UNIX Shell Dr. Tran, Van Hoai Faculty of Computer Science and Engineering HCMC Uni. of Technology hoai@cse.hcmut.edu.vn

Where is a shell ? Users Shells & Utilities kernel Hardware C programs scripts Users Shells & Utilities ksh, bash, gcc, find open(), fork(), Exec(), kernel Hardware

What is a shell ? To users: And definition A user interface to UNIX Many shells Thought harder to use than GUI, but not Harder to learn And definition Program to make the system understand commands (also called command intepreter)

Purpose of shell Interactive use Customization of UNIX session Environment variables Startup files Programming Shell scripts: executing a series of commands

Favourite shells GNU Project’s Bash (Bourne-Again SHell) (bash) Thompson shell → Bourne shell → Bash Most popular Command-line editing Korn shell (ksh) “Tenex” C shell (tcsh) C programming style

Shell for individual /etc/passwd Default shell for interactive use hoai:x:556:500:Tran Van Hoai:/home/hoai:/bin/bash Default shell for interactive use “which” to find a path of a shell hoai@moon:~> which tcsh /usr/bin/tcsh hoai@moon:~> which bash /bin/bash “exec” to change shell hoai@moon:~> exec bash

Interactive use Command history Command-line editing File/command expansion Job control

Shell scripting Script = set of shell and UNIX commands Text file Executable program Automating repetitive task and administrative tools

Simple commands Sequence of non-blank arguments separated by blanks or tabs First argument = name of command hoai@moon:~> ls -l lib.tar.gz -rwx------ 1 hoai users 80738389 2005-09-27 17:09 lib.tar.gz command arguments

Types of arguments Options/Flags Parameters -X or –longname $ tar –c –v –f archive.tar main.c main.h Options/Flags -X or –longname Parameters Strings, file names Depends on command

Getting help on UNIX “man”: display manual page Manual entry organization Commands System calls Subroutines Special files File format and conventions Games

Security fundamentals Multiple users identified by a number and and name hoai:x:556:500:Tran Van Hoai:/home/hoai:/bin/bash superuser = (id 0, name root) Set of users can form a group A user can belong to several groups

How are users/groups used? Used to determine if file or process operations can be performed Can a given file be read? Written to ? Can this program be run? Can I use this piece of hardward? Can I stop a particular running process?

Example hoai@moon:~> ls -l lib.tar.gz -rwx------ 1 hoai users 80738389 2005-09-27 17:09 lib.tar.gz read write execute

Unix file system is NEXT