POSIX is an acronym for "Portable Operating System Interface", a family of standards specified by the IEEE for maintaining compatibility between operating systems.

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What does the `set $variable` do?

I know that set is used to manipulate environment variables, but I want to make myself sure - these alterations are not permanent, right? I mean once I close shell and reopen it, no changes will be ...
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1answer
283 views

Using shell's read command with live editing functionality (readline like)

Is there a standard (POSIX) way of asking the user some data from within a shell script, with read for example, while allowing live edition of the text being typed (what readline does)? I know bash ...
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1answer
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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 ...
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1answer
184 views

indirect variable expansion in POSIX as done in bash? [closed]

Is it possible, or is there some elegant hack to do indirect variable expansion in POSIX as can be done in Bash? For context, I'm trying to do the following: for key in ${!map_*} do # do ...
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2answers
58 views

How can we obtain a temporary filename in a standard-conforming IEEE 1003.1 script?

How can we obtain a temporary file name in a standard-conforming shell script (using no commands or extensions which are not described the 2013 edition of IEEE 1003.1). There doesn't appear to be any ...
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1answer
287 views

Is the Linux implementation of the system configuration “variable” ARG_MAX different from other system variables and is it POSIX compliant?

In the shell, as explained in this this Q&A in the context of expansion, depending on the system, the maximum length of a command's argument is initially constrained by the kernel setup. The ...
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1answer
954 views

Reverse id command, I want the User/Group Name from the uid or gid

I know some uid and gids that I dont know who they belong to and they are not in /etc/passwd (could be LDAP or from another system). How do I get Id info from only uid or gid preferably without using ...
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2answers
36 views

Can I use `type` to check whether an arbitrary name is a valid command name?

The bash built-in type can be used for this purpose by checking its exit status: Exit Status: Returns success if all of the NAMEs are found; fails if any are not found. How portable is it? The ...
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1answer
78 views

Common flag designations and standards for shell scripts and functions

I have been adjusting to using GETOPT and GETOPTS in my shell scripts and custom functions to enable me to use the functions that I create more flexibly. After using a number of UNIX functions I have ...
2
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1answer
134 views

Are “mostly POSIX-compliant” systems still considered POSIX systems?

Irix and OS X are POSIX systems because they are fully-POSIX-compliant. However, operating systems like FreeBSD, Haiku, and GNU/Linux, for example, are "mostly-POSIX-compliant". Can people still ...
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125 views

Nested POSIX groups in Linux [duplicate]

If I want to add all of the members who belong to the supplemental group web_users to the apache group (as an additional supplemental group) can I just add web_users group to the apache group? Or do ...
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1answer
2k views

Obtain device name and mount point if partition label is known

How to obtain the partition device name and mount point when the file system label is known? There are several possibilities: Linux File system label to device name The device name can be obtained ...
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3answers
336 views

OS X/Linux one-liner/script to find the largest recurring group of lines in a text file?

I have a log containing an execution trace where there is infinite recursion eventually terminating when the stack is too deep. There are enough lines and valid included recursion within the larger ...
22
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3answers
713 views

shellcheck is advising not to use basename: why?

I am trying out shellcheck. I have something like that basename "${OPENSSL}" and I get the following suggestion Use parameter expansion instead, such as ${var##*/}. From the practical point of ...
3
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2answers
154 views

Shell-independent way to cp dotfiles from a folder to another [duplicate]

I have two existing directories : foo: directory with dotfiles in it foo2: empty directory I would like to have a solution to copy all dotfiles in foo to foo2. I would like a solution that is not ...
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2answers
1k views

POSIX find all local files

I'm trying to write a cross-platform command line that will find all local files on a file system. My intention is to use the find command, using only POSIX parameters. Is there a way to skip ...
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0answers
38 views

Is there a standard POSIX way of doing `tac` [duplicate]

Looks like at least IBM AIX and HP-UX don't have the tac command by default. Is there a way to do this using just basic posix standard shell utilities available on all UNIX systems? Furthermore, is ...
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1answer
42 views

Is there such a scheduler and is it feasible?

I read about a scheduling algorithm called EDD which always works on the next deadline and that is feasible. Would it also be feasible to within each time windows or slice, work on all processor or ...
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1answer
3k views

How does umask affect ACLs?

Can someone explain to me how umask affects the default mask of newly created files if ACLs are activated? Is there some documentation about this? Example: $ mkdir test_dir && cd test_dir $ ...
2
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1answer
211 views

How to negate a regex inside another regex

That is, given a POSIX regex X, what POSIX regex is the complementary of X? (in a more detailed way: how to get regex X' that matches all strings that are not matched by X, but doesn't match any ...
4
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3answers
665 views

Should a serious student of *nix choose a distribution that is POSIX-compliant to learn standard features well?

I want to learn Unix or Linux as a serious student, and specifically want to learn the basics well so that I can have proper understanding and so I can move between platforms and distributions without ...
2
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1answer
203 views

POSIX head and tail not feature equivalent

The documentation for POSIX tail's -n option says that The origin for counting shall be 1; that is, -n +1 represents the first line of the file, -n -1 the last. So tail -n 10 (used later on the ...
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2answers
249 views

How to monitor the last PID assigned by the kernel?

To monitor process creation I'd like to periodically write the last PID assigned by the kernel to any process on the system (not just processes created by the PID-monitoring program) with a time stamp ...
3
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1answer
1k views

Why is brk(0) called?

On a first shell I run the command tail with no argument. On a second shell I kill the first tail using strace to monitor the system calls. qdii@nomada ~ $ strace kill 1713 execve("/bin/kill", ...
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2answers
757 views

What would break if the C locale was UTF-8 instead of ASCII?

The C locale is defined to use the ASCII charset and POSIX does not provide a way to use a charset without changing the locale as well. What would happen if the encoding of C were switched to UTF-8 ...
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2answers
149 views

what happens when a window manager is put to background

I am trying to understand the situation why .xsession file expects the last process to be in foreground otherwise it is supposed to stop X11 session. I have a small stand-alone script. when I run ...
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1answer
41 views

IO and other shell commands when a program is not started by a terminal

I have several launchers on my desktop for various programs, and there is a "run in a terminal" setting for those launchers. When I check that setting, the program runs from a terminal, and logs its ...
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1answer
760 views

POSIX and Bash, sh shorthand, for compound if statement

I prefer to do my sh commands in shorthand, And I prefer to be in POSIX. Truthfully, if the script can run on Dash, that is good enough. Which means using the test command [, not the extended [[. I ...
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2answers
81 views

Read umask for a user

Plenty of examples on how to set umask for a user... but how do you read (or uncover) the default umask for a given user (say the logged in user)
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2answers
5k views

$VAR vs ${VAR} and to quote or not to quote

I can write VAR=$VAR1 VAR=${VAR1} VAR="$VAR1" VAR="${VAR1}" the end result to me all seems about the same. Why should I write one or the other? are any of these not portable/POSIX?
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0answers
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How do you write test cases for a web server that uses posix sockets? [closed]

I know how to test it experimentally by running the program, but ideally I would like to test each function I write as I write it. Usually I can't test my functions until I write a whole bunch of ...
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2answers
157 views

POSIX test and -a

I checked a script of mine with checkbashisms and I got the following warnings: possible bashism in check_ssl_cert line 821 (test -a/-o): if [ -n "${ALTNAMES}" -a -n "${COMMON_NAME}" ] ; then In ...
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1answer
500 views

What is difference between POSIX and bash and other shell scripts?

While going through many answers on Unix.SE, I come across many of them writing their contents w.r.t to Posix or Bash or some other standard. I know of some differences like some have arrays and some ...
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2answers
212 views

Is it better to archive/move mail based on mtime or ctime?

I have been using a perl script to archive mail found here The script uses ctime to process and decide which month sub folder to place mail. In most cases this is fine since the ctime = mtime and ...
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4answers
5k views

sed on OSX insert at a certain line

So I've been using 'sed' on linux for a while, but have had a bit of difficulty trying to use it on OSX since 'POSIX sed' and 'GNU sed' have so many little differences. Currently I'm struggling with ...
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2answers
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What is the point of the `cd` external command?

As referenced in this fine answer, POSIX systems have an external binary cd in addition to the shell builtin. On OS X 10.8 it's /usr/bin/cd. You can't use it like the builtin cd since it exits ...
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1answer
138 views

the return built-in

According to the Open Group, [t]he return utility shall cause the shell to stop executing the current function or dot script. If the shell is not currently executing a function or dot script, ...
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2answers
305 views

What is difference between [ and [[ in BASH? [duplicate]

Possible Duplicate: using single or double bracket - bash When should I use a single bracket? [ When should I use double brackets? [[ Are both POSIX compliant?
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How can I test for POSIX compliance for shell scripts?

Probably the main thing that this website has taught me so far is the importance of writing portable shell scripts. Considering that POSIX is the closest thing to a common standard between all ...
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1answer
122 views

Linking “..” do somewhere other than parent directory and other strange fs beharviors

Can the entry ".." become linked to someplace other than the parent directory? Is it possible to cause "tar" to create this weird behavior? Scenario: I have a pre-built compiler to a embeded system, ...
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1answer
1k views

Disabling readahead with hdparm or POSIX_FADV_RANDOM

From Linux kernel's doc the advice POSIX_FADV_RANDOM seems to disable readahead. But whent I disable Readhead using (sudo hdparm -A 0 /dev/sdb) I get huge degradation in performances; also noticed ...
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1answer
2k views

A couple arithmetic related commands, Dash, POSIX shell, sh, Increment

When pre/post incrementing a variable, in POSIX Shell, what is the difference between these two examples. They seem like they would have the same consequences, but would they be used differently? ...
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4answers
1k views

Is there a difference between these two commands?

cat a > b and cp a b If they are functionally the same for all intents and purposes, which one is faster?
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2answers
371 views

find command for the newest 500 files in a directory tree and also be POSIX compliant

I am looking for a single line shell script or unix command to find the newest 500 files in a directory. Major constraints are it should be POSIX complaint and the directory can have tons of files.
8
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1answer
226 views

Does POSIX guarantee that the standard utilities are in PATH?

In the "Command Search and Execution" section, the POSIX specification says that PATH is searched when finding a utility to execute (with some exceptions). Does the specification mention anywhere ...
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2answers
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Why do some applications use ~/.config/appname for their config data while others use ~/.appname?

I've noticed that some applications put their configuration files to ~/.config/appname while others use ~/.appname (the classic way, AFAIK) for this. What's the sense in this distinction and what ...
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Portability of “> /dev/stdout”

Occasionally I need to specify a "path-equivalent" of one of the standard IO streams (stdin, stdout, stderr). Since 99% of the time I work with Linux, I just prepend /dev/ to get /dev/stdin, etc., ...
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1answer
276 views

POSIX conformance: NetBSD 5.1 <sys/time.h> gettimeofday declaration

I'm compiling some code which requests POSIX 1003.1 1993 conformance via -D_POSIX_C_SOURCE=2. The code includes the header <sys/time.h> and wants to call gettimeofday, but on NetBSD 5.1 there ...
4
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3answers
556 views

Why is there a type for process ids (pid_t), but not for file descriptors (fd)?

I see that pid_t is typedef'd in unistd.h, and file descriptors are defined to be int in fcntl.h. But since they are used in similar ways, wouldn't it make sense to have a typedef for file ...
6
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1answer
734 views

A minimal POSIX conformant operating system

For educational purposes, I'd like to start with a POSIX compliant base, including a shell, basic utilities, and an ANSI C compiler, then see what kind of environments I can build by putting packages ...