Bash

Bash How to Loop through Files

You can use it for the loop. Here is the syntax: for ELEMENT in $ARRAY; do command; done

Let’s try in in live example. First, create a new directory:

mkdir /root/test
cd /root/test
touch /root/test/sample.sh
touch /root/test/example.sh
touch /root/test/document.doc

Secondly, we process all files that have suffix .sh with for loop:

for FILE in /root/test/*.sh
do echo "Processing ${FILE} file.."
done

Output: Processing /root/test/example.sh file.. Processing /root/test/sample.sh file..

Based on the results of output (above), we can see that the loop processes all files in the /root/test, which have the suffix .sh

If you want to process command-line arguments use:

for FILE in "$*"
do
echo "Processing $FILE" file.

$* expands to file names of all files in the current directory.

Bash How to Wait Seconds

How can we in the script wait for it until the system completes some tasks? The answer is to use sleep. This command suspends the script so that the script is low almost no system resources. Timing is sufficient.

Do you want to wait some seconds? In the next example, we will wait one second:

sleep 1

To turn off the script only for a split second? You can. This example shows the ingested sleep 100ms:

sleep 0.1

In this example, we will wait 20 minutes:

sleep 20m

In the next example, we will wait 8 hours:

sleep 8h

Do you want to wait several days? This is possible if you use a parameter d. However, consider using a cron scheduler. It is robust. Can you schedule to run it in your scripts at specified times and periodic runs?

In the last example, we will wait 7 days:

sleep 7d

Bash provides command wait to wait for the process/processes given as arguments:

wait $PID

PID is processing ID. ‘wait ${!}’ waits until the last background process is completed.

Bash How to Print Array

Arrays are collection of elements, The Arrays in bash are indexed from 0 (zero-based).
Below is the definition on an Array in Bash
my_array=(zero one two three four)

Now our array is defined.
Here is exactly how the my_array is stored on BASH:

my_array=([0]="zero" [1]="one" [2]="two" [3]="three" [4]="four")

You can explicit define an array:

declare -a MY_ARRAY

You can view the declarations along with other environment variables using the declare command.

declare -p my_array
declare -a my_array=([0]="zero" [1]="one" [2]="two" [3]="three" [4]="four")

Now if you try to print the array:

my_array=(zero one two three four)
echo $my_array
zero

By default only the first element value is printed which belongs to the 0 index.

To print the first element of the array using the indexing:

my_array=(zero one two three four)
echo ${my_array[0]}
zero

The change we noticed here is the use of the Curly Braces ‘{}’, its used to refer to the value of an item in the array. The curly braces are required to avoid issues with path name expansion.

To read all elements of the array use the symbols “@” or “*”.

echo ${my_array[@]}
zero one two three four

echo ${my_array[*]}
zero one two three four

The difference between “$@” and “$*” is “$@” expands each element as a separate argument, however “$*” expand to the arguments merged into one argument.

To prove this, print the index elements followed by $@ or $* format.

echo ${my_array[$*0]}
zero

echo ${my_array[$@1]}
one

echo ${my_array[$*2]}
two

Getting the Length of the Array

If you need to get the length of the array uses the symbol “#” before the name of the array:

echo "${#my_array[*]}"
5

How do I print an array in bash?

Print Bash Array

We can use the keyword ‘declare’ with a ‘-p’ option to print all the elements of a Bash Array with all the indexes and details. The syntax to print the Bash Array can be defined as: declare -p ARRAY_NAME.

How do I print in bash?

After typing in this program in your Bash file, you need to save it by pressing Ctrl +S and then close it. In this program, the echo command and the printf command is used to print the output on the console.

How do I display all array elements at once?

1. public class PrintArray {
2. public static void main(String[] args) {
3. //Initialize array.
4. int [] arr = new int [] {1, 2, 3, 4, 5};
5. System. out. println(“Elements of given array: “);
6. //Loop through the array by incrementing value of i.
7. for (int i = 0; i < arr. length; i++) {
8. System. out. print(arr[i] + ” “);

How do I create an array in bash?

1. To declare your array, follow these steps:
   Give your array a name.
2. Follow that variable name with an equal sign. The equal sign should not have any spaces around it.
3. Enclose the array in parentheses (not brackets like in JavaScript)
4. Type your strings using quotes, but with no commas between them.

How do you create an array in bash?

Define An Array in Bash

You have two ways to create a new array in bash script. The first one is to use declare command to define an Array. This command will define an associative array named test_array. In another way, you can simply create Array by assigning elements.

How does printf work in bash?

What Is the Bash printf Function? As the name suggests, printf is a function that prints formatted strings of text. That means you can write a string structure (the format) and later fill it in with values (the arguments). If you’re familiar with the C/C++ programming languages, you might already know how printf works.

How do you pass an array to a function in bash?

10 Answers

1. Expanding an array without an index only gives the first element, use copyFiles “${array[@]}” instead of copyFiles $array.
2. Use a she-bang #!/bin/bash.
3. Use the correct function syntax. Valid variants are function copyFiles {… …
4. Use the right syntax to get the array parameter arr=(“$@”) instead of arr=”$1″

How do you create an empty array in bash?

To declare an empty array, the simplest method is given here. It contains the keyword “declare” following a constant “-a” and the array name. The name of the array is assigned with empty parenthesis.

How do I get the size of an array in bash?

To get the length of an array, we can use the {#array[@]} syntax in bash. The # in the above syntax calculates the array size, without hash # it just returns all elements in the array.

Bash Vs KSH

Linux and Unix have various shells. Two kinds of these numerous shells are KSH and BASH.

KSH (The Korn Shell) was developed many years before the BASH. Ksh has associative arrays and handles loop syntax better than bash. Also ksh’s command print is better than bash’s echo command. In other way, ksh does not support history completion, process substitution and rebindable command-line editing.

Bash has more added extension than ksh. Bash has tab completion and easier method to set a prompt in order to display current directory.

Compared to ksh, bash is newer and more popular.

Example of difference ksh and bash in condition test. First bash:

if [ $i -eq 3 ]

and condition test in ksh:

if (($i==3))

Bash can handle exit codes from pipes in a cleaner way. Bash and KSH are both Bourne=compatible shells, they share common functions and features and can be interchangeable to use.

Is ksh same as bash?

KSH and Bash shells are also products of combinations of other shells’ features. Bash and KSH are both Bourne-compatible shells. Since they share common features, they can be used interchangeably.

Is ksh faster than bash?

The ksh and zsh seems about seven times faster than bash . The ksh excelled in 17 tests and the zsh in six tests.

What is the difference between ksh CSH and bash?

CSH is C shell while BASH is Bourne Again shell. 2. C shell and BASH are both Unix and Linux shells. While CSH has its own features, BASH has incorporated the features of other shells including that of CSH with its own features which provides it with more features and makes it the most widely used command processor.

Is ksh a Linux shell?

Ksh is an acronym for KornSHell. It is a shell and programming language that executes commands read from a terminal or a file. It was developed by David Korn at AT&T Bell Laboratories in the early 1980s. It is backwards-compatible with the Bourne shell and includes many features of the C shell.

Why is ksh used?

ksh is a command and programming language that executes commands read from a terminal or a file. rksh is a restricted version of the command interpreter ksh; it is used to set up login names and execution environments whose capabilities are more controlled than those of the standard shell.

How do I run a ksh script in Bash?

How do I run . sh file shell script in Linux?

• Open the Terminal application on Linux or Unix.
• Create a new script file with .sh extension using a text editor.
• Write the script file using nano script-name-here.sh.
• Set execute permission on your script using chmod command : chmod +x script-name-here.sh.
  To run your script :

Is dash faster than Bash?

If you need speed, go definitely with dash, it is much faster than any other shell and about 4x faster than bash.

What is faster than Bash?

Perl is absurdly faster than Bash. And, for text manipulation, you can actually achieve better performances with Perl than with C, unless you take time to write complex algorithms.

How much faster is dash than Bash?

Dash is not Bash compatible, but Bash tries to be mostly compatible with POSIX, and thus Dash. Dash shines in: Speed of execution. Roughly 4x times faster than Bash and others.

What is difference between sh and ksh in Unix?

sh is the original Bourne shell. On many non-Linux systems, this is an old shell without the POSIX features. Thus bash and ksh (or even csh and tcsh) are better choices than sh. … Public domain ksh (pdksh) is Bourne-compatible and mostly POSIX-compatible.

Is zsh better than Bash?

It has many features like Bash but some features of Zsh make it better and improved than Bash, such as spelling correction, cd automation, better theme, and plugin support, etc. Linux users don’t need to install the Bash shell because it is installed by default with Linux distribution.

Bash How to Read from Keyboard

To read input from the keyboard and assign input value to a variable use the read command.

Read syntax:

read options var1 var2

To write the value of first entered word use:

read var1 var2
echo $var1

If you don’t give any argument to the read command, input will assign to the environment variable REPLY.

The “s” option does not echo input while reading from a keyboard.

read -s -p "Enter password:" $password

The -p “TEXT” option displays TEXT to the user without a newline.

The -e option means that command readline is used to obtain the line.
Option -u FD reads inputs from file descriptor FD (0,1,2).
Option -t TIME causes that read returns a failure if the input is not read within TIME seconds.

Bash How to Return from Function

Use the statement “return” to return from the function. You can also specify the return value. Example:

return 1234

We returned function status 1234 from function. Usually, we use 0 value for success and 1 for failure. It is similar to command exit which we use to terminate the script.

If you don’t use the return statement in the whole function, the status of the last executed command will be returned.

To verify returned status from the last called function use “$?”.

There is a difference between commands return and exit. The exit will cause the script to end at the line where it is called. Return will cause the current function to go out of scope and continue execution command after the function.

What is Bash Script

Metaphorically speaking bash script is like a ‘to-do list’. After you read the first entry you start realizing it. After you finished first entry, you continue with the second entry and so on.

A bash script is a text file that contains a mixture of commands.

Bash script can contain also functions, loops, conditional constructs. Scripts are commonly used for administration task like change file permission, creating disk backups. Using bash scripts is often faster than using the graphical user interface.

It’s important to mention that there is no difference between putting series of 10 commands into a script file and executing that script or you entering commands one by one to the command-line interface. In both situations, the result will be exactly the same thing.

After you create your script it is good practice to add the extension ‘.sh’ to the filename, for example ‘myFirstScript.sh’.

Bash Error Output Redirect

Each open file gets assigned a file descriptor. The file descriptors for STDIN is 0, for STDOUT is 1 nad STDERR is 2.

If you want to redirect just STDERR (standard error output) to file, ju do:

cmd_name 2> /file

If you want o redirect STDOUT to other files, and STDERR to other files, just do:

cmd_name >/stdout_file 2>/stderr_file

If you want to merge both (STDERR, STDOUT) into one file, you can do:

cmd_name >/file_4_both 2>&1

For the same effect, you can also use this syntax:

cmd_name &> /file_4_both

Even more, it is very useful to use the tee command. By definition, tee read from standard input and write to standard output and files, in same time.

In next example, we merge STDOUT and STDERR of “find /” command together. Then, we pass it to STDIN of tee command. Tee command is executed with -a parameter, that means append to an existing file (if exists):

find / 2>&1 | tee -a /home/tee.output
ll /home

Output: (lines omitted) -rw-r–r–. 1 root root 2.2M Jun 10 13:16 tee.output

The child process inherits open file descriptors. If you want to prevent file descriptors from being inherited, close it. For example:

<&-

This close stddin descriptor.

Bash Function How to Return Value

Bash functions have “return” statement, but it only indicates a return status (zero for success and a non-zero value for failure).

function myfunc() {
var='some text'
echo Hello..
return 10
}

myfunc
echo "Return value of previous function is $?"

Output: Hello.. Return value of the previous function is 10

If you want to return value you can use a global variable.

var=0
string () {
var="My return value."
}
string; echo $var

Output: My return value.

It is simple, but using global variables in complex scripts or programs causes harder methods to find and fix bugs.

We can use command substitution and assign an output from function:

string () {
local local_var="Value from function."
echo $local_var
}

var=$( string )
echo $var

Output: Value from function.

It’s good practice to use within-function local variables. Local variables are safer from being changed by another part of the script.

Bash How to Compare Numbers

The first option is to use the command test to binary compare numbers. For example:

if [ $a -eq $b ]; then
echo "a == b"
else
echo a!=b
fi

An operator”-eq” is equal to, -gt is greater than. For more operators type:

man test

If you are more familiar with symbols “<, >, <=, >=, ==” use double parentheses:

if (( $a < $b )); then
echo "a < b"
fi

For POSIX shells that don’t support double parentheses use test command.

Few examples of test options:

• -ge: greater or equal
• -le: less or equal
• -lt: less than
• -ne: not equal

Bash How to Return String from Function

You can return string from function in many ways, but you can not use the command “return” to return string:

return "Hello..."

Return statement can return only an integer value.

First option uses a passing argument to the function. To assign to the first argument use in function “$1”:

eval "$1='Hello...'"

Then call the function “my_function”:

my_var=''
my_function my_var
echo $my_var

Output: Hello…

Other way is to use a global variable which you modify within the function.

You can also use command echo to write string value and use command substitution to get it:

hello() {
var='Hello friend.'
echo "$var"
}

greeting=$(hello)
echo $greeting