exec - Linux

Overview

The exec command in Linux is used to replace the current shell process with a new process specified by the command or script provided by the user without creating a new process. It is especially useful for altering the executing process directly, managing shell scripts’ behavior, or setting up environment variables globally for the shell.

Syntax

The basic syntax of the exec command is as follows:

exec [OPTIONS] [COMMAND] [ARGUMENTS...]

If COMMAND is specified, the exec command replaces the current shell with COMMAND. If no COMMAND is provided, any redirections take effect in the current shell.

Options/Flags

exec does not have many distinct options, but it prominently handles redirections and environment adjustments for the process that replaces the shell or the current shell.

Redirection: Used in the format >, <, >>, etc., which redirects the input/output accordingly.
Environment Variables: You can set environment variables that will affect the behavior of the script or command run by exec.

For instance:

exec 2> my-errors.log

Redirects standard error to my-errors.log.

Examples

Replacing the shell with a Python script:
```
exec python myscript.py
```
This command replaces the current shell with a Python script.
Changing the file descriptor:
```
exec 3< myfile.txt
```
This command opens myfile.txt for reading on file descriptor 3.
Environment modification:
```
exec PATH=$PATH:/new/path bash
```
This modifies the PATH environment variable and starts a new bash session with the changed path.

Common Issues

Exit Behavior: After exec COMMAND is run, the shell is immediately replaced and does not return to the original command line. If the command fails, you won’t return to your shell.
Script Execution Failures: If exec fails to execute the command, the script or shell using exec will also terminate.

Workaround:
Always check command availability and correct syntax before using it with exec.

Integration

The exec command is commonly integrated in shell scripts to modify the shell environment permanently during the script execution or replace the shell with other processes for efficient resource usage.

Example:

#!/bin/bash
exec java -jar myapp.jar

In this script, the Java application replaces the bash script process, freeing up any overhead of the shell.

sh – Command interpreter (shell)
bash – Bourne-Again SHell, an extension of sh
env – Runs a program in a modified environment

To learn more about the concepts behind exec and detailed behavior, the bash man page (man bash) can be consulted, particularly the sections about shell built-ins and process handling.