Bash Files – Difference Between `if [ -a FILE ]` and `if [ -e FILE ]`

bashfiles

I only want to check in bash if a certain FILE exists.
On this page two options are mentioned to use with if:

[ -a FILE ] – True if FILE exists
[ -e FILE ] – True if FILE exists

Are they equivalent or is there any difference between them?

Best Answer

What does -a do and why does it exist ?

The -a option is the same thing as -e, and exists for compatibility with Korn Shell from which Bash borrowed a lot of features.

From POSIX standard, description of test command(link):

An early proposal used the KornShell -a primary (with the same meaning), but this was changed to -e because there were concerns about the high probability of humans confusing the -a primary with the -a binary operator.

side note: binary means flag that appears between two variables [ $var1 -a $var2 ], primary means appearing in the list of arguments first as in [ -a $var ]

In fact, Korn Shell (ksh93 here) manual states:

-a file

Same as -e below. This is obsolete.

The test command that is used in the TLDP article you referenced uses bash built-in test , however the option is also present in /usr/bin/test despite missing from documentation:

$ /usr/bin/test -a /etc/passwd  && echo 1                                   
1

If functionality is the same, is it good idea to use it?

So long as you are 100% sure your scripts will be used on either Bash or Korn Shell - then yes, it is alright. However, if you strive for portability of your scripts and want to write script the Right Way^TM, you should use -e. The standard Ubuntu shell, /bin/sh, which is actually Dash - Debian Amquist Shell - doesn't recognize that as valid option:

$ dash
$ test -a /etc/passwd
dash: 1: test: -a: unexpected operator

In cases where you want to port your script to other platforms, using -a is not guaranteed to be safe with /usr/bin/test as well, and you should stick with -e because it is in fact specified by POSIX standard.

Related Solutions

bash – Difference Between ‘source x’, ‘. x’ and ‘./x’ in Bash

Before answering, I think some clarifications are needed. Let's analyze the following three lines:

source run.sh
. run.sh
./run.sh

The first two lines are exactly identical: . is in fact an alias for source. What source does is executing the shell script in the current context, hence a call to exit will quit the shell.

The third line (which is the one that confuses you) has however nothing to do with the other lines. ./run.sh is just a path, and is the same as (for example) /home/user/run.sh or /usr/bin/something. Always remember that commands in the shell are separated by a space. So, in this case, the command is not ., but is ./run.sh: this means that a sub-shell will be executed and that the exit will have effect just to the sub-shell.

Bash Scripts – Difference Between #!/bin/sh and #!/bin/bash

bash and sh are two different shells. Basically bash is sh, with more features and better syntax. Most commands work the same, but they are different.

Having said that, you should realize /bin/sh on most systems will be a symbolic link and will not invoke sh. In Ubuntu /bin/sh used to link to bash, typical behavior on Linux distributions, but now has changed to linking to another shell called dash. I would use bash, as that is pretty much the standard (or at least most common, from my experience). In fact, problems arise when a bash script will use #!/bin/sh because the script-maker assumes the link is to bash when it doesn't have to be.

For more info, http://man.cx/sh, http://man.cx/bash.