39
votes
4answers
7k views

What is the difference between a builtin command and one that is not?

Is there any intrinsic difference between a builtin command and another command which can nominally do the same thing? eg. Do builtins get "special" treatement? ... is there less overhead running ...
40
votes
5answers
9k views

Timing out in a shell script

I have a shell script that's reading from standard input. In rare circumstances, there will be no one ready to provide input, and the script must time out. In case of timeout, the script must execute ...
64
votes
17answers
111k views

SSH tunneling error: “channel 1: open failed: administratively prohibited: open failed”

When I open this ssh tunnel: ssh -nXNT -p 22 localhost -L 0.0.0.0:8984:remote:8983 I get this error when trying to access the HTTP server running on localhost:8984: channel 1: open failed: ...
60
votes
5answers
81k views

Limit memory usage for a single Linux process

I'm running pdftoppm to convert a user-provided PDF into a 300DPI image. This works great, except if the user provides an PDF with a very large page size. pdftoppm will allocate enough memory to ...
55
votes
3answers
30k views

Why can rm remove read-only files?

If I create a file and then change its permissions to 444 (read-only), how come rm can remove it? If I do this: echo test > test.txt chmod 444 test.txt rm test.txt ...rm will ask if I want to ...
52
votes
4answers
41k views

What is the difference between Halt and Shutdown commands?

What is the difference between the halt and shutdown commands?
46
votes
5answers
35k views

Using “${a:-b}” for variable assignment in scripts

I have been looking at a few scripts other people wrote (specifically Red Hat), and a lot of their variables are assigned using the following notation VARIABLE1="${VARIABLE1:-some_val}" or some expand ...
31
votes
9answers
4k views

What is the /boot partition really for?

I'm reading a relatively old text on Linux partitions and file systems (the LPIC 1 Certification Bible). It says: Some versions of the Linux boot loaders cannot access a kernel that is outside ...
49
votes
7answers
14k views

what does the @ mean in ls -l?

I am using Mac OSX. When I type ls -l I see something like drwxr-xr-x@ 12 xonic staff 408 22 Jun 19:00 . drwxr-xr-x 9 xonic staff 306 22 Jun 19:42 .. -rwxrwxrwx@ 1 xonic staff 6148 25 ...
12
votes
3answers
945 views

Order of redirections

I don't quite understand how the computer reads this command. cat file1 file2 1> file.txt 2>&1 If I understand, 2>&1 simply redirect Standard Error to Standard Output. By that ...
6
votes
6answers
2k views

How to clean up file extensions?

I have a directories with .MP3 files which I'd like to change the extensions to .mp3. What's the easiest way to do this? I'm think something along the lines of: find /RootPath -type f -iname "*.mp3" ...
5
votes
2answers
6k views

ext4: How to account for the filesystem space?

I've recently formated a 1.5 TB drive with the intention of replacing ntfs with ext4. Then I noticed that the files I saved don't fit on the new partition. df: ext4 (ext3 & ext2 show the same ...
320
votes
7answers
28k views

How can I find the implementations of Linux kernel system calls?

I am trying to understand how a function, say mkdir, works by looking at the kernel source. This is an attempt to understand the kernel internals and navigate between various functions. I know mkdir ...
197
votes
5answers
36k views

/usr/bin vs /usr/local/bin on Linux

Why are there so many places to put a binary in Linux? There are atleast these five: /bin/ /sbin/ /usr/bin/ /usr/local/bin/ /usr/local/sbin/ And on my office box, I do not have write permissions ...
115
votes
1answer
237k views

How do you empty the buffers and cache on a Linux system?

Prior to doing some benchmarking work how would one free up the memory (RAM) that the Linux Kernel is consuming for its buffers and cache?
211
votes
6answers
88k views

What does env x='() { :;}; command' bash do and why is it insecure?

There is apparently a vulnerability (CVE-2014-6271) in bash: Bash specially crafted environment variables code injection attack I am trying to figure out what is happening, but I'm not entirely sure ...
139
votes
17answers
72k views

How to have tail -f show colored output

I'd like to be able to tail the output of a server log file that has messages like: INFO SEVERE etc, and if it's SEVERE, show the line in red; if it's INFO, in green. What kind of alias can I ...
77
votes
30answers
22k views

Quick directory navigation in the terminal

I would like to frequently switch between directories that are in totally unrelated paths, for example /Project/Warnest/docs/ and ~/Dropbox/Projects/ds/test/. But I don't want to type cd ...
99
votes
7answers
38k views

Why use swap when there is more than enough free space in RAM?

Using swap space instead of RAM can drastically slow down a PC. So why, when I have more than enough RAM available, does my Linux system (Arch) use the swap? Checkout my conky output below: Also, ...
68
votes
4answers
16k views

What exactly is POSIX?

I see POSIX mentioned often and everywhere, and I had assumed it to be the baseline UNIX standard.. until I noticed the following excerpt on a Wikipedia page: The Open Group The Open Group is ...
89
votes
2answers
62k views

Getting 256 colors to work in tmux

I have 256 colors working just fine in konsole,. I thought I'd give tmux a try because, unlike screen, it seems to support vi mode. However I find that the colors of my prompt show up and this is ...
47
votes
4answers
4k views

What features are in zsh and missing from bash, or vice versa?

As a Linux user, I've always just used bash because it was the default on every distro I used. People using other Unix systems such as BSD seem to use other shells far more frequently. In the ...
40
votes
3answers
33k views

What causes this green background in ls output?

There are two directories shown by 'ls'. Normally directories anywhere are blue on black background. But the first one is blue on green and impossible to read. Why is this? How to make it blue ...
29
votes
6answers
32k views

Lightweight outgoing SMTP server

What do you use as a lightweight SMTP for outgoing e-mails only? I am running an app that sends e-mails via SMTP when events occur once in a while, and I don't need a fully featured e-mail server. ...
31
votes
7answers
10k views

Block network access of a process?

Is it possible to block the (outgoing) network access of a single process?
27
votes
4answers
2k views

Quick way to include a directory path when calling mv? [duplicate]

Often I will work out of the root of a rails directory where I find my self navigating the same path twice every time I would like to move a file: mv app/views/layouts/application.html.erb ...
17
votes
4answers
6k views

Evolution of Operating systems from Unix

Can you explain the evolution hierarchy of operating systems (Linux and Windows) from Unix?
14
votes
1answer
17k views

File descriptors & shell scripting

I am having a very hard time understanding how does one use file descriptors in shell scripts. I know the basics such as exec 5 > /tmp/foo So fd 5 is attached to foo for writing. exec 6 < ...
10
votes
3answers
11k views

ssh port forward to access my home machine from anywhere

http://serverfault.com/questions/303937/how-to-make-freebsd-box-accessible-from-internet I want to understand this whole process of port forwarding. I read so many things but failing to understand ...
34
votes
5answers
8k views

What process created this X11 window?

Given an X11 window ID, is there a way to find the ID of the process that created it? Of course this isn't always possible, for example if the window came over a TCP connection. For that case I'd ...
20
votes
7answers
1k views

Reading and searching long man pages

I finally got fed up when wanting to read about bash's read and it's -s option with man bash. I found the right spot eventually (around line 4500), but it was a frustrating as usual, since both /read ...
23
votes
2answers
4k views

Can I get individual man pages for the bash builtin commands? [duplicate]

Is there anywhere you can download a manpage for every builtin commands? I know you can just use help or man bash and search to find info about it, but I want them separated, so I can just do man ...
21
votes
2answers
17k views

How to colorize output of ls ?

I'm running CentOS in Linux text mode. When I run the command ls /usr/, the output is too hard to read (dark blue on black). How can I change the text coloring?
10
votes
3answers
2k views

Trapping errors in command substitution using “-o errtrace” (ie set -E)

According to this ref manual: -E (also -o errtrace) If set, any trap on ERR is inherited by shell functions, command substitutions, and commands executed in a subshell environment. The ERR ...
29
votes
5answers
44k views

How can I increase the number of inodes in an ext4 filesystem?

I had a problem (new to me) last week. I have a ext4 (Fedora 15) filesystem. The application that runs on the server suddenly stopped. I couldn't find the problem at first look. df showed 50% ...
33
votes
1answer
3k views

Does 'rm .*' ever delete the parent directory?

The expression .* is expanded by bash to include the current and parent directories: $ ls -la total 2600 drwxrwxrwx 2 terdon terdon 2162688 Sep 10 16:22 . drwxr-xr-x 142 terdon terdon 491520 Sep ...
10
votes
2answers
4k views

How to restrict internet access for a particular user on the lan using iptables in Linux

Let's say there are two users on the LAN, A and B. How do I restrict user A from internet access using iptables rules and saving the rules so that after reboot, they are still effective. And suppose I ...
14
votes
2answers
1k views

Manipulating X key and pointer grabs on the command line

What command line tools exist to list and manage X server grabs? (That's grab as in key and pointer grabs, i.e. restricting the use of a particular key or mouse button to a particular application, or ...
11
votes
3answers
25k views

Extracting a regex matched with 'sed' without printing the surrounding characters

To all the 'sed' doctors out there: How can you get 'sed' to extract a regular expression it has matched in a line? In other words words, I want just the string corresponding to the regular ...
16
votes
3answers
6k views

What defines the maximum size for a command single argument?

I was under the impression that the maximum length of a single argument was not the problem here so much as the total size of the overall argument array plus the size of the environment, which is ...
8
votes
3answers
2k views

Linux Bulk/Remote Administration

Beside our internal IT infrastructure, we've got around 500 Linux machines hosting our services for the on-line world. They are grouped in a bunch of clusters like Database A-n, Product A-n, NFS, ...
10
votes
5answers
1k views

What's the best way to take a segment out of a text file?

What's a good way of extracting say, lines 20 -45 out of a huge text file. Non-interactively of course!
4
votes
1answer
4k views

Rename All Files with a Certain Name

I'm trying to find certain files with the name "stringx" and replace the name (but not the extension) with "stringy". So basically for stringx.txt and stingx.cs, I'd want stringy.txt and stringy.cs. ...
217
votes
3answers
122k views

How to unfreeze after accidentally pressing Ctrl-S in a terminal?

It's a situation that has happened quite often to me: after I press (with a different intention) Ctrl-S in a terminal, the interaction (input or output) with it is frozen. It's probably a kind of ...
60
votes
6answers
52k views

What is a Superblock, Inode, Dentry and a File?

From the article Anatomy of the Linux file system by M. Tim Jones, I read that Linux views all the file systems from the perspective of a common set of objects and these objects are superblock, inode, ...
198
votes
32answers
145k views

Tracking down where disk space has gone on Linux?

When administering Linux systems I often find myself struggling to track down the culprit after a partition goes full. I normally use du / | sort -nr but on a large filesystem this takes a long time ...
56
votes
6answers
2k views

Resources for portable shell programming

What resources exist for portable shell programming? The ultimate answer is to test on all targeted platforms, but that's rarely practical. The POSIX / Single UNIX specification is a start, but it ...
123
votes
4answers
27k views

How do I clear Bash's cache of paths to executables?

When I execute a program without specifying the full path to the executable, and Bash must search the directories in $PATH to find the binary, it seems that Bash remembers the path in some sort of ...
62
votes
9answers
39k views

How can I move files and view the progress (e.g. with a progress bar)?

When moving large directories using mv, is there a way to view the progress (%)? The cp command on gentoo had a -g switch that showed the progress.
65
votes
5answers
46k views

How can I send stdout to multiple commands?

There are some commands which filter or act on input, and then pass it along as output, I think usually to stdout - but some commands will just take the stdin and do whatever they do with it, and ...

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