If you use sudo to run commands as root, you've probably run into “permission denied” problems when only part of a pipeline or part of a command is running with root permissions.
This fails with “permission denied” because the file is writable only by root:
$ echo 12000 > /proc/sys/vm/dirty_writeback_centisecs
But, this fails too:
$ sudo echo 12000 > /proc/sys/vm/dirty_writeback_centisecs
Why? The /bin/echo program is running as root, because of sudo, but the shell that's redirecting echo's output to the root-only file is still running as you. Your current shell does the redirection before sudo starts.
The solution is to run the whole pipeline under sudo. There are a couple ways to do it, but I prefer:
echo "echo 12000 > /proc/sys/vm/dirty_writeback_centisecs" | sudo sh
That way, I can type everything before the pipe character, and see what I'm about to run as root, then press the up arrow and add the | sudo sh to do it for real. This is not a big deal for short, obvious pipelines, but when you're building up a more complicated command as root, it's safer to look at it first before you run it.
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December 2009, #188
If last month's Infrastrucuture issue was too "big" for you then try on this month's Embedded issue. Find out how to use Player for programming mobile robots, build a humidity controller for your root cellar, find out how to reduce the boot time of your embedded system, and if you're new to embedded systems find out the basics that go into one. You can also read about the Beagle Board, the Mesh Potato and a spate of other interestingly named items. And along with our regular columns don't miss our new monthly column: Economy Size Geek.
Why? The /bin/echo program
On April 18th, 2009 boediger (not verified) says:
Why? The /bin/echo program is running as root, because of sudo, but the shell that's redirecting echo's output to the root-only file is still running as you. Your current shell does the redirection before sudo starts.
The solution is to run the whole pipeline under sudo. There are a couple ways to do it, but I prefer:
boediger
A whole article decribing a
On April 12th, 2009 Anonymous (not verified) says:
A whole article decribing a single command line method? Surely more thought could have gone into this.
A whole article decribing a
On April 6th, 2009 K31th (not verified) says:
A whole article decribing a single command line method? Surely more thought could have gone into this.
tee shirts?
On April 4th, 2009 felipe (not verified) says:
man 1 tee
just as the above post explains, you can use tee. It is easier to visualise what's happening.
Proto's way is better.
On April 3rd, 2009 jpenny (not verified) says:
Sorry, bad cut and paste. Proto's method shows:
Apr 3 12:00:09 xxxxxxxx sudo: xxxxxxx : TTY=pts/22 ; PWD=/tmp ; USER=root ; COMMAND=/bin/sh -c echo 0 >/proc/sys/net/ipv4/ip_forward
First set of xxxxxxxx is machine name, second is userid.
Proto's way is better
On April 3rd, 2009 jpenny (not verified) says:
It gives better logging.
Using proto's method, auth.log shows:
Apr 3 12:00:12 xxxxxxxxx sudo: xxxxxxxx : TTY=pts/22 ; PWD=/tmp ; USER=root ; COMMAND=/usr/bin/tail /var/log/auth.log
Don's way just shows:
Apr 3 11:58:02 xxxxxxxx sudo: xxxxxxxx : TTY=pts/22 ; PWD=/tmp ; USER=root ; COMMAND=/bin/sh
sudo: cd: command not found
On April 3rd, 2009 Anonymous (not verified) says:
The one that drives me crazy is when I'm trying to change to a directory with only root access:
I always just give up at that point and change to the root user account with:
but I guess I could stack up the commands as shown above.
I do the same with: sudo sh
On April 1st, 2009 Proto (not verified) says:
I do the same with:
sudo sh -c "echo 12000 > /proc/sys/vm/dirty_writeback_centisecs"
I use the same one: sudo -c
On May 5th, 2009 Barun (not verified) says:
I use the same one: sudo -c ''. It is the more direct approach available, than using echo with sudo.
how about 'sudo -i' first?
On March 31st, 2009 Gerald Marewo (not verified) says:
how about 'sudo -i' first?
I am totally agree with you,
On April 7th, 2009 day spa gold coast (not verified) says:
I am totally agree with you, It's really great.
another way
On March 30th, 2009 Joe A (not verified) says:
I like to use tee for this. Using your example, you could use
$ echo 12000 | sudo tee /proc/sys/vm/dirty_writeback_centisecsThis works for the same reason, it runs tee with sudo. You can also use
tee -ato append to file instead of overwriting it.
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