[shell] How to check if a file exists in a shell script

I'd like to write a shell script which checks if a certain file, archived_sensor_data.json, exists, and if so, deletes it. Following http://www.cyberciti.biz/tips/find-out-if-file-exists-with-conditional-expressions.html, I've tried the following:

[-e archived_sensor_data.json] && rm archived_sensor_data.json

However, this throws an error

[-e: command not found

when I try to run the resulting test_controller script using the ./test_controller command. What is wrong with the code?

Here is an alternative method using ls:

(ls x.txt && echo yes) || echo no

If you want to hide any output from ls so you only see yes or no, redirect stdout and stderr to /dev/null:

(ls x.txt >> /dev/null 2>&1 && echo yes) || echo no

The backdrop to my solution recommendation is the story of a friend who, well into the second week of his first job, wiped half a build-server clean. So the basic task is to figure out if a file exists, and if so, let's delete it. But there are a few treacherous rapids on this river:

• Everything is a file.

• Scripts have real power only if they solve general tasks

• To be general, we use variables

• We often use -f force in scripts to avoid manual intervention

• And also love -r recursive to make sure we create, copy and destroy in a timely fashion.

Consider the following scenario:

We have the file we want to delete: filesexists.json

This filename is stored in a variable

<host>:~/Documents/thisfolderexists filevariable="filesexists.json"

We also hava a path variable to make things really flexible

<host>:~/Documents/thisfolderexists pathtofile=".."

<host>:~/Documents/thisfolderexists ls $pathtofile

filesexists.json  history20170728  SE-Data-API.pem  thisfolderexists

So let's see if -e does what it is supposed to. Does the files exist?

<host>:~/Documents/thisfolderexists [ -e $pathtofile/$filevariable ]; echo $?

0

It does. Magic.

However, what would happen, if the file variable got accidentally be evaluated to nuffin'

<host>:~/Documents/thisfolderexists filevariable=""

<host>:~/Documents/thisfolderexists [ -e $pathtofile/$filevariable ]; echo $?

0

What? It is supposed to return with an error... And this is the beginning of the story how that entire folder got deleted by accident

An alternative could be to test specifically for what we understand to be a 'file'

<host>:~/Documents/thisfolderexists filevariable="filesexists.json"

<host>:~/Documents/thisfolderexists test -f $pathtofile/$filevariable; echo $?

0

So the file exists...

<host>:~/Documents/thisfolderexists filevariable=""

<host>:~/Documents/thisfolderexists test -f $pathtofile/$filevariable; echo $?

1

So this is not a file and maybe, we do not want to delete that entire directory

man test has the following to say:

-b FILE

       FILE exists and is block special

-c FILE

       FILE exists and is character special

-d FILE

       FILE exists and is a directory

-e FILE

       FILE exists

-f FILE

       FILE exists and is a regular file

...

-h FILE

       FILE exists and is a symbolic link (same as -L)

Internally, the rm command must test for file existence anyway,
so why add another test? Just issue

rm filename

and it will be gone after that, whether it was there or not.
Use rm -f is you don't want any messages about non-existent files.

If you need to take some action if the file does NOT exist, then you must test for that yourself. Based on your example code, this is not the case in this instance.

If you're using a NFS, "test" is a better solution, because you can add a timeout to it, in case your NFS is down:

time timeout 3 test -f 
/nfs/my_nfs_is_currently_down
real    0m3.004s <<== timeout is taken into account
user    0m0.001s
sys     0m0.004s
echo $?
124   <= 124 means the timeout has been reached

A "[ -e my_file ]" construct will freeze until the NFS is functional again:

if [ -e /nfs/my_nfs_is_currently_down ]; then echo "ok" else echo "ko" ; fi

<no answer from the system, my session is "frozen">