Try catch с linux

Bash Try-Catch#

if..else — это наше всё. Настоящих перехватов ошибок в баше нет, только логически, если сам напишешь.

Есть ещё альтернативная if..else конструкция, построенная на логическом «или»

 # try какая-то*команда && > ||  # catch какая-то*команда > 

Ещё люди пишут «обёртки», которые имитируют try..catch:

Пример#

#!/bin/bash function try()  [[ $- = *e* ]]; SAVED_OPT_E=$? set +e > function throw()  exit $1 > function catch()  export ex_code=$? (( $SAVED_OPT_E )) && set +e return $ex_code > function throwErrors()  set -e > function ignoreErrors()  set +e > 

А вот как это использовать#

#!/bin/bash export AnException=100 export AnotherException=101 try ( # Всё происходит в отдельной сессии баша, то есть тут тупо скобки () echo "do something" [ someErrorCondition ] && throw $AnException echo "do something more" executeCommandThatMightFail || throw $AnotherException throwErrors # Автоматически завершает try-блок, если результат не нулевой echo "now on to something completely different" executeCommandThatMightFail echo "it's a wonder we came so far" executeCommandThatFailsForSure || true # заигнорить обвалившуюся команду, если нужно ignoreErrors # Отключить перехватывание ошибок executeCommand1ThatFailsForSure local result = $(executeCommand2ThatFailsForSure) [ result != "expected error" ] && throw $AnException # Обычный контроль ошибок, на основе присваивания переменной executeCommand3ThatFailsForSure # В конце нужно очистить $ex_code, иначе сработает catch-блок, ибо он всё равно срабатывает # echo "finished" - выступает в роли "всегда срабатывающей команды" # которая очистит все возможные ошибки, которые были ранее echo "finished" ) # Сразу после того как завершается выделенная сессия баша, # мы вызываем catch, который подчищает нашу сессию и возвращает переменную ex_code, # которую мы уже анализируем в блоке "или"(||), либо пропускаем, если ошибок нет catch ||  case $ex_code in $AnException) echo "AnException was thrown" ;; $AnotherException) echo "AnotherException was thrown" ;; *) echo "An unexpected exception was thrown" throw $ex_code # тут вы можем выкинуть ошибку, завершив аварийно скрипт ;; esac > 

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Is there a TRY CATCH command in Bash?

“Bash” does not support the “try/catch” command. However, there are other ways to apply its functionalities, such as the “if/else” statements, “OR” operators, the “trap” command, or the “-x” flag.

The “try-catch” is a programming term used to handle exceptions. In simple words, the “try” block tries to do some work, and if there is an error, such as a file not found, it throws an exception which can be copied into the “catch” block.

This guide explores the approaches that can be used as a substitute for the “try/catch” command.

Check the “Exit Status”

All the commands generate a single-digit value (“0” for “true” and “1” for “false”). It is done using the “set -e” option. This option prompts the Bash to exit immediately if any command in the script exits with a non-zero code. In the below example, the script installs Firefox on the system. Once it is successfully executed, it displays the message “Command Succeeded”, as follows:

Before executing it, make sure to give it execute permissions (the above script is named “script.sh”) using the chmod command with +x flag:

The above executed command confirms that the execute permissions were granted to the file “script.sh”. However, to execute it, apply the following command:

By looking at the above image, it is evident that the command is successfully executed as the message “command succeeded” is displayed. There could be multiple scenarios where you can use the echo command to check the “exit status” right after the command is executed.

How to Make the “trap” Command Function as TRY CATCH?

The “trap” command works based on the Signals sent to it by the OS or the user (by pressing “CTRL+C” to interrupt the program). It is a trigger that is a response to a specific command. For example, the script below runs until the user presses “CTRL+C”. Once pressed, it will display the message “trap worked” and sleep for “5” seconds before giving back the control to the user:

trap ‘echo «trap worked»‘ INT

The above script is named “script.sh.” Let’s execute it to view the results:

In the above terminal, it is seen that when we pressed “CTRL+C”, it printed “trap worked”, and there can be multiple scenarios where it can be used. For example, in the below script, when the service is running, it will stop and restart that service. Let’s assume the service as “mysql” in this case:

The script is named “script.sh”. Let’s execute it to view the output:

As seen in the above terminal, it first stops the service and then starts it again. If you want to start the service immediately right after it is stopped, press “CTRL+C”:

The above examples are similar to the “try/catch” in such a way that a script with multiple commands takes a long time to execute. You can eliminate it using the “CTRL+Z” shortcut keys, but it will not display the message printed via the “echo” command. But when the “trap” command is used, it is easier to identify which command works fine and which is not.

How to Trace Output Using the “-x Flag” in Bash?

The “-x” flag is used for debugging a bash script. It interprets each line being executed and displays everything in the script. To use it, add a prior “-x” when executing the command, as seen below:

The above image displays the script’s parameters in the same format as it is executed.

How to Force Exit When Error is Detected in Bash?

The “set” is used with “errexit” or “-e” in bash to exit. What it does is the automatic termination of the command when there is an error. This option instructs “Bash” to immediately exit the script when any command returns a non-zero exit status, indicating an error.

Following is an example script in which the system repositories are updated, Python is installed, git is cloned, the requirements for Python are installed and finally, the server is launched, respectively:

sudo apt install git curl python3-pip

git clone https: // github.com / example / repo.git

pip3 install -r requirements.txt

It is named “script.sh”. To execute it, apply the below-stated command, as discussed:

The above provided “Username” and “Password” for GitHub are incorrect, which will cause an error resulting in the script’s termination indicated below:

As seen above, the script is immediately terminated once an error is popped.

Conclusion

The bash scripting does not support the “try/catch” statement like most other coding languages. However, there are other alternatives to apply the same functionality, like checking the “exit status”, applying the “trap” command, or tracing the output with the “-x” flag, which can also be useful. Also, the script can immediately be terminated once an error is popped up by using the “set -e” command. This guide discussed the status of the “try/catch” command in bash and its alternatives.

About the author

Talha Saif Malik

Talha is a contributor at Linux Hint with a vision to bring value and do useful things for the world. He loves to read, write and speak about Linux, Data, Computers and Technology.

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bash try catch

Is there a TRY CATCH command in Bash? No. . There is no try/catch in bash; however, one can achieve similar behavior using && or || .

What is a try catch?

The try statement allows you to define a block of code to be tested for errors while it is being executed. The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.

How do I stop a shell script from error?

Use the exit statement to indicate successful or unsuccessful shell script termination. The value of N can be used by other commands or shell scripts to take their own action. If N is omitted, the exit status is that of the last command executed. Use the exit statement to terminate shell script upon an error.

What is bash Error in Linux?

Most of the time in Linux OS we run commands and programs by typing the commands in the Terminal program. However, sometimes when we run the command, we receive an error “bash :command not found”. . This error also occurs if you type the command incorrectly.

What is set e bash?

set -e stops the execution of a script if a command or pipeline has an error — which is the opposite of the default shell behaviour, which is to ignore errors in scripts. Type help set in a terminal to see the documentation for this built-in command.

What is bash set?

set is a shell builtin, used to set and unset shell options and positional parameters. Without arguments, set will print all shell variables (both environment variables and variables in current session) sorted in current locale. You can also read bash documentation.

Does finally run after catch?

The Rule. The finally block on a try / catch / finally will always run — even if you bail early with an exception or a return . This is what makes it so useful; it’s the perfect place to put code that needs to run regardless of what happens, like cleanup code for error-prone IO.

Should you always use try-catch?

Use try / catch blocks around code that can potentially generate an exception and your code can recover from that exception. In catch blocks, always order exceptions from the most derived to the least derived.

How do you use try-catch?

Place any code statements that might raise or throw an exception in a try block, and place statements used to handle the exception or exceptions in one or more catch blocks below the try block. Each catch block includes the exception type and can contain additional statements needed to handle that exception type.

What is Pipefail in bash?

The bash shell normally only looks at the exit code of the last command of a pipeline. . This particular option sets the exit code of a pipeline to that of the rightmost command to exit with a non-zero status, or to zero if all commands of the pipeline exit successfully.

How do I get out of Bash shell in terminal?

While in a (bash) terminal, you can simply type exit to leave the terminal and close the shell session.

How do I run a bash script?

1. 1) Create a new text file with a . sh extension. .
2. 2) Add #!/bin/bash to the top of it. This is necessary for the “make it executable” part.
3. 3) Add lines that you’d normally type at the command line. .
4. 4) At the command line, run chmod u+x YourScriptFileName.sh. .
5. 5) Run it whenever you need!

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