How to read in bash script? (2023)

To read the Bash user input, we use the built-in Bash command called read. It takes input from the user and assigns it to the variable. It reads only a single line from the Bash shell.

In Bash, you can check if a command succeeded or failed by examining the exit status of the command. The exit status of a command is a numerical value that indicates the success or failure of the command. A command with an exit status of 0 indicates success, and a command with a non-zero exit status indicates failure.

You can pass more than one argument to your bash script. In general, here is the syntax of passing multiple arguments to any bash script: script.sh arg1 arg2 arg3 … The second argument will be referenced by the $2 variable, the third argument is referenced by $3 , .. etc.

$0 – The name of the script. $1 – The first argument sent to the script. $2 – The second argument sent to the script.

$$ is used to reference process id of any command or bash script. $0. $0 is used to get the name of the command in a bash script. $name. $name will print the value of variable “name” defined in the script.

Using a Backslash. The backslash (\) is an escape character that instructs the shell not to interpret the next character. If the next character is a newline, the shell will read the statement as not having reached its end. This allows a statement to span multiple lines.

The readLine() method of BufferedReader class reads file line by line, and each line appended to StringBuffer, followed by a linefeed. The content of the StringBuffer are then output to the console.

Bash read Syntax

The read command takes the user input and splits the string into fields, assigning each new word to an argument. If there are fewer variables than words, read stores the remaining terms into the final variable. Specifying the argument names is optional.

Overall, Bash is relatively easy to learn. Most students can learn basic, intermediate, and advanced commands within six months.

The special character $# stores the total number of arguments. We also have $@ and $* as wildcard characters which are used to denote all the arguments. We use $$ to find the process ID of the current shell script, while $? can be used to print the exit code for our script.

Instead, we use the special variable $@ , which means, ‘All of the command-line arguments to the shell script‘. We also should put $@ inside double-quotes to handle the case of arguments containing spaces ( “$@” is special syntax and is equivalent to “$1” “$2” …).

$1 means an input argument and -z means non-defined or empty. You’re testing whether an input argument to the script was defined when running the script.

In a bash script or function, $1 denotes the initial argument passed, $2 denotes the second argument passed, and so forth. This script takes a name as a command-line argument and prints a personalized greeting.

The $! variable is a special shell variable that stores the PID of the most recently executed background process. A background process, also known as a background job, allows us to continue using the command line interface for other tasks. There are different ways to start a background job in Linux.

The start-up time of a bash shell script is 2.8 mili seconds, while that of PPython is 11.1 mili seconds. Bash is a general-purpose language like Python, but both have strengths and weaknesses. Bash shell programming is the default terminal in most Linux distributions; thus, it will always perform faster.

1. Overview. While comparing variables in Bash, we generally use single brackets ([ ]) and double brackets ([[ ]]) interchangeably. For example, we can use the expressions [ 3 -eq 3 ] or [[ 3 -eq 3 ]] while checking whether 3 is equal to 3. Both will be successful in performing the comparison.

The $0 is one of the special variables you get in bash and is used to print the filename of the script that is currently being executed. The $0 variable can be used in two ways in Linux: Use $0 to find the logged-in shell. Use $0 to print the name of the script that is being executed.

We use the wc command to count the lines in a file. Adding the −l flag gives us the total line count and the filename. We can use the wc −l (word count) command to see how many lines there are in our file.

Description. The read command reads one line from standard input and assigns the values of each field in the input line to a shell variable using the characters in the IFS (Internal Field Separator) variable as separators.

If you want to break up a command so that it fits on more than one line, use a backslash (\) as the last character on the line. Bash will print the continuation prompt, usually a >, to indicate that this is a continuation of the previous line. $ printf “%s\n” “This is a very long printf. How long is it?\

In bash, a string can also be divided without using $IFS variable. The ‘readarray’ command with -d option is used to split the string data. The -d option is applied to define the separator character in the command like $IFS. Moreover, the bash loop is used to print the string in split form.

If you want to read all the lines at once, use readline(). Note − The readline() method, in contrast to its equivalent, only extracts one line from a file. A trailing newline character will also be added to the end of the string by the realine() method.

Reading of files in a Batch Script is done via using the FOR loop command to go through each line which is defined in the file that needs to be read. Since there is a no direct command to read text from a file into a variable, the ‘for’ loop needs to be used to serve this purpose.

The readlines() method reads all the lines from a file, going through the file line by line. It then returns a list of strings: with open(“example. txt”) as file: print(file.

IFS is a special variable in Bash which is used to control the field separator for hyphenation and line parsing. By default, IFS is set to a space, a tab, and a newline character, which means that fields in a string are separated by any combination of these characters.

You can use the scanf() function to read a string. The scanf() function reads the sequence of characters until it encounters whitespace (space, newline, tab, etc.).

In order to check if a String has only Unicode letters in Java, we use the isDigit() and charAt() methods with decision-making statements. The isLetter(int codePoint) method determines whether the specific character (Unicode codePoint) is a letter. It returns a boolean value, either true or false.

To find out if a bash variable is defined:

Determine if a bash variable is set or not : [[ ! -z ${PURGEIMAGE+z} ]] && echo “Set” || echo “Not defined” Return true if the variable is set on Bash version 4.2+: [ -v $VAR ] && echo “Bash \$VAR NOT set”

Bash scripting is an essential skill for any developer, system administrator, or power user who wants to automate repetitive tasks and make their workflow more efficient.

Coding is significantly more difficult and complex than scripting. For example, in order to run a program, you need thousands of lines of code. In order to automate a particular process within the program, you need scripting. Scripting requires a lot fewer lines of code, and it’s the next step after coding.

If you seek more portability for your script on Unix systems, Bash is a good option since the Bash interpreter is more widely pre-installed than Python.

Special Parameters $* and $@

Both the parameters specify the command-line arguments. However, the “$*” special parameter takes the entire list as one argument with spaces between and the “$@” special parameter takes the entire list and separates it into separate arguments.

Inside the script, we can use the $ symbol followed by the integer to access the arguments passed. For example, $1 , $2 , and so on.

If you require all arguments, then you can simply use %* in a batch script. %*refers to all the arguments (e.g. %1 %2 %3 %4 %5 …) but only arguments %1 to %9 can be referenced by number. Note: %0 is a special case, as this contains the name of the batch file itself.

The total number of arguments can be counted in Bash in two ways. One is using “$#” and another by using a loop. The methods of checking the number of arguments and using this value for different purposes are shown in this tutorial.

Arguments can be passed to the script when it is executed, by writing them as a space-delimited list following the script file name. Inside the script, the $1 variable references the first argument in the command line, $2 the second argument and so forth. The variable $0 references to the current script.

So basically, $# is a number of arguments given when your script was executed. $* is a string containing all arguments. For example, $1 is the first argument and so on. This is useful, if you want to access a specific argument in your script.

The $$ variable is the PID (Process IDentifier) of the currently running shell. This can be useful for creating temporary files, such as /tmp/my-script. $$ which is useful if many instances of the script could be run at the same time, and they all need their own temporary files.

$? determines the exit status of the executed command. $ followed by numbers (e.g. $1 , $2 , etc.) represents the parameters in the shell script.

Bash variables are case-sensitive, meaning that myvariable and MyVariable will be considered different variables. When a user wants to refer to a variable, print it out. In other words, you need to add a $ symbol before its name. This way, bash will know that the user wants to use a variable value.

To display the values of environment variables, use the printenv command. If you specify the Name parameter, the system only prints the value associated with the variable you requested.

To check if a variable is set in Bash Scripting, use-v var or-z ${var} as an expression with if command. This checking of whether a variable is already set or not, is helpful when you have multiple script files, and the functionality of a script file depends on the variables set in the previously run scripts, etc.

ReadProperty.sh

In the above script, the method getProperty() method identifies the Property file to be read and fetches the entire row using the Key (i.e., Name). Now it will have the complete string i.e., Name=Admin. Our motto is to fetch only the Value i.e., Admin.

The read command takes the user input and splits the string into fields, assigning each new word to an argument. If there are fewer variables than words, read stores the remaining terms into the final variable. Specifying the argument names is optional.

One of the quickest ways to display a single Linux environment variable is to use the echo command. The basic syntax is echo $<variable_name> . The echo $<variable_name> command also works for shell variables that are NOT environment variables.

To access the elements of a Bash Array, we can use the following syntax: echo ${ARRAY_NAME[2]}

Use the -v flag with printf to assign a shell variable to store the data instead of printing it in standard output. The output prints the stored variable contents.

Typically, when writing bash scripts, we use echo to print to the standard output. echo is a simple command but is limited in its capabilities. To have more control over the formatting of the output, use the printf command. The printf command formats and prints its arguments, similar to the C printf() function.

To check the exit code we can simply print the $? special variable in bash. This variable will print the exit code of the last run command. $ echo $?

[ $a -ne $b ] is true. Checks if the value of left operand is greater than the value of right operand; if yes, then the condition becomes true. [ $a -gt $b ] is not true. Checks if the value of left operand is less than the value of right operand; if yes, then the condition becomes true.

In Bash shell, we can only perform integer arithmetic. If we want to perform arithmetic involving a floating point or fractional values, then we will need to use various other utilities, such as awk , bc , and similar. For using the bc utility, we need to configure a scale parameter.