Bash Shell Environment Variables – Issue an Error When Using Empty Shell Variables

bashenvironment-variablesshell

Sometimes I use, $PROJECT_HOME/* to delete all files in the project. When the environment variable, PROJECT_HOME is not set (because I did su and the new user doesn't have this environment variable set), it starts deleting all files from the root folder. This is apocalyptic.

How can I configure bash to throw error, when I use an undefined environment variable in the shell?

Best Answer

In POSIX shell, you can use set -u:

#!/bin/sh

set -u
: "${UNSET_VAR}"

or using Parameter Expansion:

: "${UNSET_VAR?Unset variable}"

In your case, you should use :? instead of ? to also fail on set but empty variables:

rm -rf -- "${PROJECT_HOME:?PROJECT_HOME empty or unset}"/*

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Bash – how to prevent exported environment variables to be recognized/accessible by a child shell (bash)

You can use the env command to start a process with a clean environment. Here is an example:

env -i /bin/myprog

If you still need to export some variables, you can do so in the call:

env -i MYVAR=foo /bin/myprog

The env command is required by POSIX.

Bash – Environment Variables Not Set in Pipeline Function

Each part of a pipeline (i.e. each side of the pipe) runs in a separate process (called a subshell, when a shell forks a subprocess to run part of the script). In par_set PIPE FAILS |sed -e's/FAILS/BARFS/', the PIPE variable is set in the subprocess that executes the left-hand side of the pipe. This change is not reflected in the parent process (environment variables do not transfer between processes, they are only inherited by subprocesses.

The left-hand side of a pipe always runs in a subshell. Some shells (ATT ksh, zsh) run the right-hand side in the parent shells; most also run the right-hand side in a subshell.

If you want to both redirect the output of a part of the script and run that part in the parent shell, in ksh/bash/zsh, you can use process substitution.

par_set PROCESS SUBSTITUTION > >(sed s/ION/ED/)

With any POSIX shell, you can redirect the output to a named pipe.

mkfifo f
<f grep NAMED= &
par_set NAMED PIPE >f

Oh, and you're missing quotes around variable substitutions, your code breaks on things like par_set name 'value with spaces' star '*'.

export "${PAR}=${VAL}"
…
par_set "$@"

Best Answer

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