Bash – Color script output only when invoked from interactive shell

bashcolorsshell-script

I made a bash script producing colored output. The colors are fine when the script is called from an interactive shell. However, if the output is processed in another script, or passed through a pipe, it should not be colored (I think).

How is this handled usually? Should I provide an option to turn on/off the colored output, or is there a way to detect this automatically in the script? Preferably it works automatically and I only need a minimum of code to output colored / non-colored, depending on the script invocation…

One problem with automatic detection seems to be that a script always runs non-interactive, so I would have to know the interactive status of the parent shell instead.

Best Answer

It is common for unix application that do support colour output (such as grep and ls) to have a command line option as well as possibly automatic detection.

looking at man ls we see

--color[=WHEN]
              colorize the output.  WHEN defaults to `always' or can be `never' or `auto'.  More info below

So by default ls will always use colour, notice the auto option.

man grep shows similar

 --color[=WHEN], --colour[=WHEN]
              <snipped> WHEN is never, always, or auto.

If the auto option is enabled (which for your application could be the default) then something like the following answer (suggested by Gilles) may be what you are looking for https://unix.stackexchange.com/a/10065/4635

That is if your output is a terminal and supports colour, and the user hasn't disabled it you should use color. Of course the user using always or never skips this check.

I find it useful if programs have a --color=always even if they support automatic detection, as this means that I can force colour usage 'when I know better' (say my destination isn't a terminal but I know it supports colour'). On some systems I have an alias for ls to ls --color=always.

Related Solutions

Bash – How to pass environment variables to a non-interactive shell (with example)

If it is an sh script - as in, it explicitly references #!/bin/sh - which might still be bash but would be like invoking it with --posix --no-rc --no-profile - then you can specify the ENV file with the ENV environment variable:

ENV=/path/to/rcfile sh

Specific variables need either to be declared on the command-line - as above for $ENV - or else with export. For example, for $PATH you do:

export "PATH=$PATH:/some/more/paths"; sh

The $BASH_ENV variable you reference is not a file you need to source - and it isn't interpreted anyway when bash is invoked as sh - but is rather a path to a file that is sourced when a non-interactive bash shell is invoked - such as with a script that specifies the:

#!/bin/bash

...or whatever bang line.

Another way you might like to invoke your script/shell is with the env utility. It can be used to explicitly remove values from the environment, or else, as is usually easiest, to wipe it clean from the start:

env - BASH_ENV=/path/to/rcfile /usr/bin/bash /some/script/file

That will tell env to invoke the /usr/bin/bash command - with all its arguments appended - with the $BASH_ENV environment variable specified, but otherwise with a clean environment entirely.

Bash – Can an interactive shell become non-interactive or vice versa

The question I'd ask would be why would anyone want to do that?

You can disable some aspects of interactive shells like:

PS1= PS2= to disable the prompts
set +m to disable job control
disable history in some shells
you may be able to unload the zle and all completion modules in zsh.

But if you want the shell to stop being interactive, you could instead do:

. /some/file; exit

To tell it to get the rest of the commands from /some/file (replace with /dev/tty if you still want the commands to be read from the tty device), though there would still be some differences from non-interactive shells, like for the behaviour of return or the fact that it would still do job control or:

exec myshell /dev/tty

To replace your current interactive shell with a non-interactive one that still reads commands from the tty device.

Note that with bash 4.4, set +i returns with a bash: set: +i: invalid option like in most other shells.

Best Answer

Related Solutions

Bash – How to pass environment variables to a non-interactive shell (with example)

Bash – Can an interactive shell become non-interactive or vice versa

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