In this chapter we shall know about some special operators, which makes shell script powerful while writing and executing the scripts.
4.1. Redirectors
4.2. Pipes
4.3. exit status
4.4. Shell Special Variables
4.5. eval command
4.6. sleep command
4.7. trap command
4.1. Redirectors
Usually we give input to the script from standard input [STDIN] and get the output in standard output [STDOUT] i.e display. And write the error in standard error[STDERR].
By default, keyboard is the standard input, display is the standard output and standard error.
But with the help of redirection operator “>” it is possible to redirect the output to a file or any other location.
Linux assigns a specific number called as file descriptor number as shown below:
0 for standard input
1 for standard output
2 for standard error
4.1.1 Some of the example below:
> Redirect to standard output
2> Redirect to standard error
2>&1 Redirect standard error to standard output
< Redirect to standard input
| Pipe, used to give output of one command to input to another.
>> Append to standard output
Example 1: The “>” operator
ls > ls_list.txt
The above command will redirect the output to “ls_list.txt”
Example 2: The “2>” operator
cdls 2> err.out
The above command will redirect the STDERR to “err.out”
Example 3: The “2>&1” operator
ls 2>&1 > hello.out
The above command will redirect the STDERR and STDOUT to “hello.out”
4.2 Pipes
Pipes are used to give the output of one command to the input to another command.
Example:
ls | wc -l
Output:
24
Here ls will give its output to “wc -l”. “wc” is word count. Hence it will count the number of files present in the folder.
4.3 exit status
Any shell program after execution, it will return it’s status to the system. With the help of return status, we can know if the script is executed successfully or not. Exit status of “0” will be returned upon successful completion. a non zero status will be returned if it fails. “$?” is used to get the exit status of the last command executed.
There are some pre determined reserved meanings, programmers are recommended not to use those values in their script.
1 – general errors
2 – Misuse of shell built-ins. Usually when there is a permission problem.
126 – Command invoked cannot execute. Usually when there is a permission problem.
127 – “command not found”. Possible a typo
128 – Invalid argument to exit. Example when you return “exit 34.54”. Error because exit takes only integer args in the range 0 – 255
128+n – Fatal error signal “n”. Example kill -9 $PPID. $? returns 137 (128 + 9).
130 – Script terminated by Control-C
255\* – Exit status out of range
4.4. Shell Special Variables
In the previous chapters we have used special operators like “$0” “$1” “$?”, which did not make much sense then. In this section we shall understand in detail about these variables.
$0 It holds the name of the script currently executing
$n “n” is a place holder for integer variable. If the input has 4 arguments, then all the 4 argument can be accessed by: $1 for first argument, $2 for second argument, $3 for argument etc.
$# It outputs the number of arguments sent to the script.
$? Gives the exit status of the last command executed.
$$ Returns the process number of the current shell
$! Process number of the last background running process.
There are 2 other special variables that are tricky to understand. They are:
$*
$@
Their behaviour changes when they are used with and without quotes.
1. With quotes:
“$*”
“$@”
and call it with:
./myScript.sh “a a” “b b” “c c”
it’s equivalent to:
“$*” will return “a a b b c c”
“$@” will return “a a” “b b” “c c”
2. When used without quotes, they’re the same:
$*
$@
would be equivalent to:
$* “a” “a” “b” “b” “c” “c”
$@ “a” “a” “b” “b” “c” “c”
Example:
#!/bin/bash
echo “The script name is $0”
echo “The first argument is $1”
echo “The second argument is $2”
echo “The third argument is $3”
echo “Total number of variables entered $#”
echo “All the entered variables $*”
echo “All the entered variables $@”
echo “The process number of the current shell is $$”
Output:
sh 20_example.sh 123 345
The script name is 20_example.sh
The first argument is 123
The second argument is 345
The third argument is
Total number of variables entered 2
All the entered variables 123 345
All the entered variables 123 345
The process number of the current shell is 10774
4.5. eval command
eval command is used to execute arguments as a shell command.
For example:
a=5
b=’$a’
eval echo $b
Output:
5
In the above example, we expect the output to be “5” but instead it returns “$a”. Because the shell will take it as a string variable. To get the correct value use “eval” command.
a=5
b=’$a’
echo $b
Output:
$a
4.6. sleep command
sleep command is used to pause the execution for the given number of seconds.
4.7. trap command
trap command is used to catch a signal during the script execution.
Example:
Create a file 21_trap.sh
#!/bin/bash
#set a trap for exit with 0, when the script will exit with 0, this will be executed
trap ‘echo “Exit 0 signal detected…”‘ 0
echo “Sample text”
exit 0
Output:
Sample text
Exit 0 signal detected…