The magic of argv

Every C-Program must have a function called main() , program execution starts from it.

When a program is executed with command line arguments, those are made available to program’s main() function by 2 arguments:

• 1st argument argc of type int indicates number of arguments
• 2nd argument *argv[] an array of pointers to the command line arguments.

The 1st string in argv i.e argv[0] is the name of program itself.

Many Linux programs use it to their benefit, by creating links to these binaries with different names.

They can perform different functions from same program depending of the program that was used to invoke them.

Let’s create a small python program program1 in /usr/local/bin

cat <<EOF > /usr/local/bin/program1
#!/usr/bin/env python3
from sys import argv
print(argv[0])
EOF

chmod +x /usr/local/bin/program1

We use python’s sys module to print argv[0] i.e program’s name, since we created it in /usr/local/bin it’s already in our PATH , we simply execute by typing program1 on terminal.

output is: /usr/local/bin/program1

Now let’s create a hard link on program1 with another name program2

ln /usr/local/bin/program1 /usr/local/bin/program2

Now, when we execute program2 output is /usr/local/bin/program2

Thus the same 3 lines of code print different output depending on calling program.

If you noticed, I used ln to create a link, this creates a hard link .

What if I create a soft link, ln -s .

Let’s see:

ln -s /usr/loca/bin/program1 /usr/local/bin/program3

Run program3 in terminal, output is /usr/local/bin/program3

Makes sense, since we used a different program name to invoke it.

If you know how links work in linux:

• soft links -> are created on file names, can span across different file systems, have different inode number, link becomes dangling/unusable once original file is deleted.
• hard links -> are created on inode number thus cannot span across different file systems, continues to work even if original file is deleted.

Now that we know what’s what, let’s try to identify such existing programs on Linux, which are same programs doing different things under different names.

Let’s check inode number for programs we created above using ls -li command, we can also use stat to find this.

ls -i /usr/local/bin/program1
61573154 /usr/local/bin/program1

ls -i /usr/local/bin/program2
61573154 /usr/local/bin/program2

As we see, they both have same inode number, since program2 was a hard link on program1

What about program3

ls -i /usr/local/bin/program3
61575256 /usr/local/bin/program3

It’s inode number is different, since this was a soft link

With this understanding that hard links will have same inode number, we try to find such programs in system’s /usr/bin director:

cat <<EOF > dup.py
#!/usr/bin/env python3
import os

if __name__ == "__main__":
    inode_name = {}
    with os.scandir(path="/usr/bin") as entries:
        for entry in entries:
            if entry.is_file():
                inode_name[entry.inode()] = inode_name.get(entry.inode(), []) + [entry.name]
    print( *[ v for _,v in inode_name.items() if len(v) > 1], sep="\n" )
EOF
chmod +x dup.py
./dup.py

The output is a lot of programs, sharing a few here I was amazed to see:

['umask', 'unalias', 'alias', 'wait', 'hash', 'fc', 'read', 'type', 'getopts', 
'bg', 'fg', 'cd', 'command', 'jobs', 'ulimit']
['gzcat', 'zcat', 'gunzip']
['readlink', 'stat']
['cksum', 'sum']

cd and ulimit are same program even though both do very separate functions on a Linux system.

I wasn’t worried about soft links since they aren’t that difficult to find:

ls -l /usr/local/bin/program3

/usr/local/bin/program3 -> /usr/local/bin/program1

If you made it till here, Thanks for reading!

Bonus:

When we created a hard link and this can also be seen in stat object of the file with:

python3 -c "import os;print(os.stat('/usr/local/bin/program1').st_nlink)"

2

the inode object maintains an int attribute st_nlink indicating number of links on the file.

References:

• Chapter 6, section 6 : The Linux Programming Interface — Michael Kerrisk
• https://docs.python.org/3/library/os.html#os.scandir
• https://docs.python.org/3/library/os.html#os.DirEntry.stat