bash-variables-env-vars.md

bash variables and env vars

set vs export

set will show all variables, env will only show variables that are exported to subprocesses.

Variables assigned in .bashrc will be only available to the current shell, and not exported to subprocesses, eg: HISTSIZE

Set a variable in the context of the current shell only:

# foo1 is not exported, so isn't in the environment of a subprocess
foo1=bar && python -c 'import os; print(os.environ["foo1"])'

KeyError: 'foo1'

echo $foo1
bar

# foo1 is not exported
env | grep -i foo1

Export a shell variable and make it available to child processes:

export foo2=bar && python -c 'import os; print(os.environ["foo2"])'
bar

echo $foo2
bar

Modify the environment of a subprocess started from the shell:

foo3=bar python -c 'import os; print(os.environ["foo3"])'
bar

# equivalent
env foo3=bar python -c 'import os; print(os.environ["foo3"])'
bar

# NB: but foo3 is not set in the shell
echo $foo3

# Take the COLUMNS shell var and export it to a subprocess that is piped (defaults to 80 when stdout is piped)
COLUMNS=$COLUMNS python -c 'import shutil; print(shutil.get_terminal_size())' | head
os.terminal_size(columns=151, lines=24)

Note that echo is a bash builtin so behaves differently:

# echo can see shell variables
foo4=bar && echo $foo4
bar

# but not here because echo runs in the current process
foo4=bar echo $foo4

References

Difference between set, export and env in bash

DYLD_LIBRARY_PATH weirdness

This is strange:

$ export DYLD_LIBRARY_PATH=/usr/local/Cellar/gcc@6/6.5.0_2/lib/gcc/6/
$ set | grep -i dyld
DYLD_LIBRARY_PATH=/usr/local/Cellar/gcc@6/6.5.0_2/lib/gcc/6/

# not exported, why?
$ env | grep -i dyld

If I remove the underscores, ie:DYLDLIBRARYPATH, it exports.