Restricted Shells

Advanced Bash-Scripting Guide:

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Chapter 21. Restricted Shells

Disabled commands in restricted shells

Running a script or portion of a script in restricted mode disables certain commands that would otherwise be available. This is a security measure intended to limit the privileges of the script user and to minimize possible damage from running the script.

Using cd to change the working directory.

Changing the values of the $PATH, $SHELL, $BASH_ENV, or $ENV environmental variables.

Reading or changing the $SHELLOPTS, shell environmental options.

Output redirection.

Invoking commands containing one or more /'s.

Invoking exec to substitute a different process for the shell.

Various other commands that would enable monkeying with or attempting to subvert the script for an unintended purpose.

Getting out of restricted mode within the script.

Example 21-1. Running a script in restricted mode

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Restricted Shells

#!/bin/bash

#Starting the script with "#!/bin/bash -r"

#runs entire script in restricted mode.

echo

echo "Changing directory." cd /usr/local

echo "Now in `pwd`"

echo "Coming back home." cd

echo "Now in `pwd`" echo

# Everything up to here in normal, unrestricted mode.

set -r

# set --restricted has same effect. echo "==> Now in restricted mode. <=="

echo echo

echo "Attempting directory change in restricted mode." cd ..

echo "Still in `pwd`"

echo echo

echo "\$SHELL = $SHELL"

echo "Attempting to change shell in restricted mode." SHELL="/bin/ash"

echo

echo "\$SHELL= $SHELL"

echo echo

echo "Attempting			to redirect output in restricted mode."
ls	-l	/usr/bin >	bin.files
ls	-l	bin.files	# Try to list attempted file creation effort.

echo

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Restricted Shells

exit 0

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Process Substitution

Advanced Bash-Scripting Guide:

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Chapter 22. Process Substitution

Process substitution is the counterpart to command substitution. Command substitution sets a variable to the result of a command, as in dir_contents=`ls -al` or xref=$( grep word datafile). Process substitution feeds the output of a process to another process (in other words, it sends the results of a command to another command).

Command substitution template

command within parentheses

>(command)

<(command)

These initiate process substitution. This uses /dev/fd/<n> files to send the results of the process within parentheses to another process. [1]

There is no space between the the "<" or ">" and the parentheses. Space there would give an error message.

bash$ echo >(true)

/dev/fd/63

bash$ echo <(true)

/dev/fd/63

Bash creates a pipe with two file descriptors, --fIn and fOut--. The stdin of true connects to fOut (dup2(fOut, 0)), then Bash passes a /dev/fd/fIn argument to echo. On systems lacking /dev/fd/<n> files, Bash may use temporary files. (Thanks, S.C.)

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Process Substitution

cat <(ls -l)
# Same as	ls -l | cat

sort -k 9 <(ls -l /bin) <(ls -l /usr/bin) <(ls -l /usr/X11R6/bin)

#Lists all the files in the 3 main 'bin' directories, and sorts by filename.

#Note that three (count 'em) distinct commands are fed to 'sort'.

diff <(command1) <(command2)

# Gives difference in command output.

tar cf >(bzip2 -c > file.tar.bz2) $directory_name

#Calls "tar cf /dev/fd/?? $directory_name", and "bzip2 -c > file.tar.bz2".

#Because of the /dev/fd/<n> system feature,

#the pipe between both commands does not need to be named.

#

# This can be emulated.

#

bzip2 -c < pipe > file.tar.bz2& tar cf pipe $directory_name

rm pipe

#or

exec 3>&1

tar cf /dev/fd/4 $directory_name 4>&1 >&3 3>&- | bzip2 -c > file.tar.bz2 3>&- exec 3>&-

#Thanks, S.C.

A reader of this document sent in the following interesting example of process substitution.

# Script fragment taken from SuSE distribution:

while read des what mask iface; do

#Some commands ...

done < <(route -n)

#To test it, let's make it do something. while read des what mask iface; do

echo $des $what $mask $iface done < <(route -n)

#Output:

#Kernel IP routing table

#Destination Gateway Genmask Flags Metric Ref Use Iface

#127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo

#As S.C. points out, an easier-to-understand equivalent is:

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Process Substitution

route -n	|
while read des what		mask iface; do # Variables set from output of pipe.
echo	$des $what $mask $iface
done	# Same output	as above.

Notes

[1]This has the same effect as a named pipe (temp file), and, in fact, named pipes were at one time used in process substitution.

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