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Shell Programming and Scripting

Script does not stop when doing a read

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Top Forums Shell Programming and Scripting Script does not stop when doing a read
# 8  
Old 11-05-2007
The use of the null ":" command
The colon ":" as a NULL command can be quite useful for debug output. A line that begins with ":" is not executed, but it is evaluated. So instead of this:

Code:
echo "The value of VAL is '$VAL'."

do this:

Code:
: The value of VAL is '$VAL'.

The latter will only show when "set -x" is used, and only once; the former show up twice in the debug output.

BUT, since it is evaluated, care must be taken. For instance:

Code:
: This won't work!

The apostophe in "won't" is evaluated and treated like an unmatched single-quote.
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LEARN ABOUT LINUX

script

SCRIPT(1)						    BSD General Commands Manual 						 SCRIPT(1)

NAME

     script -- make typescript of terminal session

SYNOPSIS

     script [-a] [-c COMMAND] [-e] [-f] [-q] [-t] [file]

DESCRIPTION

     Script makes a typescript of everything printed on your terminal.	It is useful for students who need a hardcopy record of an interactive
     session as proof of an assignment, as the typescript file can be printed out later with lpr(1).

     If the argument file is given, script saves all dialogue in file.	If no file name is given, the typescript is saved in the file typescript.

     Options:

     -a      Append the output to file or typescript, retaining the prior contents.

     -c COMMAND
	     Run the COMMAND rather than an interactive shell.	This makes it easy for a script to capture the output of a program that behaves
	     differently when its stdout is not a tty.

     -e      Return the exit code of the child process. Uses the same format as bash termination on signal termination exit code is 128+n.

     -f      Flush output after each write. This is nice for telecooperation: One person does `mkfifo foo; script -f foo' and another can super-
	     vise real-time what is being done using `cat foo'.

     -q      Be quiet.

     -t      Output timing data to standard error. This data contains two fields, separated by a space. The first field indicates how much time
	     elapsed since the previous output. The second field indicates how many characters were output this time. This information can be used
	     to replay typescripts with realistic typing and output delays.

     The script ends when the forked shell exits (a control-D to exit the Bourne shell (sh(1)), and exit, logout or control-d (if ignoreeof is not
     set) for the C-shell, csh(1)).

     Certain interactive commands, such as vi(1), create garbage in the typescript file.  Script works best with commands that do not manipulate
     the screen, the results are meant to emulate a hardcopy terminal.

ENVIRONMENT

     The following environment variable is utilized by script:

     SHELL  If the variable SHELL exists, the shell forked by script will be that shell. If SHELL is not set, the Bourne shell is assumed. (Most
	    shells set this variable automatically).

SEE ALSO

     csh(1) (for the history mechanism), scriptreplay(1).

HISTORY

     The script command appeared in 3.0BSD.

BUGS

     Script places everything in the log file, including linefeeds and backspaces.  This is not what the naive user expects.

AVAILABILITY

     The script command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.

Linux								   July 30, 2000							     Linux