A simple shell script
A shell script is little more than a list of commands that are run in sequence.
Conventionally, a shellscript should start with a line such as the following:
#!/bin/bash
THis indicates that the script should be run in the bash shell regardless
of which interactive shell the user has chosen. This is very important,
since the syntax of different shells can vary greatly.
A simple example
Here's a very simple example of a shell script. It just runs a few simple commands
#!/bin/bash
echo "hello, $USER. I wish to list some files of yours"
echo "listing files in the current directory, $PWD"
ls # list files
Firstly, notice the comment on line 4. In a bash script, anything following
a pound sign # (besides the shell name on the first line) is treated as a
comment. ie the shell ignores it. It is there for the benifit of people
reading the script.
$USER and $PWD are
variables. These are standard variables defined
by the bash shell itself, they needn't be defined in the script.
Note that the variables are
expanded when the variable name is inside
double quotes. Expanded is a very appropriate word: the shell basically sees
the string $USER and replaces it with the variable's value then executes the
command.
We continue the discussion on variables below ...
Variables
Any programming language needs variables. You define a variable as follows:
X="hello"
and refer to it as follows:
$X
More specifically, $X is used to denote the value of the variable X.
Some things to take note of regarding semantics:
- bash gets unhappy if you leave a space on either side of the = sign. For
example, the following gives an error message:
X = hello
- while I have quotes in my example, they are not always necessary.
where you need quotes is when your variable names include spaces. For example,
X=hello world # error
X="hello world" # OK
This is because the shell essentially sees the command line as a pile of
commands and command arguments seperated by spaces.
foo=baris
considered a command. The problem with
foo = bar is the shell
sees the word
foo seperated by spaces and interprets it as a
command. Likewise, the problem with the command
X=hello world is
that the shell interprets
X=hello as a command, and the word
"world" does not make any sense (since the assignment command doesn't take
arguments).
Single Quotes versus double quotes
Basically, variable names are exapnded within double quotes, but not single
quotes. If you do not need to refer to variables, single quotes are good
to use as the results are more predictable.
An example
#!/bin/bash
echo -n '$USER=' # -n option stops echo from breaking the line
echo "$USER"
echo "\$USER=$USER" # this does the same thing as the first two lines
The output looks like this (assuming your username is elflord)
$USER=elflord
$USER=elflord
so the double quotes still have a work around. Double quotes are more flexible,
but less predictable. Given the choice between single quotes and double
quotes, use single quotes.
Using Quotes to enclose your variables
Sometimes, it is a good idea to protect variable names in double quotes.
This is usually the most important if your variables value either
(a) contains spaces or (b) is the empty string.
An example is as follows:
#!/bin/bash
X=""
if [ -n $X ]; then # -n tests to see if the argument is non empty
echo "the variable X is not the empty string"
fi
This script will give the following output:
the variable X is not the empty string
Why ? because the shell expands $X to the empty string. The expression
[ -n ] returns true (since it is not provided with an argument).
A better script would have been:
#!/bin/bash
X=""
if [ -n "$X" ]; then # -n tests to see if the argument is non empty
echo "the variable X is not the empty string"
fi
In this example, the expression expands to [ -n "" ] which returns false,
since the string enclosed in inverted commas is clearly empty.
Variable Expansion in action
Just to convince you that the shell really does "expand" variables
in the sense I mentioned before, here is an example:
#!/bin/bash
LS="ls"
LS_FLAGS="-al"
$LS $LS_FLAGS $HOME
This looks a little enigmatic. What happens with the last line
is that it actually executes the command
ls -al /home/elflord
(assuming that /home/elflord is your home directory).
That is, the shell simply replaces the variables with their values,
and then executes the command.
Using Braces to Protect Your Variables
OK. Here's a potential problem situation. Suppose you want to
echo the value of the variable X, followed immediately by
the letters "abc". Question: how do you do this ?
Let's have a try :
#!/bin/bash
X=ABC
echo "$Xabc"
THis gives no output. What went wrong ?
The answer is that the shell thought that we were asking for the
variable Xabc, which is uninitialised.
The way to deal with this is to put braces around X to seperate it from
the other characters. The following gives the desired result:
#!/bin/bash
X=ABC
echo "${X}abc"
Conditionals, if/then/elif
Sometimes, it's necessary to check for certain conditions.
Does a string have 0 length ? does the file "foo" exist, and is it a symbolic
link , or a real file ? Firstly, we use the if command to run a test.
The syntax is as follows:
if condition
then
statement1
statement2
..........
fi
Sometimes, you may wish to specify an alternate action when the
condition fails. Here's how it's done.
if condition
then
statement1
statement2
..........
else
statement3
fi
alternatively, it is possible to test for another condition
if the first "if" fails. Note that any number of elifs can be added.
if condition1
then
statement1
statement2
..........
elif condition2
then
statement3
statement4
........
elif condition3
then
statement5
statement6
........
fi
The statements inside the block between
if/elif and the next
elif or
fi are executed if the corresponding condition
is true. Actually, any command can go in place of the conditions, and the
block will be executed if and only if the command returns an exit status of 0
(in other words, if the command exits "succesfully" ).
However, in the course of this document, we will be only interested in
using "test" or "[ ]" to evaluate conditions.
The Test Command and Operators
The command used in conditionals nearly all the time is the test command.
Test returns true or false (more accurately, exits with 0 or non zero status)
depending respectively on whether the test is passed or failed.
It works like this:
test operand1 operator operand2
for some tests, there need be only one operand (operand2)
The test command is typically abbreviated in this form:
[ operand1 operator operand2 ]
To bring this discussion back down to earth, we give a few examples:
#!/bin/bash
X=3
Y=4
empty_string=""
if [ $X -lt $Y ] # is $X less than $Y ?
then
echo "\$X=${X}, which is smaller than \$Y=${Y}"
fi
if [ -n "$empty_string" ]; then
echo "empty string is non_empty"
fi
if [ -e "${HOME}/.fvwmrc" ]; then # test to see if ~/.fvwmrc exists
echo "you have a .fvwmrc file"
if [ -L "${HOME}/.fvwmrc" ]; then # is it a symlink ?
echo "it's a symbolic link
elif [ -f "${HOME}/.fvwmrc" ]; then # is it a regular file ?
echo "it's a regular file"
fi
else
echo "you have no .fvwmrc file"
fi
Some pitfalls to be wary of
The test command needs to be in the form "operand1<space>operator<space>operand2" or
operator<space>operand2 , in other words you really
need
these spaces, since the shell considers the first block containing no spaces
to be either an operator (if it begins with a '-') or an operand (if it doesn't). So for example; this
if [ 1=2 ]; then
echo "hello"
fi
gives exactly the "wrong" output (ie it echos "hello", since it sees an operand
but no operator.)
Another potential trap comes from not protecting variables in quotes.
We have already given an example as to why you
must wrap
anything you wish to use for a
-n test with quotes.
However, there are a lot of good reasons for using quotes all the time,
or almost all of the time.
Failing to do this when you have variables expanded inside tests can
result in
very wierd bugs. Here's an example:
For example,
#!/bin/bash
X="-n"
Y=""
if [ $X = $Y ] ; then
echo "X=Y"
fi
This will give misleading output since the shell expands our expression
to
[ -n = ]
and the string "=" has non zero length.
A brief summary of test operators
Here's a quick list of test operators. It's by no means comprehensive,
but its likely to be all you'll need to remember (if you need anything else,
you can always check the bash manpage ... )
| operator | produces true if... | number of operands |
| -n | operand non zero length | 1 |
| -z | operand has zero length | 1 |
| -d | there exists a directory whose name is operand | 1 |
| -f | there exists a file whose name is operand | 1 |
| -eq | the operands are integers and they are equal | 2 |
| -neq | the opposite of -eq | 2 |
| = | the operands are equal (as strings) | 2 |
| != | opposite of = | 2 |
| -lt | operand1 is strictly less than operand2 (both operands should be integers) | 2 |
| -gt | operand1 is strictly greater than operand2 (both operands should be integers) | 2 |
| -ge | operand1 is greater than or equal to operand2 (both operands should be integers) | 2 |
| -le | operand1 is less than or equal to operand2 (both operands should be integers) | 2 |
Loops
Loops are constructions that enable one to reiterate a procedure
or perform the same procedure on several different items.
There are the following kinds of loops available in bash
For loops
The syntax for the for loops is best demonstrated by example.
#!/bin/bash
for X in red green blue
do
echo $X
done
THe for loop iterates the loop over the space seperated items.
Note that if some of the items have embedded spaces, you need to
protect them with quotes. Here's an example:
#!/bin/bash
colour1="red"
colour2="light blue"
colour3="dark green"
for X in "$colour1" $colour2" $colour3"
do
echo $X
done
Can you guess what would happen if we left out the quotes in
the for statement ? This indicates that variable names should be
protected with quotes unless you are pretty sure that they do not
contain any spaces.
Globbing in for loops
The shell expands a string containing a *
to all filenames that "match". A filename matches
if and only if it is identical to the match string after replacing the
stars * with arbitrary strings.
For example, the character "*" by itself expands to a space seperated
list of all files in
the working directory (excluding those that start with a dot "." )
So
echo *
lists all the files and directories in the current directory.
echo *.jpg
lists all the jpeg files.
echo ${HOME}/public_html/*.jpg
lists all jpeg files in your public_html directory.
As it happens, this turns out to be very useful for performing
operations on the files in a directory, especially used in
conjunction with a for loop. For example:
#!/bin/bash
for X in *.html
do
grep -L '<UL>' "$X"
done
While Loops
While loops iterate "while" a given condition is true.
An example of this:
#!/bin/bash
X=0
while [ $X -le 20 ]
do
echo $X
X=$((X+1))
done
This raises a natural question:
why doesn't bash allow the C like for loops
for (X=1,X<10; X++)
As it happens, this is discouraged for a reason: bash is an interpreted
language, and a rather slow one for that matter.
For this reason, heavy iteration is discouraged.
Command Substitution
Command Substitution is a very handy feature of the bash shell.
It enables you to take the output of a command and treat it
as though it was written on the command line.
For example, if you want to set the variable X to the output of a command,
the way you do this is via command substitution.
There are two means of command substitution: brace expansion
and backtick expansion.
Brace expansion workls as follows:
$(commands)
expands to the output of
commands
This permits nesting, so
commands can include brace expansions
Backtick expansion expands
`commands` to the output of
commands
An example is given;:
#!/bin/bash
files="$(ls)"
web_files=`ls public_html`
echo "$files" # we need the quotes to preserve embedded newlines in $files
echo "$web_files" # we need the quotes to preserve newlines
X=`expr 3 \* 2 + 4` # expr evaluate arithmatic expressions. man expr for details.
echo "$X"
The advantage of the $() substitution method is almost
self evident: it is very easy to nest. It is supported by most of
the bourne shell varients (the POSIX shell or better is OK).
However, the backtick substitution is slightly more readable,
and is supported by even the most basic shells (any #!/bin/sh version
is just fine)
Note that if strings are not quote-protected in the above echo statement, new
lines are replaced by spaces in the output.
