-r filename && . filename

Question

i was just looking into a script i find this line. can anyone tell me what this line do. this is the code line.

[ -r "$PosConfigDir/posconfig.sh" ] && . "$PosConfigDir/posconfig.sh"

Answer 1

It checks whether you have permission to read the file $PosConfigDir/posconfig.sh, after symlink resolution, then sources that file.

It does not guarantee that you can read the file. For example, on most systems, [ -r / ] would return true but you cannot read /.

-r is a standard test operator.

. is a special builtin command of Bourne-like shells that tells the shell to read and interpret the code in the file given as argument. source is the equivalent command in csh. Many modern Bourne-like shells understand source as an alias for ..

With some shell implementations, the failure of . to open or read the file would cause the shell to abort (after having output an error message). The -r check is presumably meant to prevent that in that case.

To prevent the shell from aborting, in POSIX shells, an alternative could be

 command . "$PosConfigDir/posconfig.sh"

(Though the above construct won't work in zsh unless in sh or ksh emulation)

To avoid the error message, you'd need to redirect the stderr output to /dev/null, but that would have the unwanted side-effect of also silencing the errors of the commands in postconfig.sh.

An example:

$ [ -r /etc/os-release ] && . /etc/os-release
$ printf '%s\n' "$PRETTY_NAME"
Debian GNU/Linux 8 (jessie)

PRETTY_NAME was defined in /etc/os-release file:

$ head -n 1 /etc/os-release 
PRETTY_NAME="Debian GNU/Linux 8 (jessie)"