Bash Built-In Variables

External

• GNU manual, bash variables: https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#Bash-Variables
• bash Internal Variables http://www.tldp.org/LDP/abs/html/internalvariables.html

Internal

• bash Parameters and Variables
• bash Parameter and Variable Expansion

Overview

This page documents variables that have a special meaning to the shell and are known as internal, keyword or built-in variables. For a list of special parameters, see bash Special Parameters.

BASH

The path to the bash binary.

BASH_ENV

An environment variable pointing to a bash startup file to read when the script is invoked.

BASH_SUBSHELL

A variable indicating the subshell level. Introduced in bash 3. The main bash process reports 0. If a function is invoked with $(...), it will report BASH_SUBSHELL=1. If another function is invoked with $(...) from it, it will report BASH_SUBSHELL=2, and so on.

BASHPID

The process ID of the current instance of bash. It is different from $$ in that $$ always stay the same for a bash script execution, representing the process ID of the top level bash process, the script itself, even if invoked from subshells with (...), whereas BASHPID has different values, depending on the subshell it is dereferenced into.

BASH_SOURCE

An array variable whose members are the source filenames where the corresponding shell function names in the FUNCNAME array variable are defined. The shell function ${FUNCNAME[$i]} is defined in the file ${BASH_SOURCE[$i]} and called from ${BASH_SOURCE[$i+1]}

${BASH_SOURCE[0]} is equivalent with $0, with the observation that ${BASH_SOURCE[0]} contains the (potentially relative) path of the containing script in all invocation scenarios, notably also when the script is sourced, which is not true for $0.

When the script is sourced, $0 contains "-bash", while ${BASH_SOURCE[0]} contains the path of the script.

DIRSTACK

Directory Manipulation Built-in Commands

IFS

IFS is the internal field separator. This variable determines how bash recognizes fields (word boundaries) when it interprets character strings. IFS defaults to whitespace (space, tab and newline). This is the proof:

echo "$IFS" | cat -vte
 ^I$
$

IFS can be changed. For example, this is how you set IFS so for iterates over lines:

IFS="$(printf '\n\r')"

Note you must set IFS back to whitespace after setting it to something else, so the basic shell function work as expected. This is done as shown below: restoring the default IFS value.

To research, IFS= seems equivalent with IFS="$(printf '\n\r')" in that it recognizes '\n' and '\r' as separators but not the space ' '. It did not work for me, but why it worked in examples?

Restoring the default IFS value

IFS="$(printf ' \t\n\r')"

IFS and for

for honors the value of IFS (default the space). If you set IFS to something else, before the for statement, for will use that as field separator while iterating over the list.

Note be extremely careful when setting IFS before a for loop, even if you restore the default value after the loop: everything inside the loop will use the non-standard IFS value and it may not work as expected.

Also see:

• for
• bash set List Separator

IFS and read

IFS characters are used to split the line processed by read into words.

PATH

PATH

PIPESTATUS

See:

Pipeline

PPID

The process ID of the shell's parent. This variable is readonly.

PS1

Bash Prompt

FUNCNAME

An array containing the names of all shell functions currently in the execution call stack. Invoked as ${FUNCNAME} returns the name of the current function. See:

The FUNCNAME Array

OLDPWD

Contains directory before the last cd command.

cd directory
echo $OLDPWD
/home/user

LD_LIBRARY_PATH

https://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html

Linux Shared Libraries

LD_PRELOAD

https://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html

Linux Shared Libraries