BASH Shell Scripting. Adapted from:

Size: px

Start display at page:

Download "BASH Shell Scripting. Adapted from:"

Wilfrid Morrison
7 years ago
Views:

1 BASH Shell Scripting Adapted from:

2 The basics A shell script is a collection of Unix commands, with a little bit of programming syntax thrown in to make it all work A comment begins with '#' and can start anywhere on the line The first line of the script tells what shell to use to interpret the script, in the form '#!/bin/sh' #!/bin/bash #hash bang bin bash but this is not special echo Hello World echo Hello World echo Hello World

3 Variables Variables are assigned with the syntax $ var=hello We can interactively set variable names using the read command Note no spaces Variables are referenced by using a "$ $ echo $var $ Hello but echo $VAR won t work - its case sensitive If there is any confusion about what the variable name is, you can enclose it in braces like ${var}. $ echo ${var}99 $ Hello99

4 Quoting There are three main types of quotes Double quote " Only variables are expanded within the double quotes Single quote ' (bottom right on most keyboards) Everthing is taken literally, not interpreted, and gathered into one argument Backquotes ` (furthest top left on most keyboards) Anything in backquotes is taken as a shell command, and the output of the command is placed on the command line in the place of the backquoted text Also done as $(date) instead of `date` Usefull within double quaotes echo hello $USER today is `date` Note the more modern version to do this is not to use the backquote as in echo Hello $USER today is $(date) Example Script today=`date` #backquote => execute date command and place in variable today echo 'Running on ùname`: The value of today is $today.' #all inside single quote => taken literally and not interpreted echo "Running on ùname`: The value of today is $today. #all inside double quote => interpreted and uname is executed and substituted into the string Example output $ Running on ùname`: The value of today is $today. $ Running on Linux: The value of today is Tue Feb 22 18:19:27 PST 2000.

5 Pre-defined variables /1 There are some special variables that a pre-defined for you whenever a shell script is running. $0 $0 is the name of the currently running script, as specified to the system If you run the script as in "./script", then $0 will be "./script" If you run the script with "/home/joe/bin/script", then $0 will be "/home/joe/bin/script" $# The number of arguments that were given to the script $* All of the arguments together, but be careful, because you lose quoting (see below) $1, $2, $3,... $9 $n is the nth argument, and is undefined if there was no nth argument The shift command moves every argument "down a notch" $2 becomes $1 $3 becomes $2 $4 becomes $3 etc. (The value of the previous $1 is discarded)

6 Pre-defined variables /2 $$ $? The PID of the script Often used to assign unique temporary filenames The exit value of the last command run from the script

7 if if <<condition>> then <<one or more statements>> fi Example if [ $var -eq 0 ] then echo Go home!" fi

8 if-then-else if <<condition>> then <<one or more statements>> else <<one or more statements>> fi Example if [ $var -eq 0 ] then echo Go Home!" else echo "It's ok you can stay." fi

9 if-then-elif-fi if <<condition>> then <<one or more statements>> elif <<condition>> <<one or more statements>> else <<one or more statements>> fi Example if [ $var -eq 0 ] then echo Go Home!" elif [ $var -eq 1 ] echo Well done!." else echo Not sure." fi

10 Conditionals Any Unix command can be used as a conditional If the command returns 0, it's "true" If the command returns anything else, it's "false" One often-used command is the "test" command or this format [ $var -eq 0 ] test is aliased as "[" to make your program look more like other programming languages The test command has a huge number of options

11 Case statement Functionally equivalent to if-then-elif-elif-elifelse-fi but cleaner-looking Matching of cases is done as strings using shell pattern matching

12 for for var in <<item1>> <<item2>> <<item3>> do <<statements -- some using $var>> done

13 Example from from /etc/rc.d/init.d/misc

CS2720 Practical Software Development

Page 1 Rex Forsyth CS2720 Practical Software Development CS2720 Practical Software Development Scripting Tutorial Srping 2011 Instructor: Rex Forsyth Office: C-558 E-mail: forsyth@cs.uleth.ca Tel: 329-2496