Bash expression is the combination of operators, features, or values used to form a bash conditional statement. Conditional expression could be binary or unary expression which involves numeric, string or any commands whose return status is zero when success.
There are several conditional expressions that could be used to test with the files. Following are few conditional expressions that are helpful.
- [ -e filepath ] Returns true if file exists.
- [ -x filepath ] Returns true if file exists and executable.
- [ -S filepath ] Returns true if file exists and its a socket file.
- [ expr1 -a expr2 ] Returns true if both the expression is true.
- [ expr1 -o expr2 ] Returns true if either of the expression1 or 2 is true.
For more conditional expression to check the files, strings and numerics please refer the bash man page.
Bash Example 1. Check File Existence
The following Bash shell script code-snippet gets the filename with its absolute path, and checks if the file exists or not and it throws the appropriate information.
$ cat exist.sh #! /bin/bash file=$1 if [ -e $file ] then echo -e "File $file exists" else echo -e "File $file doesnt exists" fi $ ./exist.sh /usr/bin/boot.ini File /usr/bin/boot.ini exists
Refer to our previous article to understand the various bash if statement types.
Bash Example 2. Compare Numbers
The below script reads two integer numbers from user, and checks if both the numbers are equal or greater or lesser than each other.
$ cat numbers.sh #!/bin/bash echo "Please enter first number" read first echo "Please enter second number" read second if [ $first -eq 0 ] && [ $second -eq 0 ] then echo "Num1 and Num2 are zero" elif [ $first -eq $second ] then echo "Both Values are equal" elif [ $first -gt $second ] then echo "$first is greater than $second" else echo "$first is lesser than $second" fi $ ./numbers.sh Please enter first number 1 Please enter second number 1 Both Values are equal $ ./numbers.sh Please enter first number 3 Please enter second number 12 3 is lesser than 12
If you are new to bash scripting, refer to our Bash Introduction tutorial.
Bash Example 3. Basic Arithmetic Calculator
This examples reads input, which is a type of arithmetic operation wants to perform on bash variables (inp1 and inp2). The arithmetic operation could be addition, subtraction or multiplication..
$ cat calculator.sh #!/bin/bash inp1=12 inp2=11 echo "1. Addition" echo "2. Subtraction" echo "3. Multiplication" echo -n "Please choose a word [1,2 or 3]? " read oper if [ $oper -eq 1 ] then echo "Addition Result " $(($inp1 + $inp2)) else if [ $oper -eq 2 ] then echo "Subtraction Result " $(($inp1 - $inp2)) else if [ $oper -eq 3 ] then echo "Multiplication Result " $(($inp1 * $inp2)) else echo "Invalid input" fi fi fi $ ./calculator.sh 1. Addition 2. Subtraction 3. Multiplication Please choose a word [1,2 or 3]? 4 Invalid input
Knowing how to use the bash special parameters ( $*, $@, $#, $$, $!, $?, $-, $_ ) will make your scripting life easy.
Bash Example 4. Read and Ping IP address
The following script is used to read the IP address and check whether the IP address is reachable, and prints the appropriate message.
$ cat ipaddr.sh #!/bin/bash echo "Enter the Ipaddress" read ip if [ ! -z $ip ] then ping -c 1 $ip if [ $? -eq 0 ] ; then echo "Machine is giving ping response" else echo "Machine is not pinging" fi else echo "IP Address is empty" fi $ ./ipaddr.sh Enter the Ipaddress 10.176.191.106 Pinging 10.176.191.106 with 32 bytes of data: Reply from 10.176.191.106: bytes=32 time<1ms TTL=128 Ping statistics for 10.176.191.106: Packets: Sent = 1, Received = 1, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 0ms, Maximum = 0ms, Average = 0ms Machine is giving ping response
In this example, -z returns true if ipaddress is zero length, When the condition is preceded by ! (negate) operator, if expression is false, it enters into if part and executes. So when the IP address is not null, it enters and checks whether the ip address is reachable.
Bash Example 5. Installer Script
Installer script of most of the packages will not allow to execute those as a root user. Script checks the user who is executing and throws the error.
The following script, allows you to execute the oracle installer script only if the user who is executing is non root.
$ cat preinstaller.sh #!/bin/bash if [ `whoami` != 'root' ]; then echo "Executing the installer script" ./home/oracle/databases/runInstaller.sh else echo "Root is not allowed to execute the installer script" fi Executing the script as a root user, # ./preinstaller.sh Root is not allowed to execute the installer script
In this example the output of the command whoami is compared with the word “root”. For string comparison ==, !=, < and should be used and for numeric comparison eq, ne,lt and gt should be used.
Bash Example 6. Enhanced brackets
In all the above examples, we used only single brackets to enclose the conditional expression, but bash allows double brackets which serves as an enhanced version of the single-bracket syntax.
$ cat enhanced.sh #!/bin/bash echo "Enter the string" read str if [[ $str == *condition* ]] then echo "String "$str has the word \"condition\" fi $ ./enhanced.sh Enter the string conditionalstatement String conditionalstatement has the word "condition"
- [ is a synonym for test command. Even if it is built in to the shell it creates a new process.
- [[ is a new improved version of it, which is a keyword, not a program.
- [[ is understood by Korn and Bash.
- In the above example, if the variable $str contains the phrase “condition” anywhere, the condition is true.
- This is the shell globbing feature, which will be supported only when you use [[ (double brackets) and therefore many arguments need not be quoted.
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