CODEWITHSUNDEEP
pythonbash
MUG4N
MUG4N

Reputation: 19727

Bash script which sets pythonpath

The mssql-cli uses the following bash script to execute the actual python script. As I understand the code, the while loop determines the current directory of the script executed, this path gets then added to PYTHONPATH. There are no .py files in the current directory so why is the path added to PYTHONPATH? Could someone please explain to me what the first part of the script is doing. Thank you for helping me out here.

#!/bin/bash

SOURCE="${BASH_SOURCE[0]}"
while [ -h "$SOURCE" ]; do # resolve $SOURCE until the file is no longer a symlink
  DIR="$( cd -P "$( dirname "$SOURCE" )" && pwd )"
  SOURCE="$(readlink "$SOURCE")"
  [[ $SOURCE != \/* ]] && SOURCE="$DIR/$SOURCE" # if $SOURCE was a relative symlink, we need to resolve it relative to the path where the symlink file was located
done
DIR="$( cd -P "$( dirname "$SOURCE" )" && pwd )"

# Set the python io encoding to UTF-8 by default if not set.
if [ -z ${PYTHONIOENCODING+x} ]; then export PYTHONIOENCODING=utf8; fi

export PYTHONPATH="${DIR}:${PYTHONPATH}"

python -m mssqlcli.main "$@"

Upvotes: 0

Views: 4680

Answers (1)

Ondrej K.
Ondrej K.

Reputation: 9679

Wonder if this is still relevant for you, but I've marked it as fun puzzle to revisit later... Long story short: It adds directory of location where this script file is with all symbolic links resolved (neither the acquired filename nor a directory leading up to is is a symbolic link) to PYTHONPATH.

It's basically the same thing as doing so using readlink (or realpath):

export PYTHONPATH="$(dirname $(readlink -f ${BASH_SOURCE})):${PYTHONOATH}"

Line by line dissection:

SOURCE="${BASH_SOURCE[0]}"

This sets SOURCE to be path with which this script was called or sourced.

while [ -h "$SOURCE" ]; do # resolve $SOURCE until the file is no longer a symlink

We enter the wile loop if SOURCE path refers to a symbolic link. I.e. in the first iteration of this file was a symbolic link. Subsequently if this was a link pointing to another link.

  DIR="$( cd -P "$( dirname "$SOURCE" )" && pwd )"

This (a bit simplified explanation of -P) changes into directory where SOURCE is resolving symbolic links along the way (i.e. lands in the directory the link(s) was/were pointing to) and prints working directory after that change (absolute path). All that happens in a subshell and result is assign to variable DIR.

  SOURCE="$(readlink "$SOURCE")"

SOURCE is assigned a new value of path resulting from symlink resolution. Literally a target the link points to (as seen by for instance ls -l) relative or absolute.

  [[ $SOURCE != \/* ]] && SOURCE="$DIR/$SOURCE" # if $SOURCE was a relative symlink, we need to resolve it relative to the path where the symlink file was located

If the SOURCE value we have obtained by symbolic link resolution does not begin with / (i.e. is an absolute path), DIR (directory where the SOURCE with which we have entered the loop resides) and resolved symbolic link SOURCE are concatenated over / to form a new SOURCE (we make it into an absolute path) and we go back to the top of this loop. NOTE: escaping of / by \ seems in this case unnecessary and arbitrary.

done

When done. SOURCE points to a file that is not a symblic link. It's path may still contain symbolic links at this point which is taken care of in the next step.

DIR="$( cd -P "$( dirname "$SOURCE" )" && pwd )"

Once more, like in the loop. DIR should now be pointing to a directory where resolved (not a symlink) SOURCE file (target of what was originally called/sourced) resides.

# Set the python io encoding to UTF-8 by default if not set.
if [ -z ${PYTHONIOENCODING+x} ]; then export PYTHONIOENCODING=utf8; fi

Exports an environmental variable if a shell variable was not set or equals to an empty string. NOTE: ${PYTHONIOENCODING+x} seems to be an alternative form of ${PYTHONIOENCODING:+x} and its use seems absolutely arbitrary. There is also a test to check if variable was set (regardless of its value).

export PYTHONPATH="${DIR}:${PYTHONPATH}"

PYTHONPATH is now set to start with an absolute resolved path (no symbolic links should be anywhere along the path) of where does this very script (or file this link points to) reside.

python -m mssqlcli.main "$@"

Calls python...

Upvotes: 2

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