1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context

1.109.2 Customize or write simple scripts Weight 3
Linux Professional Institute Certification — 102

Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables

Nick Urbanik <nicku@nicku.org>
This document Licensed under GPL—see section 18

Quoting Command Substitution The if statement while statement The for statement

2005 November

The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Outline

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives

Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Quoting and Funny Chars Quoting

Command Substitution The if statement while statement The for statement The test program Conditions Arithmetic Input & Output Output with echo Input with read Alerting about problems by email License Of This Document

The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Topic 109 Shells, Scripting, Programming and Compiling [8]
Where we are up to

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables

1.109.1 Customize and use the shell environment [5] 1.109.2 Customize or write simple scripts [3]

Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Description of Objective
1.109.2 Customize or write simple scripts [3]

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context

Candidate should be able to customize existing scripts, or write simple new (ba)sh scripts. This objective includes using standard sh syntax (loops, tests), using command substitution, testing command return values, testing of file status, and conditional mailing to the superuser. This objective also includes making sure the correct interpreter is called on the first (#!) line of scripts. This objective also includes managing location, ownership, execution and suid-rights of scripts.

Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Key files, terms, and utilities include:
1.109.2 Customize or write simple scripts [3]

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

while — shell builtin: does things repetively while a condition is true for — shell builtin: does things repetively, once with each element of a list test — used to construct a condition chmod — an external command, to change the permission on a file

The Shebang: #!

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context

You ask the Linux kernel to execute the shell script kernel reads first two characters of the executable file
If first 2 chars are “#!” then kernel executes the name that follows, with the file name of the script as a parameter

Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Example: a file called find.sh has this as the first line: #! /bin/sh then kernel executes this: /bin/sh find.sh What will happen in each case if an executable file begins with:
#! /bin/rm #! /bin/ls

Making the script executable

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives

To easily execute a script, it should: be on the PATH have execute permission. How to do each of these? Red Hat Linux by default, includes the directory ∼/bin on the PATH, so create this directory, and put your scripts there: $ mkdir ∼/bin ← If your script is called script, then this command will make it executable: $ chmod +x script ←

The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Should you make a script SUID?

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context

Normally, when you run a script, the process is owned by you, and has the same access rights as you If a script has the SUID permission, then:
it does not matter who executes it! the owner of the process is the owner of the file This is very dangerous, especially if the owner of the file is root!

Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Never make a shell script SUID, unless you really, really know what the risks are and how to avoid them Instead, write it in a language such as Perl, with taint checking, and make it as simple as possible. See Topic 1.114.1 Perform security administration tasks for details of manipulating SUID/SGID permissions.

True and False

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #!

Shell programs depend on executing external programs When any external program execution is successful, the exit status is zero, 0 An error results in a non-zero error code To match this, in shell programming:
The value 0 is true any non-zero value is false

Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

This is opposite from other programming languages

Variables—1

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Variables not declared; they just appear when assigned to Assignment:
no dollar sign no space around equals sign examples: $ x=10 # correct $ x = 10 # wrong: try to execute program called ‘‘x’’

The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Read value of variable:
put a ‘$’ in front of variable name example: $ echo "The value of x is $x"

Variables—Assignments

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #!

You can put multiple assignments on one line: i=0 j=10 k=100 You can set a variable temporarily while executing a program: $ echo $EDITOR emacsclient $ EDITOR=gedit crontab -e $ echo $EDITOR emacsclient

Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Variables—Local to Script

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context

Variables disappear after a script finishes Variables created in a sub shell disappear
parent shell cannot read variables in a sub shell example: $ cat variables #! /bin/sh echo $HOME HOME=happy echo $HOME $ ./variables /home/nicku happy $ echo $HOME /home/nicku

Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Variables—unsetting Them

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables

You can make a variable hold the null string by assigning it to nothing, but it does not disappear totally: $ VAR= ← $ env | grep ’^VAR’ ← VAR= You can make it disappear totally using unset: $ unset VAR ← $ env | grep ’^VAR’ ←

Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Command-line Parameters
Command-line parameters are called $0, $1, $2, . . . Example: when call a shell script called “shell-script” like this: $ shell-script param1 param2 param3 param4 ←

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False

variable $0 $1 $2 $3 $4 $#

value shell-script param1 param2 param3 param4 number of parameters to the program, e.g., 4

Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic

Note: these variables are read-only.

Input & Output Alerting about problems by email License Of This Document

Special Built-in Variables

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context

Both $@ and $* are a list of all the parameters. The only difference between them is when they are quoted in quotes—see manual page for bash $? is exit status of last command $$ is the process ID of the current shell Example shell script: #! /bin/sh echo $0 is the full name of this shell script echo first parameter is $1 echo first parameter is $2 echo first parameter is $3 echo total number of parameters is $# echo process ID is $$

Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Special Characters
Many characters have a special meaning to the shell Character ∼ ‘ # $ & * | ( ) [ ] { ; \ ’ " < > / ? ! space or tab Meaning Home directory Command substitution. Better: $(...) Comment Variable expression Background Job File name matching wildcard Pipe Start subshell End subshell Start character set file name matching End character set file name matching Start command block Command separator Quote next character Strong quote Weak quote Redirect Input Redirect Output Pathname directory separator Single-character match in filenames Pipline logical NOT shell normally splits at white space

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting
Quoting and Funny Chars Quoting

Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This

Quoting

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives

Sometimes you want to use a special character literally; i.e., without its special meaning. Called quoting Suppose you want to print the string: 2 * 3 > 5 is a valid inequality? If you did this: $ echo 2 * 3 > 5 is a valid inequality the new file ‘5’ is created, containing the character ‘2’, then the names of all the files in the current directory, then the string “3 is a valid inequality”.

The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting
Quoting and Funny Chars Quoting

Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This

Quoting—2

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives

To make it work, you need to protect the special characters ‘*’ and ‘>’ from the shell by quoting them. There are three methods of quoting:
Using double quotes (“weak quotes”) Using single quotes (“strong quotes”) Using a backslash in front of each special character you want to quote

The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting
Quoting and Funny Chars Quoting

This example shows all three: $ echo "2 * 3 > 5 is a valid inequality" $ echo ’2 * 3 > 5 is a valid inequality’ $ echo 2 \* 3 \> 5 is a valid inequality

Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This

Quoting—When to use it?

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives

Use quoting when you want to pass special characters to another program. Examples of programs that often use special characters:
find, locate, grep, expr, sed and echo

The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting
Quoting and Funny Chars Quoting

Here are examples where quoting is required for the program to work properly: $ find . -name \*.jpg $ locate ’/usr/bin/c*’ $ grep ’main.*(’ *.c $ i=$(expr i \* 5)

Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This

More about Quoting

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context

Double quotes: "..." stop the special behaviour of all special characters, except for:
variable interpretation ($) backticks (‘) — see slide 5 the backslash (\)

Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting
Quoting and Funny Chars Quoting

Single quotes ’...’:
stop the special behaviour of all special characters

Backslash:
preserves literal behaviour of character, except for newline; see slides §4, §9 Putting “\” at the end of the line lets you continue a long line on more than one physical line, but the shell will treat it as if it were all on one line.

Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This

Command Substitution — $(...) or ‘...‘

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #!

Enclose command in $(...) or backticks:‘...‘ Means, “Execute the command in the $(...)and put the output back here.” Here is an example using expr: $ expr 3 + 2 5 $ i=expr 3 + 2 # error: try execute command ‘3’ $ i=$(expr 3 + 2) # correct $ i=‘expr 3 + 2‘ # also correct

Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Command Substitution—Example
We want to put the output of the command hostname into a variable: $ hostname nicku.org $ h=hostname $ echo $h hostname Oh dear, we only stored the name of the command, not the output of the command! Command substitution solves the problem: $ h=$(hostname) $ echo $h nicku.org We put $(...) around the command. You can then assign the output of the command.

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

if Statement
Syntax:
if test-commands then statements-if-test-commands-1-true elif test-commands-2 then statements-if-test-commands-2-true else statements-if-all-test-commands-false fi

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution

Example:
if grep nick /etc/passwd > /dev/null 2>&1 then echo Nick has a local account here else echo Nick has no local account here fi

The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

while Statement

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives

Syntax:
while test-commands do loop-body-statements done

The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables

Example:
i=0 while [ "$i" -lt 10 ] do echo -n "$i " # -n suppresses newline. let "i = i + 1" # i=$(expr $i + 1) also works done

Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

for Statement
Syntax: for name in words do loop-body-statements done Example: for planet in Mercury Venus Earth Mars \ Jupiter Saturn Uranus Neptune Pluto do echo $planet done
The backslash “\” quotes the newline. It’s just a way of folding a long line in a shell script over two or more lines.

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

for Loops: Another Example
Here the shell turns *.txt into a list of file names ending in “.txt”:
for i in *.txt do echo $i grep ’lost treasure’ $i done

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

You can leave the in words out; in that case, name is set to each parameter in turn:
i=0 for parameter do let ’i = i + 1’ echo "parameter $i is $parameter" done

Conditions—String Comparisons
All programming languages depend on conditions for if statements and for while loops Shell programming uses a built-in command which is either test or [...] Examples of string comparisons:
[ [ [ [ [

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False

"$USER" = root ] # true if the value of $USER is "root" Shell Variables "$USER" != root ] # true if the value of $USER is not "root" -z "$USER" ] # true if the string "$USER" has zero lengthSpecial Variables string1 \< string2 ] # true if string1 sorts less than string2 Quoting string1 \> string2 ] # true if string1 sorts greater than string2 Command Substitution
The if statement while statement The for statement The test program
Conditions

Note that we need to quote the ‘>’ and the ‘<’ to avoid interpreting them as file redirection. Note: the spaces after the “[“ and before the “]” are essential. Also spaces are essential around operators

Arithmetic Input & Output Alerting about problems by email License Of This Document

Conditions—Integer Comparisons

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives

Examples of numeric integer comparisons:
[ [ [ [ [ [ "$x" "$x" "$x" "$x" "$x" "$x" -eq -ne -lt -gt -le -ge 5 5 5 5 5 5 ] ] ] ] ] ] # # # # # # true true true true true true if if if if if if the value of $x is 5 integer $x is not 5 integer $x is < 5 integer $x is > 5 integer $x is ≤ 5 integer $x is ≥ 5

The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution

Note again that the spaces after the “[“ and before the “]” are essential. Also spaces are essential around operators

The if statement while statement The for statement The test program
Conditions

Arithmetic Input & Output Alerting about problems by email License Of This Document

Conditions—File Tests, NOT Operator
The shell provides many tests of information about files. Do man test to see the complete list. Some examples:
$ $ $ $ $ $ $ $ $ [ [ [ [ [ [ [ [ [ -f file ] # true if file is an ordinary file ! -f file ] # true if file is NOT an ordinary file -d file ] # true if file is a directory -u file ] # true if file has SUID permission -g file ] # true if file has SGID permission -x file ] # true if file exists and is executable -r file ] # true if file exists and is readable -w file ] # true if file exists and is writeable file1 -nt file2 ] # true if file1 is newer than file2

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement

Note again: the spaces after the “[“ and before the “]” are essential. Also spaces are essential around operators

The test program
Conditions

Arithmetic Input & Output Alerting about problems by email License Of This Document

Conditions—Combining Comparisons
Examples of combining comparisons with AND: -a and OR: -o, and grouping with \(...\)
# true if the value of $x is 5 AND $USER is not equal to root: [ "$x" -eq 5 -a "$USER" != root ] # true if the value of $x is 5 OR $USER is not equal to root: [ "$x" -eq 5 -o "$USER" != root ] # true if ( the value of $x is 5 OR $USER is not equal to root ) AND # ( $y > 7 OR $HOME has the value happy ) [ \( "$x" -eq 5 -o "$USER" != root \) -a \ \( "$y" -gt 7 -o "$HOME" = happy \) ]

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program
Conditions

Note again that the spaces after the “[“ and before the “]” are essential. Do man test to see the information about all the operators.

Arithmetic Input & Output Alerting about problems by email License Of This Document

Arithmetic Assignments
Can do with the external program expr
. . . but expr is not so easy to use, although it is very standard and portable: see man expr Easier is to use the built in let command
see help let

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False

Examples: $ $ $ $ $ $ $ $ let let let let let let let let x=1+4 ++x x=’1 + 4’ ’x = 1 + 4’ x="(2 + 3) * 5" "x = 2 + 3 * 5" "x += 5" "x = x + 5" # Now x is 6

Shell Variables Special Variables Quoting

# # # #

now now now now

x x x x

is is is is

25 Command Substitution The if statement 17 while statement 22 27; NOTE NO $ The for statement
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Notice that you do not need to quote the special characters with let. Quote if you want to use white space. Do not put a dollar in front of variable, even on right side of assignment; see last example.

Output with echo

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable

To perform output, use echo, or for more formatting, printf. Use echo -n to print no newline at end. Just echo by itself prints a newline

Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output
Output with echo Input with read

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Input: the read Command
For input, use the built-in shell command read read reads standard input and puts the result into one or more variables If use one variable, variable holds the whole line Syntax: read var1 ... Often used with a while loop like this: while read var1 var2 do # do something with $var1 and $var2 done Loop terminates when reach end of file To prompt and read a value from a user, you could do: while [ -z "$value" ]; do echo -n "Enter a value: " read value done # Now do something with $value

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output
Output with echo Input with read

Alerting about problems by email License Of This

Your Linux system has a large number of shell scripts that you can refer to as examples. I counted about 1400. Here is one way of listing their file names:

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik

$ file /bin/* /usr/bin/* /usr/sbin/* /sbin/* /etc/rc.d/* /usr/X11R6/bin/* Context | grep -i "shell script" | awk -F: ’{print $1}’
Objectives

Let’s see how this works. I suggest executing the commands separately to see what they do:

The shebang: #! Making the script executable

Should you make a $ file /bin/* /usr/bin/* script SUID? $ file /bin/* /usr/bin/* | grep -i "shell script" True and False $ file /bin/* /usr/bin/* | grep -i "shell script" | awk -F: ’{print $1}’ Shell Variables

The awk program is actually a complete programming language. It is mainly useful for selecting columns of data from text. awk automatically loops through the input, and divides the input lines into fields. It calls these fields $1, $2,. . . $NF. $0 contains the whole line. Here the option -F: sets the field separator to the colon character. Normally it is any white space. So printing $1 here prints what comes before the colon, which is the file name. Suppose you want to look for all shell scripts containing a particular command or statement? Looking for example shell scripts that use the mktemp command:

Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output
Output with echo Input with read

Alerting about problems by email License Of This

$ file /bin/* /usr/bin/* /usr/sbin/* /sbin/* /etc/rc.d/* /usr/X11R6/bin/* 1.109.2 Customize or write | grep -i ’shell script’| awk -F: ’{print $1}’ | xargs grep mktemp scripts simple
Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Alerting about problems by email

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False

#! /bin/sh # A quick script whipped up by Nick to send mail if # root file system is more than 90 per cent full. percentful=$(df / | awk ’NR > 1{sub("%", "", $5);print $5}’) if [ "$percentful" -gt 90 ] then message="root file system is $percentful% full" echo "$message" | mail -s $message root fi

Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Topics Covered

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives

Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Quoting and Funny Chars Quoting

Command Substitution The if statement while statement The for statement The test program Conditions Arithmetic Input & Output Output with echo Input with read Alerting about problems by email License Of This Document

The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

License Of This Document

1.109.2 Customize or write simple scripts Weight 3 Nick Urbanik Context Objectives The shebang: #! Making the script executable Should you make a script SUID? True and False Shell Variables Special Variables Quoting Command Substitution The if statement while statement The for statement The test program Arithmetic Input & Output Alerting about problems by email License Of This Document

Copyright c 2005 Nick Urbanik <nicku@nicku.org> You can redistribute modified or unmodified copies of this document provided that this copyright notice and this permission notice are preserved on all copies under the terms of the GNU General Public License as published by the Free Software Foundation — either version 2 of the License or (at your option) any later version.