Chapter 7: Working with the BASH Shell

escape sequences

The character sequences that have special meaning inside the echo command. They are prefixed by the \ character.

environment files

The files used immediately after login to execute commands; they are typically used to load variables into memory.

hashpling

The first line in a shell script, which defines the shell that will be used to interpret the commands in the script file.

variable identifier

The name of a variable.

file descriptors

The numeric labels used to define command input and command output.

redirection

The process of changing the default locations of Standard Input, Standard Output, and Standard Error.

shell scripts

The text files that contain a list of commands or constructs for the shell to execute in order.

user-defined variables

The variables that are created by the user and are not used by the system. These variables are typically exported to subshells.

environment variables

The variables that store information commonly accessed by the system or programs executing on the system; together, these variables form the user environment.

filter command

A command that can take from Standard Input and send to Standard Output. In other words, a filter is a command that can exist in the middle of a pipe.

alias command

A command used to create special variables that are shortcuts to longer command strings.

env command

A command used to display a list of exported variables present in the current shell, except special variables.

echo command

A command used to display or echo output to the terminal screen. It might utilize escape sequences.

read command

A command used to read Standard Input from a user into a variable.

export command

A command used to send variables to subshells.

tee command

A command used to take from Standard Input and send to both Standard Output and a specified file.

tr command

A command used to transform or change characters received from Standard Input.

set command

A command used to view all variables in the shell, except special variables.

Standard Error (stderr)

A file descriptor that represents any error messages generated by a command.

Standard Input (stdin)

A file descriptor that represents information input to a command during execution.

Standard Output (stdout)

A file descriptor that represents the desired output from a command.

<

A shell metacharacter used to obtain Standard Input from a file.

|

A shell metacharacter used to pipe Standard Output from one command to the Standard Input of another command.

>

A shell metacharacter used to redirect Standard Output and Standard Error to a file.

subshell

A shell started by the current shell.

decision construct

A special construct used in a shell script to alter the flow of the program based on the outcome of a command or contents of a variable. Common decision constructs include if, case, &&, and |.

loop construct

A special construct used in a shell script to execute commands repetitively. Common decision constructs include for and while.

test statement

A statement used to test a certain condition and generate a True/False value.

pipe

A string of commands connected by | metacharacters.

counter variable

A variable that is altered by loop constructs to ensure that commands are not executed indefinitely.

variable

An area of memory used to store information. Variables are created from entries in environment files when the shell is first created after login, and are destroyed when the shell is destroyed upon logout.