TL;DR
There is a non-standard -r option to xargs that allows it to skip executing when there are no arguments at all.
On some operating systems, the -r is default.
MacOS has no -r, but does not execute xargs if there are no arguments given.
Archive for the ‘bash’ Category
-r argument to pipe (no argument for MacOS)- If no input is given to xargs, don’t let xargs run the utility – Unix & Linux Stack Exchange
Posted by jpluimers on 2021/07/28
Posted in *nix, *nix-tools, bash, bash, Development, Power User, Scripting, Software Development, xargs | Leave a Comment »
Busybox sh (actually ash derivative dash): checking exit codes
Posted by jpluimers on 2021/04/20
Even if you include a double quotes "sh" in a Google search to force only sh (in the early days this was the Thompson shell, but nowadays usually a Bourne shell or derivative) results, almost all unix like scripting examples you find are based on bash (the Bourne again shell), so I was glad I dug a bit deeper into what the actual Busybox shell is.
I wanted to know which shell Busybox uses and what capabilities it has, as ESXi ships with this very slimmed down set of tools (called applets in Busybox speak).
It does not even include ssh: that gap is often filled by [Wayback] Dropbear SSH, which was used by ESXi and named dbclient (I think with ESXi 6.0 it was replaced with a more regular ssh implementation): [Wayback] How to compile a statically linked rsync binary for ESXi.
Busybox shell source code is at [Wayback] ash.c\shell – busybox – BusyBox: The Swiss Army Knife of Embedded Linux and indicates the shell is the ash (the Almquist shell) derivative dash (yes, you guessed it right: the Debian Almquist shell), ported from NetBSD and debianized:
* Copyright (c) 1997-2005 Herbert Xu <herbert@gondor.apana.org.au> * was re-ported from NetBSD and debianized. ... //config: The most complete and most pedantically correct shell included with //config: busybox. This shell is actually a derivative of the Debian 'dash' //config: shell (by Herbert Xu), which was created by porting the 'ash' shell //config: (written by Kenneth Almquist) from NetBSD.
nx like systems have a shell hell similar to Windows DLL hell: there are too many, and their differences and be both subtle and frustrating. To get a feel, browse through Source: Comparison of command shells – Wikipedia (yes, some shells from other operating environments like DOS, OS/2, VMS and Windows, but the majority is nx).
Since ash is sufficiently different from bash (for example [Wayback] ash – exit code for a piped process), I always want to know what shell code (which often comes from bash as it is so ubiquitous) will work.
There is hardly any shell documentation at the Busybox site. There is [Wayback] BusyBox – The Swiss Army Knife of Embedded Linux, the source code at [Wayback] ash.c\shell – busybox – BusyBox: The Swiss Army Knife of Embedded Linux does not offer much either,
A manual page of it is at [Archive.is] ash(1) [minix man page]. There you see the age: back then, “exit status” is used where nowadays many people would use “exit code”. It does not explain how to check for specific exit codes.
Because ash is derived from the Bourne shell, this page was of great help for me to grasp exit code handing: [Wayback] Exit Codes – Shell Scripting Tutorial
A Bourne Shell Programming / Scripting Tutorial for learning about using the Unix shell.
Here two examples from that page to get me going:
#!/bin/sh # Second attempt at checking return codes grep "^${1}:" /etc/passwd > /dev/null 2>&1 if [ "$?" -ne "0" ]; then echo "Sorry, cannot find user ${1} in /etc/passwd" exit 1 fi USERNAME=`grep "^${1}:" /etc/passwd|cut -d":" -f1` NAME=`grep "^${1}:" /etc/passwd|cut -d":" -f5` HOMEDIR=`grep "^${1}:" /etc/passwd|cut -d":" -f6` echo "USERNAME: $USERNAME" echo "NAME: $NAME" echo "HOMEDIR: $HOMEDIR"
and
#!/bin/sh # A Tidier approach check_errs() { # Function. Parameter 1 is the return code # Para. 2 is text to display on failure. if [ "${1}" -ne "0" ]; then echo "ERROR # ${1} : ${2}" # as a bonus, make our script exit with the right error code. exit ${1} fi } ### main script starts here ### grep "^${1}:" /etc/passwd > /dev/null 2>&1 check_errs $? "User ${1} not found in /etc/passwd" USERNAME=`grep "^${1}:" /etc/passwd|cut -d":" -f1` check_errs $? "Cut returned an error" echo "USERNAME: $USERNAME" check_errs $? "echo returned an error - very strange!"
This basically means that status code handling is the same as in bash, so constructs can be used like [Wayback] bash – How to check the exit status using an if statement – Stack Overflow:
$?is a parameter like any other. You can save its value to use before ultimately callingexit.exit_status=$? if [ $exit_status -eq 1 ]; then echo "blah blah blah" fi exit $exit_status
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Posted in *nix, *nix-tools, ash/dash, ash/dash development, bash, bash, BusyBox, Development, Power User, Scripting, Software Development, ssh/sshd | 1 Comment »
linux – How can I execute a series of commands in a bash subshell as another user using sudo? – Stack Overflow
Posted by jpluimers on 2021/03/31
Based on [WayBack] linux – How can I execute a series of commands in a bash subshell as another user using sudo? – Stack Overflow:
alias restart-spotlight-service-as-root="sudo bash -c 'echo stop;launchctl stop com.apple.metadata.mds;echo start;launchctl start com.apple.metadata.mds;echo started'"
The bold bits above sudo bash -c 'echo stop;launchctl stop com.apple.metadata.mds;echo start;launchctl start com.apple.metadata.mds;echo started' allow the commands between single quotes to executed in one new bash shell under sudo.
–jeroen
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Posted in *nix, *nix-tools, Apple, bash, bash, Development, Mac OS X / OS X / MacOS, Power User, Scripting, Software Development | Leave a Comment »
explainshell.com: parse and explain just about any shell command
Posted by jpluimers on 2021/02/17
I bumped into the tremendously site [WayBack] explainshell.com – match command-line arguments to their help text only after documenting the relevant cURL options of yesterdays post on checking your CertBot domain expiration dates.
The site allows put in a shell command-line to see the help text that, including matches for each argument.
It works so well because it parses both the shell command-line and the man pages, then constructs a web-page linking the relevant man page content to the shell command-line in the correct shell command-line order.
The
explainshellhas a counterpart showthedocs (both are open source) for explaining other languages (on the one hand more extended as it goes much deeper into parsing for instance SQL, on the other hand more limited as it only supports a few languages). More onshowthedocslater.
The links
- [WayBack] explainshell.com – about.
- [WayBack] GitHub – idank/explainshell: match command-line arguments to their help text
The parsing results
The first bit below is just the text output, and the second bit the screenshot, of a relatively simple command like [WayBack] explainshell.com – curl -fsSL example.org:
-f, --fail (HTTP) Fail silently (no output at all) on server errors. This is mostly done to better enable scripts etc to better deal with failed attempts. In normal cases when a HTTP server fails to deliver a document, it returns an HTML document stating so (which often also describes why and more). This flag will prevent curl from outputting that and return error 22. This method is not fail-safe and there are occasions where non-successful response codes will slip through, especially when authentication is involved (response codes 401 and 407).-s, --silent Silent or quiet mode. Don't show progress meter or error messages. Makes Curl mute.-S, --show-error When used with -s it makes curl show an error message if it fails.-L, --location (HTTP/HTTPS) If the server reports that the requested page has moved to a different location (indicated with a Location: header and a 3XX response code), this option will make curl redo the request on the new place. If used together with -i, --include or -I, --head, headers from all requested pages will be shown. When authentication is used, curl only sends its credentials to the initial host. If a redirect takes curl to a different host, it won't be able to intercept the user+password. See also --location-trusted on how to change this. You can limit the amount of redirects to follow by using the --max-redirs option. When curl follows a redirect and the request is not a plain GET (for example POST or PUT), it will do the following request with a GET if the HTTP response was 301, 302, or 303. If the response code was any other 3xx code, curl will re-send the following request using the same unmodified method.source manpages: curl
The screenshot is even more impressive:
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bash – How to get the primary IP address of the local machine on Linux and OS X? – Stack Overflow
Posted by jpluimers on 2021/02/05
[WayBack] bash – How to get the primary IP address of the local machine on Linux and OS X? – Stack Overflow (thanks [WayBack] Collin Anderson!):
I tried on OS X 10.13.6 (macOS High Sierra) and these two work fine:
ip route get 1 | awk '{print $NF;exit}'
and
ip route get 8.8.8.8 | head -1 | cut -d' ' -f8
The first one needs the $ back-slash escaped as bash alias; the second does not need that.
Related:
- [WayBack] scripts/internalip.md at master · rsp/scripts · GitHub
- [WayBack] internalip
- [WayBack] GitHub – rsp/scripts: Random shell scripts for different tasks
- [WayBack] Get Started | Public DNS | Google Developers
- [WayBack] linux – How can I get my external IP address in a shell script? – Unix & Linux Stack Exchange
- [WayBack] networking – How do I find my internal ip address? – Ask Ubuntu
–jeroen
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Posted in *nix, *nix-tools, Apple, bash, Mac OS X / OS X / MacOS, macOS 10.13 High Sierra, Power User | Leave a Comment »
