Bash подавление вывода ошибок

8.4.2. Подавление вывода сообщений об ошибках

Допустим, вы хотите найти запись пользователя louise в системном файле паролей:

$ grep louise /etc/passwd

louise:lxAL6GW9G.ZyY:501:501:Accounts Sect1С:/home/accts/louise:/bin/sh

He исключена возможность, что вы забудете, как называется этот файл. В таком случае воспользуйтесь следующей командой:

$ grep louise /etc/password

grep: /etc/password: No such file or directory

Команда grep выводит сообщение об ошибке, в которой говорится о том, что указанного файла не существует. Можно попробовать провести поиск во всех файлах каталога /etc:

$ grep louise /etc/*

Однако в результате будут выведены многочисленные сообщения об ошибках, гласящие, что к определенным файлам доступ запрещен, а некоторые элементы каталога являются подкаталогами и поиск в них не ведется.

В подобной ситуации можно воспользоваться опцией -s, которая подавляет вывод сообщений об ошибках:

$ grep -a louise /etc/*

Если ваша версия команды grep не поддерживает данную опцию, воспользуйтесь следующей командой:

$ grep louise /etc/* 2> /dev/null

Эта команда направляет поток ошибок (2>) в системную корзину (устройство /dev/null). На жаргоне системных администраторов это устройство называется битодробилкой.

Bash, or the Bourne-Again Shell, is a powerful command-line interface (CLI) that is commonly used in Linux and Unix systems. When working with Bash, it is important to understand how to handle errors that may occur during the execution of commands. In this article, we will discuss various ways to understand and ignore errors in Bash. Bash scripting is a powerful tool for automating and simplifying various tasks in Linux and Unix systems. However, errors can occur during the execution of commands and can cause scripts to fail. In this article, we will explore the various ways to understand and handle errors in Bash. We will look at ways to check the exit status code and error messages of commands, as well as techniques for ignoring errors when necessary. By understanding and properly handling errors, you can ensure that your Bash scripts run smoothly and achieve the desired outcome.

Step-by-step approach for understanding and ignoring errors in Bash:

Step 1: Understand how errors are generated in Bash.

• When a command is executed, it returns an exit status code.
• A successful command will have an exit status of 0, while a failed command will have a non-zero exit status.
• Error messages are generated when a command returns a non-zero exit status code.

Step 2: Check the exit status code of a command.

• To check the exit status code of a command, you can use the $? variable, which holds the exit status of the last executed command.
• For example, after executing the command ls non_existent_directory, you can check the exit status code by running echo $? The output
• will be non-zero (e.g., 2) indicating that the command failed.

Step 3: Check the error message of a command.

• To check the error message of a command, you can redirect the standard error output (stderr) to a file or to the standard output (stdout) using the 2> operator.
• For example, you can redirect the stderr of the command ls non_existent_directory to a file by running ls non_existent_directory 2> error.log. Then you can view the error message by running cat error.log.

Step 4: Use the set -e command.

• The set -e command causes the script to exit immediately if any command exits with a non-zero status. This can be useful for detecting and handling errors early on in a script.
• For example, if you run set -e followed by ls non_existent_directory, the script will exit immediately with an error message. 

Step 5: Ignore errors when necessary.

• To ignore errors, you can use the command || true construct. This construct allows you to execute a command, and if it returns a non-zero exit status, the command following the || operator (in this case, true) will be executed instead.
• For example, you can run rm non_existent_file || true to remove a file that does not exist without exiting with an error.
• Another way to ignore errors is to use the command 2> /dev/null construct, which redirects the standard error output (stderr) of a command to the null device, effectively ignoring any error messages.
• Additionally, you can use the command 2>&1 >/dev/null construct to ignore both standard error and standard output.
• You can also use the command || : construct which allows you to execute a command and if it returns a non-zero exit status, the command following the || operator (in this case, 🙂 will be executed instead. The: command is a no-op command that does nothing, effectively ignoring the error.

Practical Explanation for Understanding Errors

First, let’s examine how errors are generated in Bash. When a command is executed, it returns an exit status code. This code indicates whether the command was successful (exit status 0) or not (non-zero exit status). For example, the following command attempts to list the files in a directory that does not exist:

$ ls non_existent_directory
ls: cannot access 'non_existent_directory': No such file or directory

As you can see, the command generated an error message and returned a non-zero exit status code. To check the exit status code of a command, you can use the $? variable, which holds the exit status of the last executed command.

$ echo $?
2

In addition to the exit status code, you can also check the standard error output (stderr) of a command to understand errors. This can be done by redirecting the stderr to a file or to the standard output (stdout) using the 2> operator.

For example, the following script will redirect the stderr of a command to a file:

$ ls non_existent_directory 2> error.log
$ cat error.log
ls: cannot access 'non_existent_directory': No such file or directory

You can also redirect the stderr to the stdout using the 2>&1 operator, which allows you to see the error message along with the standard output of the command.

$ ls non_existent_directory 2>&1
ls: cannot access 'non_existent_directory': No such file or directory

Another useful tool for understanding errors is the set -e command, which causes the script to exit immediately if any command exits with a non-zero status. This can be useful for detecting and handling errors early on in a script.

$ set -e
$ ls non_existent_directory
# as soon as you hit enter this will exit shell and will close the terminal.

After this command script will exit from the shell if the exit code is nonzero.

Practical Explanation for Ignoring Errors

While it is important to handle errors in Bash scripts, there may be certain situations where you want to ignore errors and continue running the script. In this section, we will discuss different methods for ignoring errors in Bash and provide examples of how to implement them.

Heredoc

Heredoc is a feature in Bash that allows you to specify a string or command without having to escape special characters. This can be useful when you want to ignore errors that may occur while executing a command. The following example demonstrates how to use Heredoc to ignore errors.

#!/bin/bash

# Example of ignoring errors using Heredoc

# The `command` will fail but it will not stop execution
cat <<EOF | while read line; do
  echo $line
done
command that will fail
EOF

# Rest of the script

In this example, the command that is inside the Heredoc will fail, but the script will not stop execution. This is because the output of the command is piped to the while loop, which reads the output and ignores the error.

Pipefail

The pipe fails option in Bash can be used to change the behavior of pipelines so that the exit status of the pipeline is the value of the last (rightmost) command to exit with a non-zero status or zero if all commands exit successfully. This can be useful when you want to ignore errors that may occur while executing multiple commands in a pipeline. The following example demonstrates how to use the pipe fail option to ignore errors.

#!/bin/bash

# Example of ignoring errors using pipefail

# The `command1` will fail but it will not stop execution
set -o pipefail
command1 | command2

# Rest of the script

In this example, command1 will fail, but command2 will continue to execute, and the script will not stop execution.

Undefined Variables

By default, Bash will stop the execution of a script if an undefined variable is used. However, you can use the -u option to ignore this behavior and continue running the script even if an undefined variable is used. The following example demonstrates how to ignore undefined variables.

#!/bin/bash

# Example of ignoring undefined variables

set +u

echo $undefined_variable

# Rest of the script

In this example, the script will not stop execution when an undefined variable is used.

Compiling and Interpreting

When compiling or interpreting a script, errors may occur. However, these errors can be ignored by using the -f option when running the script. The following example demonstrates how to ignore errors when compiling or interpreting a script.

#!/bin/bash

# Example of ignoring errors when compiling or interpreting

bash -f script.sh

# Rest of the script

In this example, the script will continue to run even if there are errors during the compilation or interpretation process.

Traps

A trap is a way to execute a command or a set of commands when a specific signal is received by the script. This can be useful when you want to ignore errors and run a cleanup command instead. The following example demonstrates how to use a trap to ignore errors.

#!/bin/bash

# Example of ignoring errors using a trap

# Set a trap to run the cleanup function when an error occurs
trap cleanup ERR

# Function to run when an error occurs
cleanup() {
  echo "Cleaning up before exiting..."
}

# Command that will cause an error
command_that_will_fail

# Rest of the script

In this example, when the command_that_will_fail causes an error, the script will execute the cleanup function instead of stopping execution. This allows you to perform any necessary cleanup before exiting the script.

Examples of Bash for Error Handling:

Example 1: Error Handling Using a Conditional Condition

One way to handle errors in Bash is to use a conditional statement. The following example demonstrates how to check for a specific error and handle it accordingly.

#!/bin/bash

# Example of error handling using a conditional condition

file=example.txt

if [ ! -f $file ]; then
  echo "Error: $file does not exist"
  exit 1
fi

# Rest of the script

In this example, we check if the file “example.txt” exists using the -f option of the [ command. If the file does not exist, the script will print an error message and exit with a status code of 1. This allows the script to continue running if the file exists and exit if it does not.

Example 2: Error Handling Using the Exit Status Code

Another way to handle errors in Bash is to check the exit status code of a command. Every command in Bash returns an exit status code when it completes, with a code of 0 indicating success and any other code indicating an error. The following example demonstrates how to check the exit status code of a command and handle it accordingly.

#!/bin/bash

# Example of error handling using the exit status code

command1

if [ $? -ne 0 ]; then
  echo "Error: command1 failed"
  exit 1
fi

# Rest of the script

In this example, the script runs the command “command1” and then checks the exit status code using the special variable $?. If the exit status code is not 0, the script will print an error message and exit with a status code of 1.

Example 3: Stop the Execution on the First Error

When running a script, it can be useful to stop the execution on the first error that occurs. This can be achieved by using the set -e command, which tells Bash to exit the script if any command exits with a non-zero status code.

#!/bin/bash

# Stop execution on the first error

set -e

command1
command2
command3

# Rest of the script

In this example, if any of the commands “command1”, “command2” or “command3” fail, the script will exit immediately.

Example 4: Stop the Execution for Uninitialized Variable

Another way to stop execution on error is if an uninitialized variable is used during script execution. This can be achieved by using the set -u command, which tells Bash to exit the script if any uninitialized variable is used.

#!/bin/bash

# Stop execution for uninitialized variable

set -u

echo $uninitialized_variable

# Rest of the script

In this example, if the uninitialized_variable is not defined, the script will exit immediately.

Conclusion

In conclusion, understanding and ignoring errors in Bash is an important aspect of working with the command-line interface. By checking the exit status code of a command, its associated error message, and redirecting the stderr to a file or the stdout, you can understand what went wrong. And by using the command || true, command 2> /dev/null, command 2>&1 >/dev/null, and command || : constructs, you can ignore errors when necessary. It’s always a good practice to test these constructs in a testing environment before using them in production.

You’re right; pkill isn’t generating the message, bash is. 
You suggest that

$ ./test1.sh 2> /dev/null

is a possible solution. 
As UVV points out, the equivalent action from within the script is

exec 2> /dev/null

This redirects the stderr for the script to /dev/null
from this statement until it is changed back. 
Clumsy ways of changing it back include

exec 2> /dev/tty

which redirects stderr to the terminal. 
This is probably (but not necessarily) where it was originally.

Or

exec 2>&1

which sets stderr to be the same as stdout, and is likely to be wrong.

A more reliable way is

exec 3>&2
exec 2> /dev/null
(do stuff where you don't want to see the stderr.)
exec 2>&3

which saves the original stderr in file descriptor 3, and later restores it.

Other ways to suppress just the announcement of the process death include

(sleep 10 & pkill sleep) 2> /dev/null

and

{ sleep 10 & pkill sleep;} 2> /dev/null

which change the stderr for only the grouped commands.