Bash Programming

Things under legendu.net/outdated are outdated technologies that the author does not plan to update any more. Please look for better alternatives.

Fish Shell is preferred to Bash/Zsh. The following content is for Bash/Zsh only.

Parse Arguments in Bash¶

Environment Variables¶

export

unset

Tips and Traps¶

1. explainshell.com is a great place for learning shell.

2. Bash-it/bash-it is a great community driven Bash framework.

3. It is suggested that you avoid writing complicated Bash scripts. IPython is a much better alternative.

4. Do NOT use ; to delimit paths passed to a shell command because ; terminates shell commands. Use , to delimit paths if needed.

5. $(some_command) is preferred over some_command. For more details, please refer to http://mywiki.wooledge.org/BashFAQ/082.

Redirect Output¶

1. Redirect stdout to the file output and stderr to another file error.

    command > output 2> error

2. Redirect stderr to stdout (&1), and then redirect stdout to the file output.

    command > output 2>&1

3. Redirect both stdout and stderr to the file output.

    command &> output

4. It seems to me that tee does not work well sometimes. Basically, I have a Python script calling shell comamnds using subprocess.call. The output of the shell commands cannot be captured by tee. However, redirection using > partially works. Output of git commands (invoked by subprocess.call) cannot be redirected correctly. Actually, it turned out to be that both > and tee only works on a file in the current directly. And even so, some output of the git command invoked by subprocess.call is not captured. I have no idea why that happens. Not sure whether this is a bug in Mac, the Bash shell or due to the fact that I mixed shell commands and the print function in Python.

5. If there are logging from difference resources (e.g., both shell comamnd via subprocess.call and the print function in Python) to the standard output, make sure you flush the console so that output/error redirection works as expected.

References¶

http://www.legendu.net/misc/blog/bash-loops/

http://www.legendu.net/misc/blog/bash-functions/

http://www.legendu.net/misc/blog/environment-variables-in-shell/

http://www.legendu.net/misc/blog/autocomplete-for-bash-in-mac/

https://askubuntu.com/questions/625224/how-to-redirect-stderr-to-a-file

https://askubuntu.com/questions/639877/tee-doesnt-get-whole-output-from-the-pipe

https://unix.stackexchange.com/questions/306111/what-is-the-difference-between-the-bash-operators-vs-vs-vs

https://stackoverflow.com/questions/2953646/how-can-i-declare-and-use-boolean-variables-in-a-shell-script