shell scripting - Day 1
Admin
Texas state survey and Roll.
Goals
In this session you will:
- learn what a shell script is
- understand how unix treats scripted and binary executables
- learn which shell scripting language to use and when
- run a simple script
- assign and access variables (parameters)
- pass arguments to the script
what is a shell script, anyhow?
- batch files?
- no line terminator; relatively inflexible
- leverages your exiting knowledge about shells; no new info
- heavy use of variables, redirection, tempfiles and utilities
- shell built-ins v. utilities
Use a shell script when...
- you are going to do a series of commands more than 1-2x in a row; or,
- you have to use an ugly one-liner frequently; or,
- you want to do something the same way each time; or,
- you want to ease the scheduling of the job (
cron, at, batch)
Use something else when...
- the job is bigger than is practical or fun in shell
- re-usability and modularity are critical
- you need to control namespace
The bottom line is use a tool that fits. Shell scripts are a good place to start on most lightweight
scripting jobs.
Quotable quote: "About the only safe generalization to make is that Csh is the wrong choice
for any programming task:-)." - bennett.
executables, Windows and Unix approaches
- associations v. file types
file filename
.sh extensions are for you
- invoking shell interpreters explicitly (
shellname scriptname) or implicitly (shebang)
- the +x bit
- $PATH
the Usual Suspects
We are not concerned with interactive ("login") behavior of the shells here, only their scripting behavior.
| name |
executable |
pronounced |
pro |
con |
use it... |
| Bourne Shell |
sh |
born shell or just plain shell |
Everywhere; the shell of choice for portable scripts |
vanilla shell, missing turbocharged features of fancier shells |
all the time, unless you've got a specific reason not to. |
| C Shell |
csh |
sea shell |
Syntax similar to c. |
Syntax similar to c. :-), seriously though, see this anti-csh rant. |
you're most familiar with C, and you need to knock out something quick/dirty. |
| Bourne Again Shell |
bash |
bash |
extended form of sh, although most of the extensions are for interactive stuff. |
not everywhere. |
you know bash best, and the script will only be run on the present machine. Commonly used on linux machines. |
| Korn Shell |
ksh |
corn shell |
extended form of sh, although most of the extensions are for interactive stuff.
| not everywhere. Not free, although free clones like pdksh do exist. |
you know ksh best, and the script will only be run on the present machine. Commonly used on Solaris machines. |
| perl |
perl |
pearl |
so-called "swiss army chainsaw" of shell scripting.
Extremely feature rich, glomming together the greatest hits of sed, awk, grep, etc. Just-in-time compilation offers sophisticated syntactical checks, debugging, etc. Extensive unix hooks. Can be compiled. OOP. |
A bloated pig. Not everywhere. Often accused of ugliness. Not a shell in the normal sense. |
your task becomes cumbersome in a traditional shell. This class is not about perl, so we won't deal with it any further. |
your first shell script
make an initials dir...
- name
- shebang or magic line
- your programatic content
practicum: hello world
comment in function and your contact info. write in both csh and sh. test.
variables
Program variables are scoped within your script. They are traditionally lower or mixed case.
[_a-zA-Z][_a-zA-Z0-9]*
see all vars with
set
Environmental variables are available to all scripts and shells on your login. They are traditionally UPPER CASE to provide visual (not technical) distinction.
see all ENV vars with
export
|
sh |
csh |
| defining vars |
var='somevalue' |
set var='somevalue' |
| accessing vars |
echo ${var} (braces can be optional if separated and legal) |
echo ${var} (braces can be optional if separated and legal) |
| globalize vars to ENV |
export ${VAR} |
setenv VAR=value |
| reading in vars from input |
read var |
FIXME |
| defining arrays |
|
|
| accessing arrays |
|
${var[i]} |
built-in vars
|
sh |
csh |
| name of script | $0 | $0 |
| number of args | $# | ${#argv} |
| first arg | $1 | ${#argv[1]}, $argv[1} |
| all args | $* | $*, $argv[*] |
| exitcode of last command | $? | |
| PID | $$ | |
Setting multiple vars on one line: foo="lala" bar="haha"
destroying variables
positional parameters
$0 command name
$1-9 args
command lines are space-delimited unless quotes are present
You can shift to get to positions greater than 9.
shells, children, and variable scope
- all shells as child of login shell; stacked shells, invoked scripts
- populating ENV by value not by reference.
- can pass vars to subshell prior to command name.
practicum: verify how ENV parsing works.
constants
readonly foo. This attribute is not passed to subshells, and some ENVs cannot be made readonly. Can't revert.
Homework
- math, pp. 132-33
- flow control, pp. 149, 169-71
- test, pp. 153-55
http://www.mousetrap.net/syllabus/shell/day1.html
$Id: day1.orb,v 1.12 2002/09/10 01:59:16 mouse Exp $
Remember, your login is based on your machine's hostname, not on any other number.
~/[initials] refers to the subdirectory under your homedir, named after your initials. Everything except for .dotfiles will be stored in your ~/[initials] directory.
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