posix

How do I programmatically check if a Flatpak package is installed in a shell script? I’m writing a script where I want to detect whether a given Flatpak package is installed, and take a different code path depending on the answer. How can I accomplish this? Traditional measures like command -v obviously won’t work unless …

Forbids something in Posix to have an empty string as a file name? My experience is that trying to do anything with an empty string filename results file not found (ENOENT), including a ls -ld ”, touch ” or mkdir ”. But I could try only ext4. While it looks quite logical to me, does …

POSIX Shell: `exec` with changed arg0 I want to exec a program and control it’s arguments including arg0 and environment. Using C I could go for execve. Can I do this in POSIX shell? Asked By: Patrick Böker || Source According to POSIX documentation of Special Builtin Utilities, the syntax for exec is exec [command …

How do executables use POSIX to keep compatibility between different UNIX systems? In my understanding, POSIX only specifies a set of APIs that the OS needs to provide, but it doesn’t specify the implementation detail, specifically the assembly level compatibility. For example, on x86, you need to use a syscall to get the POSIX API: …

How to convert all newlines to "n" in POSIX sh strings I have a string that contains newline characters. I want to escape all newlines in that string by replacing all newline characters with a string of two characters: "n". How can I do this in POSIX sh? Here’s the goal: $ printf ‘anbncnd’ | …

Is the following `readonly` use POSIX-ly correct? I defined the following as read-only: readonly root_command=’sudo -s’ later used in my script as in: exec $root_command My question is, maybe I am slow or something, but I do not fully understand the POSIX man page, as for example if I can single quote or have to …

Why does the POSIX for-loop allow a `for name in ; do` (no words in list) syntax? I’m trying to understand the for-loop syntax in the POSIX shell grammar rules: for_clause : For name do_group | For name sequential_sep do_group | For name linebreak in sequential_sep do_group | For name linebreak in wordlist sequential_sep do_group …

Why was -H and -L choosen for –dereference-command-line and –dereference in ls as well as for example du? Why was -H and -L choosen for respectively –dereference-command-line and –dereference in ls as well as for example du? I guess -H and -L were thought to be less commonly used, so could be used in multiple …

Why is XCU the abbreviation for "Shell and Utilities" and XSH the abbreviation for "System Interfaces"? POSIX is divided into 4 major volumes, and the abbreviations XBD, XSH, XCU and XRAT are used throughtout, respectively standing for "Base Definitions", "Shell and Utilities", "System Interfaces" and "Rationale". For what it’s worth, "Shell and Utilities" and "System …

Why pipe keep sudo and redirection not? Pipe (|) and redirections (<, <<, >, >>) both using standard streams (stdin, stdout, stderr), but although only pipe can keep sudo privileges, why? Works: sudo echo "hello" | tee /root/test Doesn’t work: sudo echo "hello" > /root/test Asked By: linuxer || Source Pipe (|) and redirections (<, …

Does `return 0` equal `true` (in sourced script to shell's environment)? I am working on a highly portable script that users shall source to their shells, forcing me to use POSIX scripting. There are many useful functions in the script, one of them is special though, as it returns true or false status to the …

Exit status of pipe with timeout command behaves differently in interactive shell vs shell script Interactive sh shell session: $ sh $ timeout 1 yes | sed -n s/a/b/p ; echo $? Terminated 143 $ Non-interactive script via sh -c: $ sh -c ‘timeout 1 yes | sed -n s/a/b/p ; echo $?’ 0 $ …

Why does `trap` passthough zero instead of the signal the process was killed with? Consider the following: #!/bin/bash trap ‘echo $?’ INT kill -INT $$ Output: 0 Here I would expect 130 for my system. Of course, if I do a Ctrl + C then I get 130. The same thing happens for any other …

Remove exact line from file if present, leave the rest of lines; error-handling – how to go about it I am not exactly sure how text processing should be done in a good way, so let me ask. I have this file: ~/.config/mpv/input.conf containing possibly other options as well as v disable. If I want …

Is there a difference between file permissions and mode? The terms "file permission" and "file mode" are often used interchangeably. However, some tools exclusively use one term or the other. Interestingly, the venerable chmod tool specifically refers to "file mode". Is there a technical or historical difference between them? Asked By: lonix || Source Technically …

Expand tabs in file with utf8 characters I use expand to expand tabs to spaces. For utf8 files expand doesn’t work correctly. E.g. in ćta tab is expanded to 6 spaces while in ata to 7 spaces. How do I make it work for utf8 files? Asked By: Marcin Król || Source You could use …

adapt 'exec &> >(tee -a "$LOG_FILE")' to sh In bash, this: exec &> >(tee -a "$LOG_FILE") Will redirect stderr & stdout to tee that allows the output to be displayed on the terminal while also logged (written to $LOG_FILE), but it uses process substitution, a feature not available in sh If I run the script …

`IFS="value" command` is a popular feature of many shells, but official documentation is hard to find. Where is it documented officially? I constantly see IFS="value" command being referred to as a means of changing the IFS (Internal Field Separator) temporarily, only for the duration of the command provided, yet there are many situations where what …

Is `command -v program` always executable? I am using the command -v "program" >/dev/null 2>&1 construct if I need to POSIX-ly find out if such program is installed in an if-statement. From its help page is not clear, whether it checks only for path or also for the executable bit? $ command –help command: command …

Bash posix regex optional groups Trying to match some Gentoo package naming components from strings in forms category/name-version, due to its complexity, I have come to this: if [[ "$1" =~ ^([<>]?=?)(([^/]+)/)?([^[:space:]]+)-(([[:digit:]]+)?(.([[:digit:]]+))*([a-z])?(_(alpha|beta|pre|rc|p)([[:digit:]]*))*(-(r([[:digit:]]+))?)?)?$ ]]; then # use "${BASH_REMATCH[n]}" here to capture groups contents fi It splits strings like <category/package-name-12345.25b_rc10-r7 as expected: Version specifier: < Category: category …