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Full Discussion: Bash: How can a script create a script?
Top Forums Shell Programming and Scripting Bash: How can a script create a script? Post 302604892 by LessNux on Tuesday 6th of March 2012 09:05:06 AM
Old 03-06-2012
Question Bash: How can a script create a script?
I need to write a bash script that will generate another bash script. Furthermore, the child script is required to be hard-coded into the parent script. The parent script is not allowed to import the child script from an external file at run time. If you have a better solution than the following, please let me know.

I tried "cat" in the following manner.

cat > child.sh << EOF
The content of child script
...
...
EOF


However, if the child script contains variables, command substitutions and backtick pipes as shown below, then bash will expand them when the parent script is executed. Thus, the resultant child script will be corrupted and differ from the original child script.

Code:
cat > child.sh << EOF
#...
#...
OriginalDir=$(pwd)  #command substitution
cd $CloudDir        #variable
Array=`ls -1 *.txt` #backtick pipe
cd $OriginalDir     #variable
#...
#...
EOF


So, I have to somehow disable variable expansion, command substitution and backtick pipe. I thought about single quotes to disable expansion.

echo '.......' > child.sh
echo '.......' >> child.sh
echo '.......' >> child.sh

However, if the original child script contains single quotes, they will interfere with the outer single quotes, and hence the resultant child script will corrupt. Single quotes do not work.

In the above cat method, the expansion were interfering with $ and `. Now, my workaround is to temporarily remove $ and ` by applying percent-encoding to $ and `. That is, $ is replaced with %24, and ` is replaced with %60, (after % is replaced with %25 first) before the child script is embedded into the parent script. All the other characters are intact. Thus, the child script is prepared by sed like below.

sed "s/%/%25/g" | sed 's/\$/%24/g' | sed 's/`/%60/g'

At run time, the parent script will decode percents and restore the original version of the child script by piping the output from cat to sed in the following manner.

Code:
cat << EOF | sed 's/%60/`/g' | sed "s/%24/$/g" | sed "s/%25/%/g" > child.sh
#...
#...
OriginalDir=%24(pwd)    #command substitution
cd %24CloudDir          #variable
Array=%60ls -1 *.txt%60 #backtick pipe
cd %24OriginalDir       #variable
#...
#...
EOF


In this last method, the percent-encoding protects the variables, command substitutions and backtick pipes in the child script from expansion. However, the percent-encoding modifies the child script, even though the modification is temporary and the original version will be restored at the end. I prefer not to modify the child script at all.

In bash, how can a parent script generate a child script without the kind of preparation that modifies the child script? Does anyone know how to protect the variables, command substitutions and backtick pipes in the child script from expansion without encoding the child script? Note again that the child script is required to be hard-coded into the parent script. The parent script is not allowed to import the child script from an external file at run time. Any idea?

Many thanks in advance.
 

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LEARN ABOUT PLAN9

script

SCRIPT(1)							   User Commands							 SCRIPT(1)

NAME

       script - make typescript of terminal session

SYNOPSIS

       script [options] [file]

DESCRIPTION

       script makes a typescript of everything displayed 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 the dialogue in this file.  If no filename is given, the dialogue is saved in  the  file  type-
       script.

OPTIONS

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

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

       --force
	      Allow  the  default output destination, i.e. the typescript file, to be a hard or symbolic link.	The command will follow a symbolic
	      link.

       -q, --quiet
	      Be quiet (do not write start and done messages to standard output).

       -t[file], --timing[=file]
	      Output timing data to standard error, or to file when given.  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.

       -V, --version
	      Display version information and exit.

       -h, --help
	      Display help text and exit.

NOTES

       The script ends when the forked shell exits (a control-D for 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.

       It is not recommended to run script in non-interactive shells.  The inner shell of script is always interactive, and  this  could  lead	to
       unexpected  results.   If  you  use  script in the shell initialization file, you have to avoid entering an infinite loop.  You can use for
       example the .profile file, which is read by login shells only:

	      if test -t 0 ; then
		  script
		  exit
	      fi

       You should also avoid use of script in command pipes, as script can read more input than you would expect.

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.

       script  is  primarily  designed for interactive terminal sessions.  When stdin is not a terminal (for example: echo foo | script), then the
       session can hang, because the interactive shell within the script session misses EOF and script has no clue when to close the session.  See
       the NOTES section for more information.

AVAILABILITY

       The  script  command  is  part of the util-linux package and is available from Linux Kernel Archive <https://www.kernel.org/pub/linux/utils
       /util-linux/>.

util-linux							     June 2014								 SCRIPT(1)
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