BASH Shell: Input and Output Redirection, Environment Variables, and Shell Scripts, Slides of Advanced Operating Systems

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Chapter 8

Working with the BASH Shell

Objectives

• Redirect the input and output of a command
• Identify and manipulate common shell environment variables
• Create and export new shell variables
• Edit environment files to create variables upon shell startup

Command Input and Output

• BASH shell responsible for:
  • Providing user interface
  • Interpreting commands
  • Manipulating command input and output
    • Provided user specifies certain shell metacharacters with command
• File descriptors: Numeric labels that define command input and command output

Command Input and Output (continued)

• Standard Input (stdin): File descriptor representing command input
• Standard Output (stdout): File descriptor representing command output
• Standard Error (stderror): File descriptor representing command error messages

Redirection

• Redirect stdout and stderr from terminal screen to a file - Use “>” shell metacharacter - Can redirect stdout and stderr to separate files
• Use separate filenames for stdout and stderr
• Otherwise, use the &
• Ex: .$ls – l /etc/host /etc/h >jj 2>&

Redirection (continued)

• Redirecting stdin to a file:
  • Use “<“ shell metacharacter
• tr command: Replace characters in a file sent via stdin
• tr accepts files not file name so you need to use input redirection
• The shell always clears the content of the file specified in the output redirection

Pipes

• Send stdout of one command to another command as stdin
• Pipe: String of commands connected by “|” metacharacters - stdout on left, stdin on right
• Commonly used to reduce amount of information displayed on terminal screen (using grep)

Pipes (continued)

Figure 8-2: Piping information from one command to another

Pipes (continued)

Figure 8-3: Piping several commands

Pipes (continued)

Table 8-2: Common filter commands

Shell Variables

• Variable: A reserved portion of memory containing accessible information
• BASH shell has several variables in memory
• Environment variables: Contain information that system and programs access regularly
• User-defined variables: Custom variables define by users
• Special variables
  • Useful when executing commands and creating new files and directories

Environment Variables

• set command: Lists environment variables and current values
• echo command: View contents a specified variable - Use $ shell metacharacter
• Changing value of a variable:
  • Specify variable name followed by equal sign (=) and new value

Environment Variables (continued)

Table 8-3 (continued): Common BASH environment variables

Environment Variables (continued)

Table 8-3 (continued): Common BASH environment variables