There are many shell built-in variables which are used for administrating and writing shell scripts. In this post we will see important built-in variables and their importance. Please add any other built-ins which are useful for administration and for shell scripting.

Note: in order get present values of these variables use echo command to get them. For example if you want to see present shell then you can give echo $SHELL to get the value.

$SHELL –Gives present shell

$HISTSIZE –Gives the size of number of Commands that can be accommodated in History file

$HISTFILE –To get the history file location

$USER –Displays username

$EUID –Display UserID.

$GROUPS – To get GID info.

$PWD –Display’s present working Directory.

$HOSTNAME –Displays hostname.

$HOME –Give you user’s home directory.

$HOSTTYPE or $MACHTYPE – To get host architecture ie 32bit or 64 bit.

$OSTYPE –Detect OS, such as gnu Linux, sun sol etc. Useful for controlling the shell script flow.

$TERM –Gives you terminal name.

$TMOUT –Exit the shell, if no activity was there on terminal, useful for securing the server.

$PATH –Get path to all the binaries ie commands

$PIPESTATUS –To get exit status of piped output.

$BASH_VERSION –To get bash version. Useful for controlling the shell script flow.

$PPID –Get parent process ID.

$PS1, $PS2, $PS3, $PS4 –Different prompts. These are useful for menu driven scripts.

In Scripting

$RANDOM –To get a random number, very much useful for giving random passwords for new users.

$LINENO –To get what is the present line number the script is executing. Useful for debugging a shell script.

$REPLY –REPLY holds last read value.

$SECONDS –To get number of seconds the script is running.

Positional parameters: $1, $2, $3, $4, $5, $6, $7, $8, $9

$0 –Gives script name

$* –All positional parameters

$@ All positional parameters with spaces

$# –Number of Parameters

$$ –Current Process ID

$! –ID of Background job

$? –Error or exit status

$_ –Get the previous command last argument

 

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Surendra Anne

Mr Surendra Anne is from Vijayawada, Andhra Pradesh, India. He is a Linux/Open source supporter who believes in Hard work, A down to earth person, Likes to share knowledge with others, Loves dogs, Likes photography. He works as Devops Engineer with Taggle systems, an IOT automatic water metering company, Sydney . You can contact him at surendra (@) linuxnix dot com.