Testing Files

Usually, the test command is not used in full, but rather as a notation using square brackets and matching options:

if [ $? -eq 0 ];
# is the same as
if test $? -eq 0;

test is used to evaluate the type and timestamps of objects in the directory tree. It returns   if the tested condition is true. See Table 3 for test's options.

The test command is useful for making a distinction in an if construct. The range of applications is extensive. In a script for saving data, for example, it would be possible to check whether a file named BACKUP.INFO exists. If so, the script creates a copy of all files that are newer than this file's timestamp. Otherwise, the script creates a full backup.

Listing 3 shows the code for a script that creates a directory named BACKUPTEST if it doesn't already exist, quickly performing a common task (Figure 12).

#! /bin/bash
uvznew () {
  # FILE?
  if [ -f BACKUPTEST ]; then
    read -p "File with same name exists! Rename (y)? " we
    if [ "$we" = "y" ]; then
      mv BACKUPTEST BACKUPTEST.file
    else
      echo "Either delete or move the BACKUPTEST file!"
      echo "END OF SCRIPT"
    fi
  fi
  if [ -d BACKUPTEST ]; then
    cd BACKUPTEST
    echo -n "The current directory is $PWD"
    echo " "
  else
    echo "Creating BACKUPTEST"
    mkdir BACKUPTEST
  fi
  return 0
}
uvznew

Figure 12: With automation, the tester.sh script in Listing 3 handles tasks more quickly and reliably.

Timestamps and Rights

The find tool not only searches for file and directory names, but it also includes timestamps, access rights, and the file size as a filter if required. See find's man page to learn about the full scope of this command. For the most important find options, see Table 4.

If necessary, you can forward find's output to a pipe or process it using xargs, which passes the result to other commands, like tar. As an example, Listing 4 creates subdirectories for files as a function of their modification date and moves them there. Figure 13 shows the directory's contents before running the script. Figure 14 shows the script in action, and Figure 15 shows the output.

01 #!/bin/bash
02 # $1 = directory to process
03 if [ -z $1 ]; then
04   echo "No input"
05   exit
06 fi
07
08 cd $1
09
10 for i in $(stat -c %y:%n * | sort -r | tr \  \:); do
11   # Populate variables subdir (subdirectory)
12   # and fn (filename)
13   subdir=$(echo $i | cut -d \: -f1)
14   fn=$(echo $i | cut -d \: -f6)
15   # Do not process if directory
16   # restart loop
17   if [ -d $fn ]; then
18     echo "$fn: Skipping directory"
19     continue
20   fi
21   # Create subdir if needed, and
22   # move file to it
23   if [ -d $subdir ]; then
24     mv -v $fn $subdir
25   else
26     mkdir $subdir
27     mv -v $fn $subdir
28   fi
29 done

Figure 13: The directory content before running sortme.sh.

Figure 14: sortme.sh evaluates the metadata and sorts files into the appropriate directory structure.

Figure 15: Files end up in the corresponding subdirectories in the new directory structure.

The for loop receives the data courtesy of stat. Since spaces are used as separators, it replaces them with colons. The output is sorted by date, starting with the newest files.

In the for loop, the subdir variable contains the subdirectory to be created and fn takes the file name. The script evaluates whether or not fn is a directory and, in this instance, aborts processing. The loop then begins with a new pass. This prevents the script from processing a directory.

If, on the other hand, fn is a file, the routine then checks again by means of a test whether the subdirectory already exists. If this is not the case, it creates the directory and moves the file to it. If the directory already exists, it simply does the latter. Figure 15 shows a visual overview of tree.

Conclusions

With the appropriate shell commands, you can identify, evaluate, and change various file and directory metadata. As a result, many operations can be simplified using scripts, which avoid errors and save valuable time.