Author: Richard Dzreke Please look at Raw for accurately formatted examples.

Custom Shell

Shell:

The outermost layer of the operating system is called the shell. In Unix-based systems, the shell is generally a command-line interface. Most Linux distributions ship with bash as the default (there are several others: csh, ksh, sh, tcsh, zsh).

• Runs commands with exec, and if the commands are not supported, it prints out an error message Example of shell running: [0]-[1]─[/]$ ./hello Hello world! [0]-[2]─[/]$ ls /dir (directory contents are shown by ls) [0]-[3]─[/]$ echo hello there! hello there! [0]-[4]─[/]$ cd /this/does/not/exist chdir: no such file or directory: /this/does/not/exist [1]-[5]─[/]$ cd /this/does/exist [0]-[6]─[/this/does/exist]$
• Prompt: shell prompt displays some helpful information, such as: [0]-[6]─[/this/does/exist]$ - Exit status of last command - Command number (begins at 1) - The current working directory

Scripting Support:

• supports scripting mode which is triggered when argv[1] is a file
• When argv[1] is a file commands are read from it instead of from stdin Example of Shell Scripting: $ cat test.sh ls / echo hi mkdir d cd d ls /ls /echo bye exit $ crash test.sh (ls / contents appear) hi exec ls failed . 1 35 32 .. 1 1 1024 bye

Executable Scripts:

• Supports scripts that begin with shebang (#!) Example: $ cat test.sh #!/crash ls / echo hi mkdir d cd d ls /ls /echo bye exit $ ./test.sh # notice how we didn't have to run 'crash' here (ls / contents appear) hi exec ls failed . 1 35 32 .. 1 1 1024 bye

Built-in Commands:

• Shell supports these built-in commands: cd to change the CWD.

  (comments): strings prefixed with # will be ignored by the shell

  history, which prints the last 100 commands entered with their command numbers ! (history execution): entering !39 will re-run command number 39, and !! re-runs the last command that was entered. !ls re-runs the last command that starts with ‘ls.’ Note that command numbers are NOT the same as the array positions; e.g., you may have 100 history elements, with command numbers 600 – 699. exit to exit the shell.

• Example of History Command: [0]-[142]─[/]$ history 42 ls 43 cat long-text.txt 44 echo "hi" # This prints out 'hi'

  ... (commands removed for brevity) ...

  140 ls /bin 141 gcc -g crash.c

• History -t (Shows the amount of time it takes to complete a command): [0]-[142]─[/]$ history -t [42|68ms] ls [43|982ms] cat long-text.txt [44|13ms] echo "hi" # This prints out 'hi' ... [140|32ms] ls /bin [141|1ms] gcc -g crash.c

IO Redirection:

• Shell supports input/output redirection and pipes Example Execution: [0]-[142]─[/]$ echo "hello world!" > my_file.txt [0]-[143]─[/]$ cat my_file.txt hello world!

  Append text with '>>':

  [0]-[144]─[/]$ echo "hello world!" >> my_file.txt [0]-[145]─[/]$ cat my_file.txt hello world! hello world!

  Pipe:

  [0]-[146]─[/]$ cat other_file.txt | sort (sorted contents shown) [0]-[147]─[/]$ seq 100000 | wc -l 10000 [0]-[148]─[/]$ cat /etc/passwd | sort > sorted_pwd.txt (sorted contents written to 'sorted_pwd.txt')

  This is equivalent, but uses input redirection instead of cat:

  [0]-[149]─[/]$ sort < /etc/passwd > sorted_pwd.txt

  This is equivalent as well. Order of < and > don't matter:

  [0]-[150]─[/]$ sort > sorted_pwd.txt < /etc/passwd

  Here's input redirection by itself (not redirecting to a file):

  [0]-[151]─[/]$ sort < sorted_pwd.txt (sorted contents shown)

Background Jobs:

• If a command ends in &, then the command is run in the background

Path Execution:

• shell supports path execution, which searches for paths or a set of paths for binaries. This allows for the execution of commands outside of the current directory
• a new function, execvp(pathname, args) is implemented to search for binaries based on priority. Priority List:
  1. Absolute or relative paths are run first. E.g., ../ls runs the ls binary in the directory one level above, /cat runs cat from the root directory, etc.
  2. If an absolute/relative path wasn’t given, and the user simply typed a program’s name, then search the / (root) directory for it by default.
  3. If the previous step failed (the program doesn’t exist in /), then attempt to execute it from the current directory instead.

Signature Feature:

• The shell's signature feature is allowing multiple commands to be run [0]-[150]─[/]$ echo "hello world" > my_file.txt
  echo "goodbye world" >> my_file.txt [0]-[151]─[/]$ cat my_file.txt hello world goodbye world