Essential System Administration, 3rd Edition

Other Useful Commands

This section briefly describes other commands that you may encounter in system scripts.

set

The set command sets the values of $1 through $n to the words given as its arguments. It is often used with a backquoted command to assign the argument identifiers to the command’s output. Here is an example of its use:

$ who -r 
.  run-level 2  Aug 21 16:58   2   0    S 
$ set `who -r` 
$ echo $6 
16:58

The unset command may be used to remove a defined variable.

eval

The eval command executes its argument as a shell command. It is used to execute commands generated by multiple levels of indirection. Here is a silly example:

$ a=c; b=m; c=d; cmd=date 
$ echo $a$b$c
cmd
$ eval $`echo $a$b$c` 
Sun Jun  3 19:37:30 EDT 2001

Here is a real example that we looked at in Chapter 12:

$ eval `tset -sQ -m ":?vt100"`

This eval command runs the commands generated by tset -s. As we say, they are used to set the TERM and TERMCAP environment variables.

The command eval resize provides a similar example for xterm windows.

printf

The printf command is used to produce formatted output strings, and you will occasionally see it used in system scripts. It takes two arguments: a format-specification string and a list of items to be printed using that format. Here is an example command used to create a record in a printer accounting file:

# pages=21; host=hamlet; user=chavez
# printf '%7.2f\t%s:%s\n' "$pages" "$host" "$user"
   21.00       hamlet:chavez

This command creates a line in which the number of pages is printed as a floating ...