Shell Programming Erwin Earley ([email protected]) Sr. Professional Services Consultant @erwinephp @RougeWaveInc @Zend © 2018 Rogue Wave Software, Inc. All Rights Reserved. 1 Agenda • What is a Shell? • Working with the shell • Programming Constructs © 2018 Rogue Wave Software, Inc. All Rights Reserved. 2 What is a Shell? • The command line used on “Unix™” systems (as well as Unix-like systems) • Like CL it can be used interactively, or run as a Program • Like CL most commands are actually Programs that get called – There are some “built in” commands • Unlike CL there are a number of varieties of shell – sh bourne shell – csh c shell – ksh korn shell – bash bourne again shell – qsh Q shell • There are some similarities and some differences • Most of the discussion here is not oPerating system sPecific – Will worK on AIX, Linux, QSH in OS/400, other nasty Unix variants, etc © 2018 Rogue Wave Software, Inc. All Rights Reserved. 3 Why do we care about the shell? • All system configuration operations can be done through the shell – often more quickly then through a GUI • It may be the only environment available in the case of a system crash • Shell scripts can automate most routine tasks such as backups, scheduled emails, etc. • GUI can be used for a great amount of admin activities – However, the shell tends to be a comfort zone providing ability to fix things in case something goes wrong © 2018 Rogue Wave Software, Inc. All Rights Reserved. 4 Different Types of Shells • A number of shells are available each providing function/usability customized to a particular type of user: • Popular shells include: – BASH (Bourne Again Shell) – PDKsh (Public Domain Korn Shell) – csh (C shell) – mc (Midnight Commander) – QSHELL (PASE shell) – ksh (Korn shell, default on AIX) • Difference tends to be in scripting capabilities and user interface – Items such as command recall and file name completion are typically different © 2018 Rogue Wave Software, Inc. All Rights Reserved. 5 Login Shell vs. Non-Login Shell • Login Shell: Shell executed when user logs in to the system. – Typically performs general setup – initializes the terminal, sets some variables, etc. • Non-login shell: Either subshells (started from the login shell), shells started by the GUI desktop, or disconnected shells started by a command – Remember that shell is simply another command on the system • Login shells unlike Non-login shells read a series of configuration files: Login Shell: Non Login Shell: On login On startup if "/etc/profile”" source it if "~/.bashrc" exists, source it if "~/.bash_profile" exists, source it else if "~/.bash_login" exists, source it if "~/.profile" exists, source it On exit if "~/.bash_logout" exists, source it © 2018 Rogue Wave Software, Inc. All Rights Reserved. 6 Starting with bash • bash stands for Bourne Again Shell Available in PASE, more – Developed by Brian Fox in 1987 on that coming up! – One of the most popular shells available in Linux • Bash incorporates features of the Korn and C shell (ksh and csh) • Bash configuration files: /bin/bash – Bash executable /etc/profile – System wide initialization file for login shells ~/.bash_profile – Personal initialization file for login shells ~/.bashrc – Personal per-interactive-shell startup file ~/.bash_logout – Login shell clean file that executes when shell exits © 2018 Rogue Wave Software, Inc. All Rights Reserved. 7 A little more on BASH • Default Linux shell – This can be changed in a variety of ways • /etc/profile – login shell • $HOME/.profile • Magic string within the script (more on this in a moment) • Very powerful as a command line shell – Recall previous commands – Command and file completion with the <TAB> key • Many programming features – Loops and conditionals © 2018 Rogue Wave Software, Inc. All Rights Reserved. 8 Shell Environment • The shell is an environment where commands can be entered and the Operations system can respond to them • A key concept to the environment is environment variables – There are a large number of environment variables – HISTFILE: points to file containing the shell history, defaults to ~/.bash_history – HISTFILESIZE: how man last commands you wish to have in history – HOME: points to your home directory – PATH: set of directories to search when trying to execute a command – PS1: Prompt variable NOTE: All of these environment variables – USER: username are available when running bash in the PASE environment. © 2018 Rogue Wave Software, Inc. All Rights Reserved. 9 Shell metacharacters Symbol Meaning > Output redirection >> Output redirection (append) < Input redirection * File substitution wildcard; zero or more characters ? File substitution wildcard; one character [ ] File substitution wildcard; any character between brackets `cmd` Command substitution $(cmd) Command substitution | The pipe (connect output of command on right to input on command on left) ; Command Sequence || OR conditional execution && AND conditional execution © 2018 Rogue Wave Software, Inc. All Rights Reserved. 10 Shell metacharacters (continued) Symbol Meaning ( ) Group commands & Run command in the background # Comment $ Expand the value of a variable \ Prevent or escape interpretation of the next character << Input redirection " $val " Literal with variable substitution ' $val ' Literal without variable substitution © 2018 Rogue Wave Software, Inc. All Rights Reserved. 11 Useful Shell Constructs • Arrow Up & Down: Scroll through recent commands used • &&: command is only executed if preceding command was successful: command1 && command2 • alias: sets a command alias or prints defined aliases alias wrklnk=ls • bg[jobid]: Resumes the suspended job in the background • cd: changes current directory to directory indicated cd /home © 2018 Rogue Wave Software, Inc. All Rights Reserved. 12 Useful Shell Constructs (continued) • echo: Outputs the arguments echo “hello world” • find [path][expression]: searches the directory indicating looking for files that match expression: find / -name passwd –print • pwd – Prints the absolute pathname of the current working directory • unalias – Removes an alias • history – displays command history with line numbers • umask – User file creation mask • logout: exits the shell environment • exit [n]: exits shell environment with exit status n © 2018 Rogue Wave Software, Inc. All Rights Reserved. 13 More information on Bash • Just enter the command – $ man Bash • Manual page for Bash is aBout 5,000 lines!! NOTE: the manual pages are not yet availaBle in the PASE environment. © 2018 Rogue Wave Software, Inc. All Rights Reserved. 14 "bash"ing PASE – yes I went there ;-) • The bash shell is available for PASE – It is part of the RPM pile • Step 1: Install the RPM pile bootstrap • Step 2: Install bash yum install bash • Step 3: Create a home directory: PATH=/QOpenSys/pkgs/bin:$PATH mkdir /home/<username> export PATH • Step 4: Create a .profile • Step 5: Install nano bash yum install nano © 2018 Rogue Wave Software, Inc. All Rights Reserved. 15 qsh • A version of a shell for System i • Has many of the same features as bash • RTFM This is an excellent and comprehensive book! © 2018 Rogue Wave Software, Inc. All Rights Reserved. 16 Programming Constructs © 2018 Rogue Wave Software, Inc. All Rights Reserved. 17 The First Shell Program #!/bin/bash NOTE: on IBM i the path to bash is echo Hello World /QOpenSys/pkgs/bin/bash However, symbolic links have been created to /bin/bash and /usr/bin/bash This means that many existing scripts can be brought over from Linux environments and run unchanged in PASE under the bash shell • This is called a shell script • The first line "#!/bin/bash" is a magic string • When a file is loaded if the first two characters are “#!” (called shebang) then the remainder of the line is used as the program to run the file • "#!/home/erwin/my_nice_program" is a valid shell script • echo is a built-in command that Just displays the rest of the line © 2018 Rogue Wave Software, Inc. All Rights Reserved. 18 Creating a shell program • Use a text editor –Traditional Unix editors include vi and emacs –More user friendly editors include “nano” • On IBM i –Install and use nano from a PASE session © 2018 Rogue Wave Software, Inc. All Rights Reserved. 19 Writing Your Shell Script Using nano © 2018 Rogue Wave Software, Inc. All Rights Reserved. 20 Running Your Shell Script earley@testbed:~$ ls –l test.sh -rw-r—r– 1 earley earley 32 2006-09-11 08:54 test.sh earley@testbed:~$ test.sh -bash: test.sh: command not found earley@testbed:~$ • We have a "test.sh" program in our directory • Lets try and run it • Doesn’t work! Why not!?!? © 2018 Rogue Wave Software, Inc. All Rights Reserved. 21 Running Your Shell Script earley@testbed:~$ echo $PATH /home/earley/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/ usr/bin:/sbin:/bin:/usr/bin/X11 earley@testbed:~$ . test.sh Hello World earley@testbed:~$ • The program is not in the “path” – The current directory is not in the path • Can run a shell script by typing “.” – Means load the file into the current shell © 2018 Rogue Wave Software, Inc. All Rights Reserved. 22 Running Your Shell Script earley@testbed:~$ mv test.sh ~/bin/ earley@testbed:~$ echo $PATH /home/earley/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/ usr/bin:/sbin:/bin:/usr/bin/X11 earley@testbed:~$ test.sh -bash: /home/earley/bin/test.sh: Permission denied earley@testbed:~$ • Move the shell script into a directory in the path • Try and execute it again • Now we get “Permission denied”! Why!?!? © 2018 Rogue Wave Software, Inc. All Rights Reserved. 23 Running Your Shell Script earley@testbed:~$ ls –l ~/bin/test.sh -rw-r--r-- 1 earley earley 32 2006-09-11 08:54 /home/earley/bin/test.sh earley@testbed:~$ chmod u+x ~/bin/test.sh earley@testbed:~$ ls –l ~/bin/test.sh -rwx--r-- 1 earley earley 32 2006-09-11 08:54 /home/earley/bin/test.sh earley@testbed:~$ test.sh Hello World earley@testbed:~$ • The file is not marked with "executable" permissions • "chmod u+x …" says "make it executable by the user who owns it" • NOW it works © 2018 Rogue Wave Software, Inc.