Question

I’m trying to check the type of a file using file, and then using the cut command to get only the type such as JPEG in my case, and then using it to check whether or not the file is the desired type. However whenever I run this in Shell it spits out pic1.jpg: Unexpected Operator. I’m not sure where the problem is and it’s been boggling me for a while now.

!#/bin/sh

file=$(file -F " " $1)

if [ $file = ERROR: ] || [ $file = empty ] then
    echo "$1 is not a valid jpeg file." >&2 >> error.log
else
    extension=$(file -F " " $1 | cut -f 3 -d " ")

Answer 1

Note that you’ll have to enclose $file in double quotes in the if statement. Otherwise the contents of $file will be interpreted as commands by the shell. This is one of the major pitfalls of shell coding you should be aware of.

In addition you where missing the ; between the ] and the then statements. Note that [ ... ] is the same as the test command. ] is just the last argument to it. Not ‘just a bracket’ like you would expect in ‘normal languages’ :). So, if there are two expressions on the same line the will have to be separated by an ;

Although I could suggest some enhancements to the I kept my example as much the same as yours to point you to the error. The following code will work:

#!/bin/sh

file=$(file -F " " $1)

if [ "$file" = "ERROR:" ] ||  [ "$file" = "empty" ] ; then
    echo "$1 is not a valid jpeg file." 1>&2 >> error.log
else
    extension=$(file -F " " $1 | cut -f 3 -d " ")
fi

echo "$extension"

Note that without double quotes your script would lead to the following if statement:

if [ test.sh  POSIX shell script, ASCII text executable = Error: ] || [ test.sh  POSIX shell script, ASCII text executable = empty ]