Linux-Programming-Interface

Unix Utilities in C

A collection of simple Unix command-line utilities implemented in C, mimicking the behavior of standard Unix tools.

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Utilities

Utility	Description
pwd	Print the current working directory
echo	Print arguments to standard output
cp	Copy a file to a destination
mv	Move/rename a file to a destination

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Files

unix-utilities/
├── pwd.cpp
├── echo.cpp
├── cp.cpp
├── mv.cpp
└── README.md

Files use the .cpp extension due to a platform limitation, but are written in pure C.

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Compilation

Each utility is compiled separately using gcc:

gcc -o pwd  pwd.cpp
gcc -o echo echo.cpp
gcc -o cp   cp.cpp
gcc -o mv   mv.cpp

Or compile all at once:

for f in pwd echo cp mv; do gcc -o $f $f.cpp; done

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Usage & Example Output

pwd

Prints the absolute path of the current working directory.

$ ./pwd
/home/user/unix-utilities

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echo

Prints all arguments to stdout, space-separated, with a trailing newline.

$ ./echo Hello World
Hello World

$ ./echo
(blank line)

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cp

Copies a source file to a destination path. Creates the destination if it does not exist; truncates it if it does. Preserves the source file's permissions.

$ echo "Hello" > file.txt
$ ./cp file.txt /tmp/file_copy.txt
$ cat /tmp/file_copy.txt
Hello

Error case:

$ ./cp nonexistent.txt /tmp/out.txt
cp: cannot open source: No such file or directory

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mv

Moves a file to a new location or renames it. Uses rename() for same-filesystem moves (atomic, instant). Falls back to copy + delete for cross-device moves.

$ echo "Hello" > /tmp/file.txt
$ ./mv /tmp/file.txt /tmp/new_name.txt
$ cat /tmp/new_name.txt
Hello
$ ls /tmp/file.txt
ls: cannot access '/tmp/file.txt': No such file or directory

Cross-device move:

$ ./mv /tmp/file.txt /home/user/file.txt
(file is copied to destination, then removed from source)

Error case:

$ ./mv nonexistent.txt /tmp/out.txt
mv: No such file or directory

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System Calls & Library Routines Used

Utility	Key calls
pwd	getcwd(), free()
echo	printf()
cp	open(), fstat(), read(), write(), close()
mv	rename(), stat(), open(), read(), write(), close(), unlink()

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Notes

• All error messages are printed to stderr using perror() or fprintf(stderr, ...)
• All system call return values are checked
• mv handles cross-device moves transparently via a copy-then-delete fallback (triggered when rename() returns EXDEV)
• cp and mv preserve source file permissions on the destination