Questions tagged [null]
For questions on how programs deal with the NULL character ("\0" or "0x00"), or the special "/dev/null" device to which program output can be redirected so that it disappears.
54
questions
0
votes
0
answers
34
views
What does it mean if /dev/null is not a character device? [duplicate]
I'm experiencing "weird" behavior on a Linux PC -- I don't know how to qualify the problem exactly, other than that it seems to have something to do with /dev/null, based on the error ...
- 1,615
1
vote
1
answer
224
views
Error redirection fail with bind - dig
I'm trying to test some nameservers against a domain name.
For that, I created a script that reads a list of nameservers and asks for a domain name.
Something basic like this:
#!/bin/bash
domain=$1
[ ...
- 325
3
votes
4
answers
444
views
Empty string breaks command line
I am having trouble trying to describe my issue. Please bear with me. I have a script that calls a command. I need to optionally include an extra argument in the command line depending on the input ...
- 29
0
votes
1
answer
220
views
Trying to rotate logs however tee command fails to return after execution
My logs nohup.out is owned by root user while I m trying to rotate the logs using system which has privileged access using sudo
I have written the below script to rotate logs.
cat rotatelog.sh
cp /var/...
- 489
0
votes
1
answer
196
views
Why is /dev/null needed to run asynchronous jobs in busybox sh?
I'm curious why this special device is needed to fork the command and run it asynchronously in the minimal Busybox shell.
BusyBox v1.30.1 (Debian 1:1.30.1-4) built-in shell (ash)
Enter 'help' for a ...
- 37
1
vote
0
answers
177
views
How to suppress php output in one-line chained terminal command
I use the following &&-chained one-line command in Linux terminal (Ubuntu desktop):
cd /opt/lampp/htdocs/detailed_payouts && for FILE in *; do dos2unix $FILE; done && php /opt/...
- 11
-1
votes
1
answer
63
views
Can any one explain what the purpose of this "dd" command that writes to /dev/null is?
dd if=/dev/sda7 of=/dev/null bs=1024 skip=276892816 count=420 iflag=direct
We are having some issues backing up a Linux server. Anyone, please explain to me what this command does in that case.
...
0
votes
0
answers
532
views
"> $logfile" does not truncate, file size goes to 0 and a second later, is back to full size [duplicate]
I have a script that writes to a logfile like this:
$ nohup myscript.sh > myscript.out 2>&1 &
when the log file gets very large, I need to truncate it like this:
> myscript.out
I ...
- 1
3
votes
1
answer
742
views
awk with null record separator printing just one file
How does the following code print just a single file?
find "$fdir" -type f -name "${fnam}-*.png" -print0 | awk -v RS='\0' -F'[-.]' '{print $(NF-1), $0}' | cat -vet
which gives me
...
- 1,183
0
votes
1
answer
807
views
null characters at the beginning of an ASCII log file
We have an java application that log4j2 to generate log file and have script to stop the process before restart it with another script. There is 5 minutes pause between stop and restart at the ...
- 1
1
vote
0
answers
179
views
How do I set up null-sink and capture to route audio into camilladsp?
I am trying to set up an assistant on my raspberry pi, but I need the audio to route through camilladsp before it's outputted to my DAC. I figured a pulseaudio null-sink and null-sink capture would be ...
1
vote
1
answer
130
views
How to empty a file with terminal?
All my sites include a script that saves in a file all the fake google IPs that are not in google IP range.
Then with a script once a day I add all the IPs to the firewall.
while read line; do sudo ...
- 111
2
votes
1
answer
632
views
bash mapfile NUL bug?
bash's mapfile seems to be broken when handling NUL separated input. In particular, it isn't handling minus characters (-) correctly, treating the empty string after one as an end-of-line marker.
For ...
- 77.7k
0
votes
1
answer
341
views
Why xargs -0 adds an extra blank line?
In the example, why xargs -0 adds an extra blank line, and how to avoid it?
$ touch a b c
$ find . -print0
../a./c./b
$ find . -print0 | sort --zero-terminated | xargs -0
. ./a ./b ./c
$ find . -...
- 1,802
0
votes
1
answer
871
views
Remove all zero bytes from file in Unix
I've got a file of data from a prototype hardware RNG, however, for some reason it's producing a lot of 0x00 bytes. I want to delete all of these 0x00 bytes so I can test if the rest of the data is ...
- 133
1
vote
1
answer
4k
views
If Bash arrays can't contain a null string, how does this array do it?
I've recently discovered that you can't pass a null string to a Bash array. This is tricky, as I'm trying to insert an empty string value into an array.
I've noticed that the Bash completion variable ...
- 205
1
vote
1
answer
332
views
how to write a pgrep pattern that (never) matches a zero byte?
I'd like to kill a process with a long name that ends with foo. Currently my plan is to use pkill (though for testing, to be friendly to the process, I've been checking my patterns with pgrep instead)....
- 152
2
votes
3
answers
787
views
awk: Exact string match on field not working with NUL as record separator
Given a file with newlines in fields (embedded by double quotes), I tried to use NUL as record separator and then select desired records.
For this I have replaced the ends of lines with NUL and then ...
- 13.3k
10
votes
1
answer
4k
views
Send null byte in unix pipe
I am trying to redirect python generated input to ELF 64-bit executable in bash 5.0.3. I am getting:
> ./bf <<< $(python2 -c "print('c'*6+b'\x00'+'c'*6)")
bash: warning: command ...
- 255
2
votes
1
answer
3k
views
Docker: bash: /dev/null: Permission denied
I use docker to unpack a linux chroot and then execute commands into it, but I get this inside the chroot.
echo ciao > /dev/null
bash: /dev/null: Permission denied
I thought that the null device ...
- 141
6
votes
2
answers
1k
views
Are shells allowed to ignore NUL bytes in scripts?
Because that's what some of them are doing.
> echo echo Hallo, Baby! | iconv -f utf-8 -t utf-16le > /tmp/hallo
> chmod 755 /tmp/hallo
> dash /tmp/hallo
Hallo, Baby!
> bash /tmp/hallo
/...
user313992
0
votes
5
answers
346
views
How to monitor changes in file filled by null bytes?
I have a 10Mb file filled by null bytes. A program is accessing it and changes zeros to specific strings up to the end of the file.
I've tried to use tail -F | grep wanted_text | grep -v "...
- 819
15
votes
2
answers
2k
views
Create virtual block device which writes to /dev/null
I want to test some physical links in a setup. The software tooling that I can use to test this require a block device to read/write from/to. The block devices I have available can't saturate the ...
- 337
0
votes
1
answer
414
views
How to nullify the /var/log/message file as a normal user
Is it possible to truncate the /var/log/messages file as a normal user? If yes what permissions do we need to mention in sudoers file for this?
0
votes
1
answer
454
views
command stdout to /dev/null
I have very simple command which generating STDOUT which i want to do /dev/null but somehow it's not working or i am missing something here.
$ ldapsearch -Y GSSAPI -b "cn=users,cn=accounts,dc=...
- 1,632
2
votes
2
answers
2k
views
Printing only the value and excluding null
I'm trying to print the json output using jq, But I'm getting null
How can I print only access_key and secret_key, but not null?
$ cat sample.json | jq '.'
{
"access_key": "...
- 177
2
votes
2
answers
215
views
Remove values from NUL separated file
I have an array of values v that I want to remove from the file f. f is NUL delimited. How do I proceed? I tried using sd, but it didn't work.
Example:
I have this file:
# cat -v $attic
^@this is 1....
- 1,543
1
vote
1
answer
2k
views
How to assign a zero-length, non-null string
Based on this, amongst many other things I've read, my understanding is that a='' makes a both null and zero-length. But, then, how does one create a zero-length, non-null string? Or, is there no ...
- 370
25
votes
2
answers
3k
views
Why does tar appear to skip file contents when output file is /dev/null?
I have a directory with over 400 GiB of data in it. I wanted to check that all the files can be read without errors, so a simple way I thought of was to tar it into /dev/null. But instead I see the ...
- 3,310
0
votes
0
answers
126
views
Compare file compression methods by sending to /dev/null
I'd like to compare the time to completion for compression algorithms and parameters. I'm confident that the inputs will be set up properly. In order to save space and avoid some variability my ...
- 177
2
votes
5
answers
826
views
remove all lines with nullbytes/corrupt data
So I recovered a text file from an old hdd, but I failed to completely recover all of the data. The data that wasn't correctly recovered has returned as null bytes. How can I remove every line from ...
- 21
26
votes
1
answer
3k
views
Why don't reads from /dev/zero count as IO_RBYTES?
I am emptying out a hard drive on some Linux 4.x OS using this command:
sudo sh -c 'pv -pterb /dev/zero > /dev/sda'
And I opened another tty and started sudo htop and noticed this:
PID USER ...
- 3,468
1
vote
0
answers
390
views
Odd Characters in Log File
I checked my cron log files today for a script that I have scheduled and the saw the following errors/characters:
\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\
...
- 125
4
votes
1
answer
1k
views
With gnu find, what is the difference between -print0 and -printf "%p\0"
In a terminal I can run...
find . -type f -print0
./testdir/testfile2.txt./testdir/testfile.txt
And then...
find . -type f -printf "%p\0"
./testdir/testfile2.txt./testdir/testfile.txt
They both ...
- 145
6
votes
6
answers
8k
views
How to use the NUL character as a separator in sed substitute and delete commands?
This is what I tried, when intending to replace /path/to/a with /path/to/b using NUL as the separator/delimiter:
$ cat pathsList| sed -r -e 's\0/path/to/a\0/path/to/b\0g'
sed: -e expression #1, char ...
- 812
117
votes
9
answers
25k
views
Why is /dev/null a file? Why isn't its function implemented as a simple program?
I am trying to understanding the concept of special files on Linux. However, having a special file in /dev seems plain silly when its function could be implemented by a handful of lines in C to my ...
- 1,218
1
vote
1
answer
835
views
random “NULL character(s) present” error in exim
Sometimes we are getting NULL character(s) present errors on incoming mail. It looks like this:
2018-03-01 12:26:51.244 [22933] SMTP syntax error in "RCPT TO:" H=mail-wm0-f56.google.com [74.125.82.56]...
- 11
6
votes
6
answers
7k
views
Remove null bytes from the end of a large file
I just backed up the microSD card from my Raspberry Pi on my PC running a Linux distro using this command:
dd if=/dev/sdx of=file.bin bs=16M
The microSD card is only 3/4 full so I suppose there's a ...
- 3,468
1
vote
2
answers
3k
views
Working with /dev/null with cat or cp vs touch
In a tutorial about public key infrastructure the author sets up a database to work with when setting up a Root Certificate Authority:
cp /dev/null ca/root-ca/db/root-ca.db
cp /dev/null ca/root-ca/db/...
- 2,958
0
votes
1
answer
5k
views
What happens when you write to /dev/null? What’s the point? [duplicate]
Tried echo "Some text" >> /dev/null
then cat null with no output. Can someone explain why this is?
- 31
7
votes
3
answers
5k
views
Using binary data as a parameter in bash - any way to allow nuls?
So I'd like to pass the first 512 bytes of binaryFile.dd as the second parameter to myProgram but bash strips out all the NUL chars. Is there any way to avoid this in bash or am I on a hiding to ...
- 303
6
votes
3
answers
2k
views
How to prevent word splitting without preventing empty string removal?
I need to pass as a program argument a parameter expansion. The expansion results in a filename with spaces. Therefore, I double-quote it to have the filename as a single word: "$var".
As long as $...
- 447
4
votes
1
answer
9k
views
Can I safely ignore: "warning: command substitution: ignored null byte in input"?
Is it possible to safely ignore the aforementioned error message? Or is it possible to remove the null byte? I tried removing it with tr but I still get the same error message.
this is my script:
#!/...
- 803
1
vote
2
answers
18k
views
cp large file to /dev/null to reduce size to zero [closed]
I just came across the advice that if you want to get rid of a large file and a process has the file handle open you should copy it to /dev/null and its size will be reduce to zero.
How does this ...
- 121
4
votes
2
answers
142
views
Invert matching lines, NUL-separated
I’m writing something that deals with file matches, and I need an inversion operation. I have a list of files (e.g. from find . -type f -print0 | sort -z >lst), and a list of matches (e.g. from ...
- 1,723
2
votes
2
answers
94
views
How can I get FreeBSD `patch` to tolerate null bytes?
The patch of the final line fails on FreeBSD 10.2, but works on GNU:
#!/bin/sh
echo a > f1
printf '\0' >> f1
echo c >> f1
cp f1 f2
echo d >> f2
diff -au f1 f2 > diff
patch <...
- 1,473
-1
votes
3
answers
394
views
Null value than rename file
I have a shell to a task but I should handle the null value in this.
So in the task i cut the part of a word and i delete or rename the file, but need another if element for null.
For example:
I ...
10
votes
4
answers
41k
views
Using/reading/writing the null and escape characters
I know that a filename in Linux has no restriction whatsoever except for two characters '/' and '\0'. I know that '/' is prohibited because it is a directory separator but is there any other reason ?
...
- 1,469
5
votes
2
answers
21k
views
Meaning of `cat /dev/null > file`
In a document created by a former coworker there is this command:
cat /dev/null > /var/spool/mail/root
It says next to it that it will clean out mailbox.
Can someone please explain how/why ...
- 721
44
votes
1
answer
40k
views
How do I use null bytes in Bash?
I've read that, since file-paths in Bash can contain any character except the null byte (zero-valued byte, $'\0'), that it's best to use the null byte as a separator. For example, if the output of ...
- 1,477