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3. Bash Operators in Linux

Although Linux Distributions provide a Graphical User Interface just like any other operating system, the ability to control the system via the command line interface (CLI) has many benefits. A way to control how tasks are executed or how input and output is redirected, can be done using operators.

1) Command “chaining”

• Sequence: ;
  

Executing multiples tasks, one after another. After the first task is completed, the next task will execute. After the second is completed, the third will execute and so on:

• Parallel execution: &
  

Execute tasks at the same time.

2) Conditional execution:

• And operator: &&
  

Task 2 will be executed only if Task 1 was successful

• Or operator: ||
  

Task 2 will be executed only if Task 1 was unsuccessful

3) Pipelines: |, |&

• Pipe operator: |
  

Used to send output from Task 1 (stdout) to input of Task 2 (stdin)

• Pipe error operator: |&
  

Error output of Task1 (stderr) is also redirected to input of Task 2 (stdin)

 

4) Input / output redirection

• Redirecting input: < 
  

Used for command that read their input from the terminal. Input is read from from file, instead of terminal

Syntax: command < file

• Redirecting input – here documents: <<
  

Used when the input of a command should be taken directly from the terminal, rather that from a file

• Redirecting input – here string:  <<<
  

Used when a single string should be redirected to the input of the command

• Redirecting output – stdout: >, >>
  

Used for commands that write output to the terminal.

Syntax: 

command > file – write output to file. If file doesn’t exist, it is created. If it exists, it will be truncated

command >> file – write output to file. If file doesn’t exists, it is created. If it exists, output will be appended

Note: Stderr is still written to the terminal

• Redirecting output – stderr: 2>, 2>>
  

Sends stderr to file.

Syntax: 

command 2> file – write stderr to file. . If file doesn’t exist, it is created. If it exists, it will be truncated

command 2>> file – write stderr to file. If file doesn’t exist, it is created. If it exists, output will be appended

Note: Output (stdout) is still written to the terminal

• Redirecting output – stdout and stderr: &>, &>>
  

Used to redirect both stdout and stderr for the same command

Syntax: 

command &> file – send stdout and stderr to file. If file doesn’t exist, it is created. If it exists, it will be truncated

command &>> file – send stdout and stderr to file. If file doesn’t exist, it is created. If it exists, output will be appended

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