Bourne shell commands

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You can issue commands in the Bourne shell.

When you issue a command in the Bourne shell, it first evaluates the command and makes all indicated substitutions. It then runs the command provided that:
  • The command name is a Bourne shell special built-in command.

    OR

  • The command name matches the name of a defined function. If this is the case, the shell sets the positional parameters to the parameters of the function.
If the command name matches neither a built-in command nor the name of a defined function and the command names an executable file that is a compiled (binary) program, the shell (as parent) creates a new (child) process that immediately runs the program. If the file is marked executable but is not a compiled program, the shell assumes that it is a shell procedure. In this case, the shell creates another instance of itself (a subshell), to read the file and execute the commands included in it. The shell also runs a parenthesized command in a subshell. To the user, a compiled program is run in exactly the same way as a shell procedure. The shell normally searches for commands in file system directories in this order:
  1. /usr/bin
  2. /etc
  3. /usr/sbin
  4. /usr/ucb
  5. $HOME/bin
  6. /usr/bin/X11
  7. /sbin
  8. Current directory
The shell searches each directory, in turn, continuing with the next directory if it fails to find the command.
Note: The PATH variable determines the order in which the shell searches directories. You can change the particular sequence of directories searched by resetting the PATH variable.

If you give a specific path name when you run a command (for example, /usr/bin/sort), the shell does not search any directories other than the one you specify. If the command name contains a slash (/), the shell does not use the search path.

You can give a full path name that begins with the root directory (such as /usr/bin/sort). You can also specify a path name relative to the current directory. If you specify, for example: 
bin/myfile
the shell looks in the current directory for a directory named bin and in that directory for the file myfile.
Note: The restricted shell does not run commands containing a slash (/).

The shell remembers the location in the search path of each executed command (to avoid unnecessary exec commands later). If it finds the command in a relative directory (one whose name does not begin with /), the shell must redetermine the command's location whenever the current directory changes. The shell forgets all remembered locations each time you change the PATH variable or run the hash -r command.