [file]

Example:

Display the first 10 lines of a file:
```
head file.txt
```
Display the last 10 lines of a file:
```
tail file.txt
```

10. Dates

First run the date command in the terminal.
```
$ date
```
Output: will be something like this...
```
Mon 27 Feb 13:58:41 GMT 2023
```
So how do we get different formats?

Well if in doubt you can check the man pages or use the commands --help flag
```
$ date --help
```
Looking at the output workout how to produce this format:
```
> 20230227
```
Answer
```
date +"%Y%m%d"
```

Now modify this output to display:

> Year: 2023, Month: 02, Day: 27

Answer

date +"Year: %Y, Month: %m, Day: %d"

The most common formatting characters:
- %D – Display date as mm/dd/yy
- %Y – Year (e.g., 2020)
- %m – Month (01-12)
- %B – Long month name (e.g., November)
- %b – Short month name (e.g., Nov)
- %d – Day of month (e.g., 01)
- %j – Day of year (001-366)
- %u – Day of week (1-7)
- %A – Full weekday name (e.g., Friday)
- %a – Short weekday name (e.g., Fri)
- %H – Hour (00-23)
- %I – Hour (01-12)
- %M – Minute (00-59)
- %S – Second (00-60)
Using the above information and what previous steps reproduce the ISO 8601 standard for datetime.
```
> 2023-02-27T12:08:45Z
```
Answer
```
$ date +"%Y-%m-%dT%H:%M:%S:%NZ"
```
- %Y – Year (e.g., 2020)
- %m – Month (01-12)
- %d – Day of month (e.g., 01)
- T – self delimeter
- %H – Hour (00-23)
- : – self delimeter
- %M – Minute (00-59)
- : – self delimeter
- %S – Second (00-60)
- Z – self delimeter
Amend the last command so that after seconds you get nano seconds.
```
> 2023-02-27T12:14:03:281999558Z
```
Answer
```
$ date +"%Y-%m-%dT%H:%M:%S:%NZ"
```
- %Y – Year (e.g., 2020)
- %m – Month (01-12)
- %d – Day of month (e.g., 01)
- T – self delimeter
- %H – Hour (00-23)
- : – self delimeter
- %M – Minute (00-59)
- : – self delimeter
- %S – Second (00-60)
- : – self delimeter
- %N – nanoseconds (000000000..999999999)
- Z – self delimeter

11 Manipulating inputs and outputs

working with the previous command lets strip out the T,:s and Z so that we are left with only numnbers.
Using awk we can manipulate the output by piping | the output of date into awk - first run this command
```
$ date +"%Y-%m-%dT%H:%M:%S:%NZ" | awk "{print $1}" 
> 2023-02-27T12:20:40:005619503Z   
```
- awk with the argument print $1 will print the first argument... ie the output of date

Now lets remove the T

$ date +"%Y-%m-%dT%H:%M:%S:%NZ" | awk -F 'T' '{print $1}'
> 2023-02-27

What happens when you do $2 on the same command as above?
Answer
```
> 12:50:48:341942600Z
```
This is because the input has been split and now
- $1 = 2023-02-27
- $2 = 12:20:40:005619503Z

Change the delimeter -F 'T' to -F ':' and repeat both steps as before:

Answer

$  date +"%Y-%m-%dT%H:%M:%S:%NZ" | awk -F ':' '{print $1}'
>  2023-02-27T12

$ date +"%Y-%m-%dT%H:%M:%S:%NZ" | awk -F ':' '{print $2}'
  55
$ date +"%Y-%m-%dT%H:%M:%S:%NZ" | awk -F ':' '{print $3}'
  32
$ date +"%Y-%m-%dT%H:%M:%S:%NZ" | awk -F ':' '{print $4}'
  473072761Z

You can put these together using the print {$2$3$4$}, try. sh date +"%Y-%m-%dT%H:%M:%S:%NZ" | awk -F ':' '{print $2$3$4}' 5719707652042Z
Experiment with this and consider piping with awk to see if you can get:
```
20230227130026355420256
```
Answer
```
$ date +"%Y-%m-%dT%H:%M:%S:%NZ" | awk -F '[-T:Z]' '{print $1$2$3$4$5$6$7$8$9}'
20230227130026355420256
```
Created with ❤️ by CompEng0001

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