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harsh banthiya
harsh banthiya

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Writing my own minimal shell in C - Part 2 (The Builtin Functions and general interpretation)

DAY 2 - Understanding the scope of each builtin

Quick recap of the builtin functions that we need to make

  • echo with the option -n
  • cd with only a relative or absolute path
  • pwd without any options
  • export without any options
  • unset without any options
  • env without any options and any arguments
  • exit without any options

cd function

cd can be used with these characters and the sequence matters.

  • " . " - indicates the current folder.
  • " .. " - indicates the parent folder.
  • " ~ " - indicates the absolute path to the user.
  • " / " - indicates separation between folders.

" ~ " can only be entered as the first argument to cd

cd ~ (this is okay)
cd ./~ (this won't work)
cd dir/~ (this won't work)

" / " it can be used as a separator between folders.

cd srcs/ (this is okay)
cd ../ (this is okay)
cd /srcs (this won't work)

Functions required to write cd function

getcwd - Get the current path
chrdir - if you put a path in chdir, it moves to that path

Handling Quotes

Constraint as discussed over quote handling

' inhibit all interpretation of the following sequence of characters.
" inhibit all interpretation of the following sequence of characters expect for $

Understanding Quotes and how they behave

  • Double quotes must be closed whenever you open one.
  • But if you open with a single quote and close with a single quote, any number of double quotation marks can be inserted between them.
  • Similarly if you open with double quotes and close with double quotes, any number of single quotes can be inserted between them.

Valid quote handling for us includes

  • " "
  • " ' "
  • ' '
  • ' " '

Invalid quote handling for us includes

  • "
  • '
  • " " '
  • ' ' "
  • ' " "
  • " ' '
  • ' ' '
  • " " "
Lets look at quotes between executable and variables

Valid quote handling includes:

  • echo "abc"
  • echo "a"bc
  • echo "ab"c
  • echo ""abc
  • echo abc""
  • ech"o" abc
  • echo"" abc
  • ""echo abc
  • "echo" abc
  • "echo""" abc

Invalid quote handling includes:

  • echo" abc
  • echo a"bc
  • echo" a"bc
  • echo" "abc
  • ech"o "abc
  • "echo abc"

echo function

  • Execute echo command with or without args
  • Repeat with other arguments
  • Restart the shell once done
  • If there is conflict, use the conflict flag
  • if something doesn't work, use the incomplete flag

exit function

  • Execute with or without args
  • exit shell
  • same use a conflict flag, incase of a conflict
  • if something doesn't work, use a incomplete job flag

env function

  • display current env variables

export function

  • Export environment variables, create new ones and replace old ones
  • export without options prints a list of env variables
  • check with env

unset function

  • remove environment variables using unset
  • check with result in env

Other Basic information to be aware of

Simple commands and global

  • Shell must be good with simple commands, and commands like /bin/ls and no options
  • Test for when empty command line is inserted.
  • Test with inserting spaces and tabs at random points
  • If something is colliding use a conflict flag
  • If something doesn't work, use the incomplete flag.

Arguments and History

  • Must be able to execute simple commands with an absolute path such as /bin/ls with args. Without single and double quotes.
  • Repeat with each command and argument
  • Use relevant flags for conflict and incomplete.

Return value of a process

  • To mimic the working of $?, execute a simple command with an absolute path or any other command with args but without quotes and double quotes then execute echo $?
  • Now try something similar as above but with single quotes and double quotes and check echo $? after that
  • Try commands that will fail like bin/ls nonexitantfile and see the value of $?
  • Maybe try $? + $?


  • ctrl-c : SIGINT
  • ctrl-\ : SIGQUIT
  • ctrl-c : Stop execution with an interrupt signal from the keyboard
  • ctrl-\ : A stop signal from keyboard, which terminates the process and dumps the core.

    • Pressing ctrl-c at an empty prompt should create newline
    • If you hit ctrl-\ at the empty prompt, nothing should happen
    • Press ctrl-d at an empty prompt to close the shell
    • After writing a few things, press ctrl-c to show a new prompt line
    • After writing a few things, press ctrl-d, nothing happens
    • after writing a few things, press ctrl-\ to turn off the shell
    • Block ctrl-c, ctrl-d, ctrl-\ while cat or grep is running.
  • Incase of conflict, use conflict and incase it doesn't work use the incomplete flag.

Single Quotes

  • Try an empty parameter
  • Test with env variables, whitespace, pipes and redirects.
  • '%USER' should print $USER

Relative Path

  • Repeat with multiple directories with complex relative paths
  • Set $PATH to a multiple directory value(dir1:dir2) and check those directories in left to right order.

Next up, Lets try to find the relevant modules to fit all this information and to inbuilt checks at the right spots and the general flow of entire shell.

Top comments (1)

fousse24 profile image

I think making a list of said modules (and/or functions) with their return values would be the best. If we know what our modules/functions should return and what we expect them to do, it makes it much more easier for us to split the work