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• Preet Harquissandas (CID: 02589338)
• Mikhail Agakov (CID: 02560202)

s3 is a custom Unix-like shell implemented in C.It supports execution of external programs, built-in commands, redirection, pipelines, batched commands, and optional extensions such as subshells.

This README document describes the features implemented, the architectural decisions made, and any additional enhancements beyond the base requirements.

✔ Parsing of command input using strtok

✔ Execution of external programs using fork() + execvp()

✔ Parent waits using wait() / custom reap()

✔ Support for multiple arguments

✔ Implemented using open() + dup2()

✔ > overwrites file

✔ >> appends to file

✔ Implemented using open(O_RDONLY) + dup2()

• Detection of redirection operator using a custom parser that respects parenthesis nesting
• Launch handled through launch_program_with_redirection()
• While the specification only required handling a single redirection operator, we extended this to support multiple operators (see Section 8).

✔ Implemented directly in parent process via chdir()

✔ Prompt displays current working directory

✔ Uses getcwd() inside construct_shell_prompt()

Example prompt:[/home/user/projects/s3 s3]$

✔ Full support for pipelines of arbitrary length

✔ Implemented using:

• pipe()
• dup2() in each child
• Multiple fork() calls in a loop

✔ Redirection allowed on pipeline endpoints

✔ Splits the command line into batch units

✔ Each batch unit is processed independently

✔ Supports pipelines or plain commands inside batch elements

✔ Support for grouped commands in parentheses

✔ Each subshell executes in its own process environment

✔ Allows redirection & pipelines involving subshells

✔ Recursive parsing of nested parentheses

✔ Stack-based matching for ( and )

✔ Support for redirecting both standard input and standard output in a single command

✔ Shell parses and records all redirection operators before executing the command

✔ Input and output redirections are applied by configuring the appropriate file descriptors

✔ Support for applying redirection to entire logical blocks (like subshells) rather than just simple binaries.

✔ Achieved a recursive architecture: the shell detects redirection at the top level, sets up the file descriptors, and then recursively calls the executor for the command string inside.

✔ Compatible with other features such as pipelines and subshell execution

We used a set of commands covering all features:

• ls
• wc txt/phrases.txt
• man cat
• grep burn txt/phrases.txt

• grep June < calendar.txt
• tr a-z A-Z < phrases.txt

• cat txt/phrases.txt | sort | wc -l
• ps aux | grep python | sort -k 3 -nr | head
• tr a-z A-Z < txt/phrases.txt | grep BURN

• echo A ; echo B ; ls -l
• mkdir results ; cat txt/phrases.txt | sort > results/sorted_phrases.txt ; echo "Done"

• (cd txt ; ls)
• (cat txt/phrases.txt | sort) | head
• Nested:((echo A))
• (echo a ; (echo b ; (echo c)))

• (tr a-z A-Z < txt/phrases.txt) > txt/phrases_upper.txt
• sort < unsorted.txt > sorted.txt
• ((tr a-z A-Z < txt/phrases.txt) | head) > txt/new_file.txt

gcc *.c -o s3

./s3