ls /tmp
How can I run the same command but using reverse polish notation?
Is there a mode that would allow me to do this or something similar to that?
I could use xargs but that's a lot more typing:
echo /tmp | xargs ls
This would be ideal:
/tmp ls
or
/tmp | ls
Bash (I assume you are using it) is a shell for unixoid systems.
As far as I know, bash doesn't provide such a mode. You could use a different shell that provides this feature. Searching in the web, this was my first result: https://github.com/iconmaster5326/RPOS, but maybe it is far from stable ;)
Alternatively, you can make a command that reverses it's argument list and execute it.
The usage would be like this:
reversex /tmp ls
reversex A.txt B.txt cp
Here is an example of such a command:
#!/bin/bash
for i in "$#"
do
CMDLINE="$i $CMDLINE"
done
$CMDLINE
If you name it /usr/local/bin/reversex and make it executable, you should be able to use simple reverse commands with the prefix reversex. I can not give a warranty that it works. Note that the arguments are parsed twice and have to be escaped twice, too.
Related
I am developing on mac and use the following command to confirm file names to a json array:
ls **/*.test.json | jq -R -s -c 'split("\n")[:-1]'
Which gives me the json array:
['folder1/a.test.json', 'folder2/b.test.json', 'c.test.json']
Which is exactly what I want. However, when executing on github action (with linux), the above command produces outcome:
['c.test.json]
And the files within folders are not included.
I confirmed that the folders where checkout successfully because echo $(ls folder1) gives a.test.json.
What is the best way to achieve what I want for the command?
** is not a standard sh feature; your Github action probably requires a POSIX shell script.
To traverse arbitrarily deep directory structure with a shell which doesn't support **, try find:
find . -name '*.test.json' -print |
jq -R -s -c 'split("\n")[:-1]'
If there is only a limited set of directory levels, maybe try
printf '%s\n' */*.test.json *.test.json | jq ...
(Also don't use ls in scripts and perhaps see also useless use of echo.)
Both of these have some gnarly corner cases if you have file names which contain newlines; find could probably be coerced to handle that case, too, but I'll not complicate this answer further; perhaps see https://mywiki.wooledge.org/BashFAQ/020 for a fuller discussion.
If your shell is Bash, ** is available, but typically not enabled out of the box; you enable it with shopt -s globstar ... but perhaps it's better to stick to proper sh in case Github changes the default shell for Actions.
Hi I was wondering how to read the argument after "|" pipe from shell script.
For example, when I run ./tmp.sh ls -la | sort
I could only get 2 arguments, which is "ls" and "-la".
Is there any way to read "| sort" without modifying the command, and realize only with shell script?
Thanks a lot!!
One way would be to pass the entire command as a string to your script.
./tmp.sh -c "ls -la | sort"
...or without a flag...
./tmp.sh "ls -la | sort"
Afterward, you can split the string into an array in your script.
I guess you could check ps axf, but part of the beauty of pipes is the loose coupling they give, because bash knows what is in your pipeline, not the individual pieces of the pipeline. This makes writing filters simple.
I am trying to get the most recent .CSV or .csv file name among other comma separated value files where the extension name is case insensitive.
I am achieving this with the following command, provided by someone else without any explanation:
ls -t ~(i:*.CSV) | head -1
or
ls -t -- ~(i:*.CSV) | head -1
I have two questions:
What is the use of ~ and -- in this case? Does -- helps here?
How can I get a blank response when there is no .csv or .CSV file in
the folder? At the moment I get:
/bin/ls: cannot access ~(i:*.CSV): No such file or directory
I know I can test the exit code of the last command, but I was wondering maybe there is a --silent option or something.
Many thanks for your time.
PS: I made my research online quite thorough and I was unable to find an answer.
The ~ is just a literal character; the intent would appear to be to match filenames starting with ~ and ending with .csv, with i: being a flag to make the match case-insensitive. However, I don't know of any shell that supports that particular syntax. The closest thing I am aware of would be zsh's globbing flags:
setopt extended_glob # Allow globbing flags
ls ~(#i)*.csv
Here, (#i) indicates that anything after it should be matched without regard to case.
Update: as #baptistemm points out, ~(i:...) is syntax defined by ksh.
The -- is a conventional argument, supported by many commands, to mean that any arguments that follow are not options, but should be treated literally. For example, ls -l would mean ls should use the -l option to modify its output, while ls -- -l means ls should try to list a file named -l.
~(i:*.CSV) is to tell to shell (this is only supported apparently in ksh93) the enclosed text after : must be treated as insensitive, so in this example that could all these possibilites.
*.csv or
*.Csv or
*.cSv or
*.csV or
*.CSv or
*.CSV
Note this could have been written ls -t *.[CcSsVv] in bash.
To silent errors I suggest you to look for in this site for "standard error /dev/null" that will help.
I tried running commands like what you have in both bash and zsh and neither worked, so I can't help you out with that, but if you want to discard the error, you can add 2>/dev/null to the end of the ls command, so your command would look like the following:
ls -t ~(i:*.CSV) 2>/dev/null | head -1
This will redirect anything written to STDERR to /dev/null (i.e. throw it out), which, in your case, would be /bin/ls: cannot access ~(i:*.CSV): No such file or directory.
In bash (or zsh), is there a way to 'look back' on previous output?
Say I have the following:
$ find . -depth 1 -name "d*"
dir1
dir2
dir3
$ cd [3rd line]
Can I pluck 'dir3' from the previous output without repeating the previous command and using a pipe? Just sort of "grab it" from onscreen?
The shell doesn't see that output at all. When findruns, it uses the file handle that it inherits from the shell to write its output, but that isn't visible to the shell (though even if it were, the shell isn't caching it).
However, given that you are using zsh, the first question you should ask involving an external command is, "Do I even need the external command?" As is often the case, you don't here. Instead of using find, just use print and a glob to get the same listing:
% print -l d*
then use the same glob again with a qualifier to select the 3rd result.
% cd d*([3])
(This assumes that the expansion of d* won't change between running the print command and running the cd command.)
Or, you can cache the result yourself:
% choices=( d* )
% print -l $choices
dir1
dir2
dir3
% cd $choices[3]
The array-based solution could work in bash as well, though with slightly different syntax (and assuming your find command is simple enough to be replaced with one of bash's far less powerful globs.)
Finally, there is the select command:
select d in d*; do
cd "$d"; break;
done
which will also work as-is in bash, again assuming a valid glob.
A cursory search reveals neither bash nor zsh provide this functionality. Happy to be corrected if there's a way to do this. :)
I'm using ls -a command to get the file names in a directory, but the output is in a single line.
Like this:
. .. .bash_history .ssh updater_error_log.txt
I need a built-in alternative to get filenames, each on a new line, like this:
.
..
.bash_history
.ssh
updater_error_log.txt
Use the -1 option (note this is a "one" digit, not a lowercase letter "L"), like this:
ls -1a
First, though, make sure your ls supports -1. GNU coreutils (installed on standard Linux systems) and Solaris do; but if in doubt, use man ls or ls --help or check the documentation. E.g.:
$ man ls
...
-1 list one file per line. Avoid '\n' with -q or -b
Yes, you can easily make ls output one filename per line:
ls -a | cat
Explanation: The command ls senses if the output is to a terminal or to a file or pipe and adjusts accordingly.
So, if you pipe ls -a to python it should work without any special measures.
Ls is designed for human consumption, and you should not parse its output.
In shell scripts, there are a few cases where parsing the output of ls does work is the simplest way of achieving the desired effect. Since ls might mangle non-ASCII and control characters in file names, these cases are a subset of those that do not require obtaining a file name from ls.
In python, there is absolutely no reason to invoke ls. Python has all of ls's functionality built-in. Use os.listdir to list the contents of a directory and os.stat or os to obtain file metadata. Other functions in the os modules are likely to be relevant to your problem as well.
If you're accessing remote files over ssh, a reasonably robust way of listing file names is through sftp:
echo ls -1 | sftp remote-site:dir
This prints one file name per line, and unlike the ls utility, sftp does not mangle nonprintable characters. You will still not be able to reliably list directories where a file name contains a newline, but that's rarely done (remember this as a potential security issue, not a usability issue).
In python (beware that shell metacharacters must be escapes in remote_dir):
command_line = "echo ls -1 | sftp " + remote_site + ":" + remote_dir
remote_files = os.popen(command_line).read().split("\n")
For more complex interactions, look up sftp's batch mode in the documentation.
On some systems (Linux, Mac OS X, perhaps some other unices, but definitely not Windows), a different approach is to mount a remote filesystem through ssh with sshfs, and then work locally.
you can use ls -1
ls -l will also do the work
You can also use ls -w1
This allows to set number of columns.
From manpage of ls:
-w, --width=COLS
set output width to COLS. 0 means no limit
ls | tr "" "\n"
Easy, as long as your filenames don't include newlines:
find . -maxdepth 1
If you're piping this into another command, you should probably prefer to separate your filenames by null bytes, rather than newlines, since null bytes cannot occur in a filename (but newlines may):
find . -maxdepth 1 -print0
Printing that on a terminal will probably display as one line, because null bytes are not normally printed. Some programs may need a specific option to handle null-delimited input, such as sort's -z. Your own script similarly would need to account for this.
-1 switch is the obvious way of doing it but just to mention, another option is using echo and a command substitution within a double quote which retains the white-spaces(here \n):
echo "$(ls)"
Also how ls command behaves is mentioned here:
If standard output is a terminal, the output is in columns (sorted
vertically) and control characters are output as question marks;
otherwise, the output is listed one per line and control characters
are output as-is.
Now you see why redirecting or piping outputs one per line.