Assuming:
the path of file f is ~/f
"which f" shows "~/f",
Then,
which f | cat shows ~/f. So cat here is applied to the quotation of ~/f, which is different with cat ~/f.
My question is: how I could use one command composed of which and cat to achieve the result of cat ~/f? When I don't know the result of which f in advance, using this composition can be very convenient. Currently, if I don't know the result of which f in advance, I have to invoke which f first, and copy-paste the result to feed less.
A related question is: how can I assign the result of which f to a variable?
Thanks a lot!
Try:
cat `which ~/f`
For the related question:
foo=`which ~/f`
echo $foo
cat "`which f`"
Like so in bash:
cat "$(which f)"
var="$(which f)"
What you want is:
cat `which f`
In which f | cat the cat program gets the output of which f on standard input. That then just passes that standard input through, so the result is the same as a plain which f. In the call cat ~/f the data is passed as a parameter to the command. cat then opens the file ~/f and displays it's contents.
To get the output of which f as a parameter to cat you can, as others have answered, use backticks or $():
cat `which f`
cat $(which f)
Here the shell takes the output of which f and inserts it as a parameter for cat.
In bash, you can use:
cat "$(which f)"
to output the contents of the f that which finds. This, like the backtick solution, takes the output of the command within $(...) and uses that as a parameter to the cat command.
I prefer the $(...) to the backtick method since the former can be nested in more complex situations.
Assigning the output of which to a variable is done similarly:
full_f="$(which f)"
In both cases, it's better to use the quotes in case f, or it's path, contains spaces, as heinous as that crime is :-)
I've often used a similar trick when I want to edit a small group of files with similar names under a given sub-directory:
vim $(find . -type f -name Makefile)
which will give me a single vim session for all the makefiles (obviously, if there were a large number, I'd be using sed or perl to modify them en masse instead of vim).
cat echos the contents of files to the standard output. When you write stuff | cat, the file cat works on is the standard input, which is connected to the output of stuff (because pipes are files, just like nearly everything else in unix).
There is no quoting going on in the sense that a lisp programmer would use the word.
Related
I have to echo an output of a command substitution concatenated with string. The string to be prepended is in fact the string of pathname. The need for use of absolute path together with filename arises due to filename containing special character,-, at the beginning of it. I've come up with a draft that only works as planned for the first line of output. How do I expand it to other lines as well?
The example scenario is as provided below.
Inside /tmp directory the files are:
-foo 1.txt
-bar 1.txt
Command and the output is:
$ echo "$PWD/$(ls | grep "^-")"
/tmp/-bar 1.txt
-foo 1.txt
While I want it to be like
/tmp/-bar 1.txt
/tmp/-foo 1.txt
I read about this brace expansion feature but I'm not sure if it works for variables, or command substitution for that matter, as stated here. I also want the separate lines for each files and the filename words unsplitted, which is suggested at when the brace expansion is carried out. (Honestly, I don't understand much of the literature about the features such as brace expansion!)
Also, are there other more convenient ways to do this? Any help is appreciated.
To do what you're asking, getting a list of full paths for files starting with -, you can use readlink:
$ readlink -f ./-*
/tmp/-bar 1.pdf
/tmp/-foo 1.pdf
However, to do directly what you mentioned in comments (using filenames starting with - as arguments for pdfgrep), you can take advantage of the common convention that -- marks the end of options, so everything after it is recognized as a filename:
pdfgrep 'pattern' -- -*
See also POSIX Utility Syntax Guidelines (Guideline 10):
The first -- argument that is not an option-argument should be accepted as a delimiter indicating the end of options. Any following arguments should be treated as operands, even if they begin with the - character.
Don't use ls at all. You can use a glob, and your loop can be implicit
$ printf '%s\n' /tmp/-*
/tmp/-foo 1.txt
/tmp/-bar 1.txt
or explicit
$ for f in /tmp/-*; do echo "$f"; done
/tmp/-foo 1.txt
/tmp/-bar 1.txt
Use this
for f in /tmp/-*
do
echo $f
done
I'm pretty sure I've seen this done before but I can't remember the exact syntax.
Suppose you have a couple of files with different file extensions:
foo.txt
bar.rtf
index.html
and instead of doing something with all of them (cat *), you only want to run a command on 2 of the 3 file extensions.
Can't you do something like this?
cat ${*.txt|*.rtf}
I'm sure there's some find trickery to identify the files first and pipe them to a command, but I think bash supports what I'm talking about without having to do that.
The syntax you want is cat *.{txt,rft}. A comma is used instead of a pipe.
$ echo foo > foo.txt
$ echo bar > bar.rft
$ echo "bar txt" > bar.txt
$ echo "test" > index.html
$ cat *.{txt,rft}
bar txt
foo
bar
$ ls *.{txt,rft}
bar.rft bar.txt foo.txt
But as Anthony Geoghegan said in their answer there's a simpler approach you can use.
Shell globbing is much more basic than regular expressions. If you want to cat all the files which have a .txt or .rtf suffix, you'd simply use:
cat *.txt *.rtf
The glob patterns will be expanded to list all the filenames that match the pattern. In your case, the above command would call the cat command with foo.txt and bar.rtf as its arguments.
Here's a simple way i use to do it using command substitution.
cat $(find . -type f \( -name "*.txt" -o -name "*.rtf" \))
But Anthony Geoghegan's answer is much simpler. I learned from it too.
I have a shell-script which lists all the file names in a directory and store them in a new file.
The problem is that when I execute this script with the nohup command, it lists the first name four times instead of listing the correct names.
Commenting the problem with other programmers they think that the problem may be the ls command.
Part of my code is the following:
for i in $( ls -1 ./Datasets/); do
awk '{print $1}' ./genes.txt | head -$num_lineas | tail -1 >> ./aux
let num_lineas=$num_lineas-1
done
Do you know an alternative to ls that works well with nohup?
Thanks.
Don't use ls to feed the loop, use:
for i in ./Datasets/*; do
or if subdirectories are of interest
for i in ./Datasets/*/*; do
Lastly, and more correctly, use find if you need the entire tree below Datasets:
find ./Datasets -type f | while IFS= read -r file; do
(do stuff with $file)
done
Others frown, but there is nothing wrong with also using find as:
for file in $(find ./Datasets -type f); do
(do stuff with $file)
done
Just choose the syntax that most closely meets your needs.
First of all, don't parse ls! A simple glob will suffice. Secondly, your awk | head | tail chain can be simplified by only printing the first column of the line that you're interested in using awk. Thirdly, you can redirect the output of your loop to a file, rather than using >>.
Incorporating all of those changes into your script:
for i in Datasets/*; do
awk -v n="$(( num_lineas-- ))" 'NR==n{print $1}' genes.txt
done > aux
Every time the loop goes round, the value of $num_lineas will decrease by 1.
In terms of your problem with nohup, I would recommend looking into using something like screen, which is known to be a better solution for maintaining a session between logins.
Hi my question is simple. I want to do this in a command prompt.
var="ls | cat"
$var
Now I know that when I try to do this manually
ls | cat
Bash takes | as a special thing. I don't know how its called, I know | it's called a pipe but I mean that bash takes | as a ... and actually makes a pipe. I also figured that when I try to do $var bash actually takes | as a string and not as a pipe. Well, my question is How can I make bash to realize that | is actually a pipe and not a string. Thanks, I hope I am clear about my point.
Simple solution: use eval:
var="ls | cat"
eval $var
bash interprets the arguments to eval as if you had typed that on the command line.
Of course, keep in mind the security risks to using eval with user input, in case that's an issue for your program.
This may or may not apply - but it sounds like you may be looking for the alias command. You can do alias var="ls | cat" and then in your command prompt you can do var and it treats it as if you wrote ls | cat
Rather than trying to embed executable code into a variable (which should be used to hold data, not code), use a shell function, which is intended to hold code:
my_func () {
ls | cat
}
| is called a pipe, I haven't heard any other naming. Basically the stream output by the command on its left goes as the input to the command on its right. In your case, ls output goes into a stream (i.e. a temporary file), and that stream is fed to cat. cat prints the content of a file, and ls stream is very much like a file.
Now, you are trying to make bash interpret your variable var. To do this, try:
var=`ls | cat`
$var
On my computer I get this:
-bash: Applications: command not found
Because in my case, $var is expanded to Applications Documents Downloads, the output of my ls.
Given crudely as is, bash believes this is a command I want him to execute.
If your intention is not to execute $varcontent but print it, try:
var=`ls | cat`
echo $var
The cat is not needed here, just use ls -1 and as other answers say you can alias it or put it in a function.
For example, if you want to override ls to print each file on a new line do something like
> alias ls='command ls -1'
> ls
file1
file2
etc...
And put it in a bash init file like ~/.bashrc if you want to make the change permanent
1) Functions are suitable for such tasks:
func (){
ls | cat
}
Invoke it by saying func
2) Also another suitable solution could be eval:
eval takes a string as its argument, and evaluates it as if you'd typed that string on a command line. (If you pass several arguments, they are first joined with spaces between them.)
var="ls | cat"
eval $var
This is what I used:
for i in `find some -type f -name *.class`
I got:
some/folder/subOne/fileOne.class
some/folder/subOne/fileTwo.class
some/other/sub/file.class
next, I would like to get rid of the "some/" for each value of $i. What command can I use? Do I HAVE to save them into a file first?
Thanks
$ i=some/other/sub/file.class
$ echo ${i#some/}
other/sub/file.class
Bash has simple string manipulation built in. See also ${i%.class} and the basename and dirname commands.
awk :)
http://en.wikipedia.org/wiki/AWK
EDIT: Oh and you can pipe commands together, so the output of the first command acts as the input for the second. Like 'cat example.txt | less' will output the file through a paginator.