This post will help administrator and novice UNIX programmer to understand and write strong as well as useful shell programs. I will talks on shell syntax and few simple day-to-day utilities for UNIX. Will discussing mainly Bourn shell because of its widely acclaimed popularity and usage among the UNIX world.
Many standard utilities (rdist, make, cron, etc.) allow you to specify a command to run at a certain time. Usually, this command is simply passed to the Bourne shell, which means that you can execute whole scripts, should you choose to do so. Steve Bourne, wrote the Bourne shell which appeared in the Seventh Edition Bell Labs Research version of Unix.
Lastly, UNIX runs Bourne shell scripts when it boots. If you want to modify the boot-time behavior of a system, you need to learn to write and modify Bourne shell scripts. It said there are 95% shell code written in Bourne shell. Code written in Bourne shell is compatible with shell scripting like ksh, bash & zsh. That means with minimal or no change one could port shell script written in Bourne shell to ksh or bash or zsh.
*Bash is popular among Linux users.
There are couple of other shells available which I will not be talking are C shell ( Used by C program to create programming rich shell) this is incompatible to Bourne shell. The C shell, csh, and its variant tcsh is a fine interactive shell (I use tcsh), but is a lousy shell for writing scripts.
Lets understand what is an executable files, they also know as program or binary executables, This files are machine readable files and human eyes can not make sense out of it. Others only contain text, knows as script. They are interpreter scripts like awk, sed, perl and many more.
Learning Shell Scripting
#!/bin/sh
The first line of any script must begin with #!, followed by the name of the interpreter. Some versions of UNIX allow whitespace between #! and the name of the interpreter. Others do not. Hence, if you want your script to be portable, leave out the blank.
A script, like any file that can be run as a command, needs to be executable: save this script as rotatelog and run
chmod +x rotatelog
to make it executable.
You can now run it by running
./rotatelog
Unlike some other operating systems, UNIX allows any program to be used as a script interpreter. This is why people talk about “a Bourne shell script” or “an awk script.” One might even write a more script, or an ls script (though the latter wouldn’t be terribly useful). Hence, it is important to let Unix know which program will be interpreting the script.
When Unix tries to execute the script, it sees the first two characters (#!) and knows that it is a script. It then reads the rest of the line to find out which program is to execute the script. For a Bourne shell script, this will be /bin/sh. Hence, the first line of our script must be
#!/bin/sh
After the command interpreter, you can have one, and sometimes more, options. Some flavors of Unix only allow one, though, so don’t assume that you can have more.
Variables
sh allows you to have variables, just like any programming languages. Variables do not need to be declared.
To set a sh variable, use VAR=value
And to use the value of the variable later, use $VAR or ${VAR}
The latter syntax is useful if the variable name immediately followed by other text:
#!/bin/sh
COLOR=yellow
echo This looks $COLORish
echo This seems ${COLOR}ish
prints
This looks
This seems yellowish
There is only one type of variable in sh: strings.
This is somewhat limited, but is sufficient for most purposes.
Local vs. environment variables
A sh variable can be either a local variable or an environment variable. They both work the same way; the only difference lies in what happens when the script runs another program (which, as we saw earlier, it does all the time).
Environment variables are passed to subprocesses. Local variables are not.
By default, variables are local. To turn a local variable into an environment variable,
use
export VAR
Here’s a simple wrapper for a program:
#!/bin/sh
NETSCAPE_HOME=/usr/imports/libdata
CLASSPATH=$NETSCAPE_HOME/classes
export CLASSPATH $NETSCAPE_HOME/bin/netscape.bin
Here, NETSCAPE_HOME is a local variable; CLASSPATH is an environment variable. CLASSPATH will be passed to netscape.bin (netscape.bin uses the value of this variable to find Java class files); NETSCAPE_HOME is a convenience variable that is only used by the wrapper script; netscape.bin doesn’t need to know about it, so it is kept local.
The only way to unexport a variable is to unset it:
unset VAR
This removes the variable from the shell’s symbol table, effectively making as if it had never existed; as a side effect, the variable is also unexported.
Also, if you have a function by the same name as the variable, unset will also delete that function. Since you may want to use this variable later, it is better not to define it in the first place.
Also, note that if a variable was passed in as part of the environment, it is already an environment variable when your script starts running. If there is a variable that you really don’t want to pass to any subprocesses, you should unset it near the top of your script. This is rare, but it might conceivably happen.
If you refer to a variable that hasn’t been defined, sh substitutes the empty string.
#!/bin/sh
echo aaa $FOO bbb
echo xxx${FOO}yyy
prints
aaa bbb xxxyyy
Special variables
sh treats certain variables specially: some are set for you when your script runs, and some affect the way commands are interpreted.
Command-line arguments
The most useful of these variables are the ones referring to the command-line arguments. $1 refers to the first command-line argument (after the name of the script), $2 refers to the second one, and so forth, up to $9.
If you have more than nine command-line arguments, you can use the shift command: this discards the first command-line argument, and bumps the remaining ones up by one position: $2 becomes $1, $8 becomes $7, and so forth.
The variable $0 (zero) contains the name of the script (argv[0] in C programs).
Often, it is useful to just list all of the command-line arguments.
For this, sh provides the variables $* (star) and $@ (at). Each of these expands to a string containing all of the command-line arguments, as if you had used $1 $2 $3…
The difference between $* and $@ lies in the way they behave when they occur inside double quotes: $* behaves in the normal way, whereas $@ creates a separate double-quoted string for each command-line argument.
That is, “$*” behaves as if you had written “$1 $2 $3″, whereas “$@” behaves as if you had written “$1″ “$2″ “$3″.
Finally, $# contains the number of command-line arguments that were given.
Other special variables
$? gives the exit status of the last command that was executed. This should be zero if the command exited normally.
$- lists all of the options with which sh was invoked. See sh(1) for details.
$$ holds the PID of the current process.
$! holds the PID of the last command that was executed in the background.
$IFS (Input Field Separator) determines how sh splits strings into words.
Quasi-variable constructs
The ${VAR} construct is actually a special case of a more general class of constructs:
${VAR:-expression}
Use default value: if VAR is set and non-null, expands to $VAR. Otherwise, expands to expression.
${VAR:=expression}
Set default value: if VAR is set and non-null, expands to $VAR. Otherwise, sets VAR to expression and expands to expression.
${VAR:?[expression]}
If VAR is set and non-null, expands to $VAR. Otherwise, prints expression to standard error and exits with a non-zero exit status.
${VAR:+expression}
If VAR is set and non-null, expands to the empty string. Otherwise, expands to expression.
${#VAR}
Expands to the length of $VAR.
The above patterns test whether VAR is set and non-null. Without the colon, they only test whether VAR is set.
Pattern-matching
sh supports a limited form of pattern-matching. The operators are
* Matches zero or more characters.
? Matches exactly one character.
[range] Matches any character in range.
Range can be either a list of characters that match, or two endpoints separated by a dash: [ak3] matches either a, k, or 3; [a-z] matches any character in the range a through z; [a-mz] matches either a character in the range a through m, or z. If you wish to include a dash as part of the range, it must be the first character, e.g., [-p] will match either a dash or p.
When an expression containing these characters occurs in the middle of a command, sh substitutes the list of all files whose name matches the pattern. This is known as “globbing.” Otherwise, these are used mainly in the case construct.
As a special case, when a glob begins with * or ?, it does not match files that begin with a dot. To match these, you need to specify the dot explicitly (e.g., .*, /tmp/.*).
Note to MS-DOS users: under MS-DOS, the pattern *.* matches every file. In sh, it matches every file that contains a dot.
Quoting
If you say something like
echo * MAKE $$$ FAST *
it won’t do what you want: first of all, sh will expand the *s and replace them with a list of all the files in the current directory. Then, since any number of tabs or blanks can separate words, it will compress the three spaces into one. Finally, it will replace the first instance of $$ with the PID of the shell. This is where quoting comes in.
sh supports several types of quotes. Which one you use depends on what you want to do.
Backslash
Just as in C strings, a backslash (“\”) removes any special meaning from the character that follows. If the character after the backslash isn’t special to begin with, the backslash has no effect. The backslash is itself special, so to escape it, just double it: \\.
Single quotes
Single quotes, such as ‘foo’ work pretty much the way you’d expect: anything inside them (except a single quote) is quoted.
You can say
echo ‘* MAKE $$$ FAST *’
and it’ll come out the way you want it to.
Note that a backslash inside single quotes also loses its special meaning, so you don’t need to double it. There is no way to have a single quote inside single quotes.
Double quotes, such as
“foo”
preserve spaces and most special characters. However, variables and backquoted expressions are expanded and replaced with their value.
If you have an expression within backquotes (also known as backticks), e.g.,
`cmd`
the expression is evaluated as a command, and replaced with whatever the expression prints to its standard output. Thus,
echo You are `whoami`
prints
You are irfan
Built-in commands
sh understands several built-in commands, i.e., commands that do not correspond to any program. These commands include:
{ commands ; }, ( commands )
Execute commands in a subshell. That is, run them as if they were a single command. This is useful when I/O redirection is involved, since you can pipe data to or from a mini-script inside a pipeline.
The { commands; } variant is somewhat more efficient, since it doesn’t spawn a true subshell. This also means that if you set variables inside of it, the changes will be visible in the rest of the script.
: (colon)
Does nothing. This is generally seen as
: ${VAR:=default}
. filename
The dot command reads in the specified filename, as if it had occurred at that place in the script.
bg [job], fg [job]
bg runs the specified job (or the current job, if none is specified) in the background. fg resumes the specified job (or the current job, if none is specified) in the foreground.
Jobs are specified as %number. The jobs command lists jobs.
cd [dir]
Sets the current directory to dir. If dir is not specified, sets the current directory to the home directory.
pwd
Prints the current directory.
echo args
Prints args to standard output.
eval args
Evaluates args as a sh expression. This allows you to construct a string on the fly (e.g., using a variable that holds the name of a variable that you want to set) and execute it.
exec command
Runs the specified command, and replaces the current shell with it. That is, nothing after the exec statement will be executed, unless the exec itself fails.
exit [n]
Exit the current shell with exit code n. This defaults to zero.
kill [-sig] %job
Send signal sig to the specified job. sig can be either numeric or symbolic. kill -l lists all available signals. By default, sig is SIGTERM (15).
read name…
Reads one line from standard input and assigns it to the variable name. If several variables name1, name2, name3 etc. are specified, then the first word of the line read is assigned to name1, the second to name2, and so forth. Any remaining words are assigned to the last variable.
set [+/-flag] [arg]
With no arguments, prints the values of all variables.set -x turns on the x option to sh; set +x turns it off.set args… sets the command-line arguments to args.
test expression
Evaluates a boolean expression and exits with an exit code of zero if it is true, or non-zero if it is false. See test for more details.
trap [command sig]…
If signal sig is sent to the shell, execute command. This is useful for exiting cleanly (e.g., removing temporary files etc.) when the script is interrupted.
ulimit
Print or set system limits on resource usage.
umask [nnn]
Sets the umask to nnn (an octal number). With no argument, prints the current umask. This is most useful when you want to create files, but want to restrict who can read or write them.
wait [n]
Wait for the background process whose PID is n to terminate. With no arguments, waits for all of the background processes to terminate.
Bear in mind that the list of builtins varies from one implementation to another, so don’t take this list as authoritative.
Flow control
sh supports several flow-control constructs, which add power and flexibility to your scripts.
if
The if statement is a simple conditional. You’ve seen it in every programming language. Its syntax is
if condition ; then
commands
[elif condition ; then
commands]…
[else
commands]
fi
That is, an if-block, optionally followed by one or more elif-blocks (elif is short for “else if”), optionally followed by an else-block, and terminated by fi.
The if statement pretty much does what you’d expect: if condition is true, it executes the if-block. Otherwise, it executes the else-block, if there is one. The elif construct is just syntactic sugar, to let you avoid nesting multiple if statements.
#!/bin/sh
myname=`whoami`
if [ $myname = root ]; then
echo “Welcome to FooSoft 3.0″
else
echo “You must be root to run this script”
exit 1
fi
The more observant among you (or those who are math majors) are thinking, “Hey! You forgot to include the square brackets in the syntax definition!”
Actually, I didn’t: [ is actually a command, /bin/[, and is another name for the test command. See below for details.
This is why you shouldn't call a test program test: if you have ``.'' at the end of your path, as you should, executing test will run /bin/test.
The condition can actually be any command. If it returns a zero exit status, the condition is true; otherwise, it is false. Thus, you can write things like
#!/bin/sh
user=arnie
if grep $user /etc/passwd; then
echo "$user has an account"
else
echo "$user doesn't have an account"
fi
while
The while statement should also be familiar to you from any number of other programming languages. Its syntax in sh is
while condition; do
commands
done
As you might expect, the while loop executes commands as long as condition is true. Again, condition can be any command, and is true if the command exits with a zero exit status.
A while loop may contain two special commands: break and continue.
break exits the while loop immediately, jumping to the next statement after the done.
continue skips the rest of the body of the loop, and jumps back to the top, to where condition is evaluated.
for
The for loop iterates over all of the elements in a list. Its syntax is
for var in list; do
commands
done
list is zero or more words. The for construct will assign the variable var to each word in turn, then execute commands. For example:
#!/bin/sh
for i in foo bar baz "do be do"; do
echo "$i"
done
will print
foo bar baz do be do
A for loop may also contain break and continue statements. They work the same way as in the while loop.
Case
The case construct works like C's switch statement, except that it matches patterns instead of numerical values. Its syntax is
case expression in
pattern)
commands
;;
...
esac
expression is a string; this is generally either a variable or a backquoted command.
pattern is a glob pattern (see globbing).
The patterns are evaluated in the order in which they are seen, and only the first pattern that matches will be executed. Often, you'll want to include a ``none of the above'' clause; to do this, use * as your last pattern.
IO
A command's input and/or output may be redirected to another command or to a file. By default, every process has three file descriptors: standard input (0), standard output (1) and standard error (2). By default, each of these is connected to the user's terminal.
However, one can do many interesting things by redirecting one or more file descriptor:
< filename
Connect standard input to the file filename. This allows you to have a command read from the file, rather than having to type its input in by hand.
> filename
Connect standard output to the file filename. This allows you to save the output of a command to a file. If the file does not exist, it is created. If it does exist, it is emptied before anything happens.
(Exercise: why doesn't cat * > zzzzzzz work the way you'd expect?)
>> filename
Connects standard output to the file filename. Unlike >, however, the output of the command is appended to filename.
<<word
This construct isn't used nearly as often as it could be. It causes the command's standard input to come from... standard input, but only until word appears on a line by itself. Note that there is no space between << and word.
This can be used as a mini-file within a script, e.g.,
cat > foo.c <<EOT
#include <stdio.h>
main() {
printf("Hello, world!\n");
} EOT
It is also useful for printing multiline messages, e.g.:
line=13 cat <<EOT An error occurred on line $line. See page 98 of the manual for details. EOT
As this example shows, by default, << acts like double quotes (i.e., variables are expanded). If, however, word is quoted, then << acts like single quotes.
<&digit
Use file descriptor digit as standard input.
>&digit
Use file descriptor digit as standard output.
<&-
Close standard input.
>&-
Close standard output.
command1 | command2
Creates a pipeline: the standard output of command1 is connected to the standard input of command2. This is functionally identical to
command1 > /tmp/foo
command2 < /tmp/foo
except that no temporary file is created, and both commands can run at the same time
There is a proverb that says, ``A temporary file is just a pipe with an attitude and a will to live.'' Any number of commands can be pipelined together.
command1 && command2
Execute command1. Then, if it exited with a zero (true) exit status, execute command2.
command1 || command2
Execute command1. Then, if it exited with a non-zero (false) exit status, execute command2.
If any of the redirection constructs is preceded by a digit, then it applies to the file descriptor with that number, rather than the default (0 or 1, as the case may be). For instance,
command 2>&1 > filename
associates file descriptor 2 (standard error) with the same file as file descriptor 1 (standard output), then redirects both of them to filename.
This is also useful for printing error messages:
echo "Danger! Danger Will Robinson!" 1>&2
Note that I/O redirections are parsed in the order they are encountered, from left to right. This allows you to do fairly tricky things, including throwing out standard output, and piping standard output to a command.
Functions
When a group of commands occurs several times in a script, it is useful to define a function. Defining a function is a lot like creating a mini-script within a script.
A function is defined using
name () {
commands
}
and is invoked like any other command:
name args...
You can redirect a function's I/O, embed it in backquotes, etc., just like any other command.
One way in which functions differ from external scripts is that the shell does not spawn a subshell to execute them. This means that if you set a variable inside a function, the new value will be visible outside of the function.
A function can use return n to terminate with an exit status of n. Obviously, it can also exit n, but that would terminate the entire script.
Function arguments
A function can take command-line arguments, just like any script. Intuitively enough, these are available through $1, $2... $9 just like the main script.
Useful utilities
There are a number of commands that aren't part of sh, but are often used inside sh scripts. These include:
basename
basename pathname prints the last component of pathname:
basename /foo/bar/baz
prints
baz
dirname
The complement of basename: dirname pathname prints all but the last component of pathname, that is the directory part: pathname:
dirname /foo/bar/baz
prints
/foo/bar
/bin/[ is another name for /bin/test. It evaluates its arguments as a boolean expression, and exits with an exit code of 0 if it is true, or 1 if it is false.
If test is invoked as [, then it requires a closing bracket ] as its last argument. Otherwise, there must be no closing bracket.
test understands the following expressions, among others:
-e filename
True if filename exists.
-d filename
True if filename exists and is a directory.
-f filename
True if filename exists and is a plain file.
-h filename
True if filename exists and is a symbolic link.
-r filename
True if filename exists and is readable.
-w filename
True if filename exists and is writable.
-n string
True if the length of string is non-zero.
-z string
True if the length of string is zero.
string
True if string is not the empty string.
s1 = s2
True if the strings s1 and s2 are identical.
s1 != s2
True if the strings s1 and s2 are not identical.
n1 -eq n2
True if the numbers n1 and n2 are equal.
n1 -ne n2
True if the numbers n1 and n2 are not equal.
n1 -gt n2
True if the number n1 is greater than n2.
n1 -ge n2
True if the number n1 is greater than or equal to n2.
n1 -lt n2
True if the number n1 is less than n2.
n1 -le n2
True if the number n1 is less than or equal to n2.
! expression
Negates expression, that is, returns true iff expression is false.
expr1 -a expr2
True if both expressions, expr1 and expr2 are true.
expr1 -o expr2
True if either expression, expr1 or expr2 is true.
( expression )
True if expression is true. This allows one to nest expressions.
Note that lazy evaluation does not apply, since all of the arguments to test are evaluated by sh before being passed to test. If you stand to benefit from lazy evaluation, use nested ifs.
echo
echo is a built-in in most implementations of sh, but it also exists as a standalone command.
echo simply prints its arguments to standard output. It can also be told not to append a newline at the end: under BSD-like flavors of Unix, use
echo -n “string”
Under SystemV-ish flavors of Unix, use
echo “string\c”
awk
Awk (and its derivatives, nawk and gawk) is a full-fledged scripting language. Inside sh scripts, it is generally used for its ability to split input lines into fields and print one or more fields. For instance, the following reads /etc/passwd and prints out the name and uid of each user:
awk -F : ‘{print $1, $3 }’ /etc/passwd
The -F : option says that the input records are separated by colons. By default, awk uses whitespace as the field separator.
sed
Sed (stream editor) is also a full-fledged scripting language, albeit a less powerful and more convoluted one than awk. In sh scripts, sed is mainly used to do string substitution: the following script reads standard input, replaces all instances of “foo” with “bar”, and writes the result to standard output:
sed -e ’s/foo/bar/g’
The trailing g says to replace all instances of “foo” with “bar” on a line. Without it, only the first instance would be replaced.
tee
tee [-a] filename reads standard input, copies it to standard output, and saves a copy in the file filename.
By default, tee empties filename before it begins. With the -a option, it appends to filename.
Debugging
Unfortunately, there are no symbolic debuggers such as gdb for sh scripts. When you’re debugging a script, you’ll have to rely the tried and true method of inserting trace statements, and using some useful options to sh:
The -n option causes sh to read the script but not execute any commands. This is useful for checking syntax.
The -x option causes sh to print each command to standard error before executing it. Since this can generate a lot of output, you may want to turn tracing on just before the section that you want to trace, and turn it off immediately afterward:
set -x # XXX – What’s wrong with this code? grep $user /etc/passwd 1>&2 > /dev/null set +x
I ma LILO fan so all below configuration and are Linux based. But based on this understanding one can quickly start writing on any other OS too.
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