exec

Understanding exec

Unlike commands that run as separate processes, exec replaces the current shell process with the specified command. This means that after exec is executed, the original shell process no longer exists. This affects script execution and resource management. The primary benefit is efficiency: it avoids the overhead of creating and managing a new process.

The basic syntax is straightforward:

exec command [arguments]

where command is the command to be executed, and arguments are any parameters passed to the command.

Executing a Single Command

Let’s start with a simple example: replacing the current shell with the date command:

exec date

After executing this command, your current shell session will terminate, and the output of date will be displayed. No further commands can be run in the same shell instance.

Redirecting Output with exec

exec works seamlessly with input/output redirection. For instance, to redirect the output of a command to a file:

exec ls -l > file_listing.txt

This replaces the current shell process with ls -l, and its output is redirected to file_listing.txt. The file will contain the long listing of the current directory.

Executing Scripts with exec

exec can be used to run shell scripts, replacing the current shell with the script’s process:

exec ./my_script.sh

This assumes my_script.sh is an executable shell script in the current directory. The script will run, and once it completes, the shell session will end.

Combining exec with other commands

The power of exec is amplified when combined with other shell features. Consider this example using a loop and exec to run a series of commands:

for i in {1..5}; do
  exec echo "Iteration: $i"
done

This loop will print “Iteration: 1” to “Iteration: 5”, each on a new line, and the shell will terminate after the fifth iteration. Note that each iteration replaces the previous one completely.

exec and File Descriptors

exec is also useful for manipulating file descriptors. You can redirect standard input, output, and error using the <, >, and 2> operators respectively.

exec 3> my_error_log.txt   #Redirect stderr to file descriptor 3
exec 2>&3                  #Redirect stderr (fd 2) to fd 3 (which points to the log file)
exec my_command_that_might_error  #This command's error output goes to my_error_log.txt

This example redirects standard error to a separate file, allowing for better error handling and debugging.

Replacing a single file descriptor

Instead of replacing the entire shell process, exec can also be used to replace a specific file descriptor. For example to redirect stdout to a file without replacing the current process:

exec 1> my_output.txt
echo "This will be written to my_output.txt"

The possibilities with exec are numerous. It’s a powerful tool for streamlining shell scripts and fine-tuning process behavior. Its ability to replace the current shell process must be carefully considered, as it results in the termination of that shell session. Remember to use it judiciously and understand it before incorporating it into your scripts.