Loop through * files using $1 in bash script

Question

I am writing a script to act on files given to the script using the $1 variable. What is the best way to loop through these files. At the moment I have hard coded the script using something like

for file in *.pdf
do 
   act on $file
done

However I would rather do this using the $1 variable Thanks Barry

Answer 1

$1 is simply the first parameter passed to your shell script. So if you have a script myscript and I call it like this:

myscript somefile

Then $1 will be somefile. If I call it like this:

myscript *

And there happen to be files in my current directory named:

foo bar baz

Then $1 will be foo. In either case, $1 only has a single value and there is nothing to loop over.

There are a couple of ways to loop over the arguments. The most common is:

for file in "$@"; do
  echo doing something with "$file"
done

Note that $@ is not the same as "$@"; the latter is a special construct that will correctly deal with filenames that contain spaces. You could also do something like:

while [ -n "$1" ]; do
  echo doing something with "$1"
  shift
done

Here, the shift command drops the first parameter, and $2 becomes $1 (and so on). This will loop as long as $1 is non-empty. You could also use $# for this (a variable which contains the number of command line arguments):

while [ $# -gt 0 ]; do
    echo arg: "$1"
    shift
done

Answer 2

If you do

script *.pdf

*.pdf is already expanded on the command line, and $1 will only refer to the first file

for file in $1

will therefore not do what you need. You could use

for file in "$@"

instead.

See the section Special Parameters in the man bash