Shell Script Basics


$(command) or `command` – the command substitution

${variable_name} or $variable_name – the parameter substitution/variable expansion

IFS - Internal Field Separator

The IFS is a special shell variable.

You can change the value of IFS as per your requirments.

The Internal Field Separator (IFS) that is used for word splitting after expansion and to split lines into words with the read builtin command.

The default value is . You can print it with the following command:

IFS=’|’ $@ expanded as “$1” “$2” “$3” … “$n” $* expanded as “$1y$2y$3y…$n”, where y is the value of IFS variable i.e. “$*” is one long string and $IFS act as an separator or token delimiters.

$@ and $* are special command line arguments shell variables. The $@ holds list of all arguments passed to the script. The $* holds list of all arguments passed to the script.

To print array



For loop

for i in 1 2 3 4 5
  echo "Looping ... number $i"

source file_name

The source command reads and executes commands from the file specified as its argument in the current shell environment. It is useful to load functions, variables, and configuration files into shell scripts.

source is a shell built-in in Bash and other popular shells used in Linux and UNIX operating systems. Its behavior may be slightly different from shell to shell.

To check file exits or not

[ -e FILE ] True if FILE exists.

This will return true for both /etc/hosts and /dev/null and for directories.

[ -f FILE ] True if FILE exists and is a regular file.

This will return true for /etc/hosts and false for /dev/null (because it is not a regular file), and false for /dev since it is a directory.

To get filename without extension

$(basename $PACKAGE_NAME .tgz)

To encode base64

echo data | base64

To decode base64

echo encoded_value(eg. ZGVmYXVsdA==) | base64 -d

To untar a file

tar -xvzf file.tgz

To compress and generate .tgz file

tar -cvzf file_name.tgz folder_name/

Special symbols in shellscript

$? –> takes the result of last command executed

If we dont want to print the output of command to screen/stdout, we can write the output to this file /dev/null

ping -c1 &> /dev/null

To print a value

echo “This is my data” printf “Hello shellscript \n” –> we have to add \n in printf or else next command output will print in the same line

To read data from user and store to variable

read variable_name

To store command error to variable

Here helm tool is not installed it will return error status, so by default the error message printed to stdout and not stored to variable

helm_cmd_output=$(helm 2>&1)

To check the command is executed successfully or not

Here helm tool is not installed it will return error status

helm version
if [[ $? != 0 ]]; then
    curl | bash