I would like to know what the difference between the two commands below is ?
ubuntu:~/bin$ (ls -A1 /home/ | wc -l)
1
ubuntu:~/bin$ $(ls -A1 /home/ | wc -l)
1: command not found
If I put dir_count=(ls -A1 /home/ | wc -l) in a script i get the following error.
./two_args: line 24: syntax error near unexpected token `|'
./two_args: line 24: `dir_1_count=(ls -A1 "$dir_1" | wc -l)'
where as the following works:
dir_count=$(ls -A1 /home/ | wc -l)
$(command), is command substitution. It simply executes the command and substitutes the standard output of the command.
So if you want to set the variable, simply: dir_count=$(ls -A1 /home/ | wc -l)
About the rest of your code:
(ls -A1 /home/ | wc -l)
this one executes the command in a subshell. You probably don't want those parentheses.
$(ls -A1 /home/ | wc -l)
this one just doesn't make any sense, you substitute the result, so you get 1, and the shell will try to execute the command called 1.
To simplify the variable management in the script, I recommend you try the following:
dir_1_count=`ls -A1 ${dir1} | wc -l`
Use the character "`" to delimitate variables and store the result of a usual shell command in your script.
Remember to use the inverted apostrophe, not the single quote, copy the variable definition I just gave you.
(...) is just a special grouping operator. It runs ... in a subshell, meaning that it cannot modify the parent's execution environment (so, for example, (foo=bar) is useless, because the assignment will not survive past the end of the command), but is otherwise treated pretty normally (its standard output goes to standard output, etc.).
$(...) is a substitution; just like how $foo gets replaced with the value of the variable foo, $(...) gets replaced with the output of the command .... More precisely . . . like (...), it also runs ... in a subshell, but in addition, it captures the standard output of ..., and then ends up getting replaced with that output. So, for example, this:
"$(echo cd)" "foo$(echo bar)"
runs echo cd and captures the cd, and runs echo bar and captures the bar, and then runs the combined command cd foobar.
Related
I am new to bash and scripting and I am trying to create a simple script but for some reason it won't let me run this:
fileCount= ls -1 | wc -l
#echo $fileCount
for (( i=0; i<$fileCount; ++i )) ; do
echo item: $i
done
Whenever I try to run this it just gives me an error message saying it expected an operand.
I am really confused on the error here and any help would be greatly appreciated!
To get your code running with minimal change, replace:
fileCount= ls -1 | wc -l
With:
fileCount=$(ls -1 | wc -l)
$(...) is called command substitution. It is what you use when you want capture the output of a command in a variable.
It is very important that there be no spaces on either side of the equal sign.
Improvements
To speed up the result, use the -U option to turn off sorting.
To prevent any attempt to display special characters, use -q.
Thus:
fileCount=$(ls -1Uq | wc -l)
Lastly, when ls is writing to something other than a terminal, such as, in this command, a pipeline, it prints one file name per line. This makes -1 optional.
you missed to assign the output of wc -l to your variable. Try this:
fileCount=$(ls | wc -l)
(option "-1" is not needed, because ls writes one file per line if its stdout is not a terminal)
How can I perform an operation for each item listed by grep individually?
Background:
I use grep to list all files containing a certain pattern:
grep -l '<pattern>' directory/*.extension1
I want to delete all listed files but also all files having the same file name but a different extension: .extension2.
I tried using the pipe, but it seems to take the output of grep as a whole.
In find there is the -exec option, but grep has nothing like that.
If I understand your specification, you want:
grep --null -l '<pattern>' directory/*.extension1 | \
xargs -n 1 -0 -I{} bash -c 'rm "$1" "${1%.*}.extension2"' -- {}
This is essentially the same as what #triplee's comment describes, except that it's newline-safe.
What's going on here?
grep with --null will return output delimited with nulls instead of newline. Since file names can have newlines in them delimiting with newline makes it impossible to parse the output of grep safely, but null is not a valid character in a file name and thus makes a nice delimiter.
xargs will take a stream of newline-delimited items and execute a given command, passing as many of those items (one as each parameter) to a given command (or to echo if no command is given). Thus if you said:
printf 'one\ntwo three \nfour\n' | xargs echo
xargs would execute echo one 'two three' four. This is not safe for file names because, again, file names might contain embedded newlines.
The -0 switch to xargs changes it from looking for a newline delimiter to a null delimiter. This makes it match the output we got from grep --null and makes it safe for processing a list of file names.
Normally xargs simply appends the input to the end of a command. The -I switch to xargs changes this to substitution the specified replacement string with the input. To get the idea try this experiment:
printf 'one\ntwo three \nfour\n' | xargs -I{} echo foo {} bar
And note the difference from the earlier printf | xargs command.
In the case of my solution the command I execute is bash, to which I pass -c. The -c switch causes bash to execute the commands in the following argument (and then terminate) instead of starting an interactive shell. The next block 'rm "$1" "${1%.*}.extension2"' is the first argument to -c and is the script which will be executed by bash. Any arguments following the script argument to -c are assigned as the arguments to the script. This, if I were to say:
bash -c 'echo $0' "Hello, world"
Then Hello, world would be assigned to $0 (the first argument to the script) and inside the script I could echo it back.
Since $0 is normally reserved for the script name I pass a dummy value (in this case --) as the first argument and, then, in place of the second argument I write {}, which is the replacement string I specified for xargs. This will be replaced by xargs with each file name parsed from grep's output before bash is executed.
The mini shell script might look complicated but it's rather trivial. First, the entire script is single-quoted to prevent the calling shell from interpreting it. Inside the script I invoke rm and pass it two file names to remove: the $1 argument, which was the file name passed when the replacement string was substituted above, and ${1%.*}.extension2. This latter is a parameter substitution on the $1 variable. The important part is %.* which says
% "Match from the end of the variable and remove the shortest string matching the pattern.
.* The pattern is a single period followed by anything.
This effectively strips the extension, if any, from the file name. You can observe the effect yourself:
foo='my file.txt'
bar='this.is.a.file.txt'
baz='no extension'
printf '%s\n'"${foo%.*}" "${bar%.*}" "${baz%.*}"
Since the extension has been stripped I concatenate the desired alternate extension .extension2 to the stripped file name to obtain the alternate file name.
If this does what you want, pipe the output through /bin/sh.
grep -l 'RE' folder/*.ext1 | sed 's/\(.*\).ext1/rm "&" "\1.ext2"/'
Or if sed makes you itchy:
grep -l 'RE' folder/*.ext1 | while read file; do
echo rm "$file" "${file%.ext1}.ext2"
done
Remove echo if the output looks like the commands you want to run.
But you can do this with find as well:
find /path/to/start -name \*.ext1 -exec grep -q 'RE' {} \; -print | ...
where ... is either the sed script or the three lines from while to done.
The idea here is that find will ... well, "find" things based on the qualifiers you give it -- namely, that things match the file glob "*.ext", AND that the result of the "exec" is successful. The -q tells grep to look for RE in {} (the file supplied by find), and exit with a TRUE or FALSE without generating any of its own output.
The only real difference between doing this in find vs doing it with grep is that you get to use find's awesome collection of conditions to narrow down your search further if required. man find for details. By default, find will recurse into subdirectories.
You can pipe the list to xargs:
grep -l '<pattern>' directory/*.extension1 | xargs rm
As for the second set of files with a different extension, I'd do this (as usual use xargs echo rm when testing to make a dry run; I haven't tested it, it may not work correctly with filenames with spaces in them):
filelist=$(grep -l '<pattern>' directory/*.extension1)
echo $filelist | xargs rm
echo ${filelist//.extension1/.extension2} | xargs rm
Pipe the result to xargs, it will allow you to run a command for each match.
Here's a snippet:
var=`ls | shuf | head -2 | xargs cat | sed -e 's/\(.\)/\1\n/g' | shuf | tr -d '\n'`
This will select two random files from the current directory, combine their contents, shuffle them, and assign the result to var. This works fine most of the time, but about once in a thousand cases, instead just the output of ls is bound to var (It's not just the output, see EDIT II). What could be the explanation?
Some more potentially relevant facts:
the directory contains at least two files
there are only text files in the directory
file names don't contain spaces
the files are anywhere from 5 to about 1000 characters in length
the snippet is a part of a larger script that it ran two instances in parallel
bash version: GNU bash, version 4.1.5(1)-release (i686-pc-linux-gnu)
uname: Linux 2.6.35-28-generic-pae #50-Ubuntu
EDIT: I ran the snippet by itself a couple of thousand times with no errors. Then I tried running it with various other parts of the whole script. Here's a configuration that produces errors:
cd dir_with_text_files
var=`ls | shuf | head -2 | xargs cat | sed -e 's/\(.\)/\1\n/g' | shuf | tr -d '\n'`
cd ..
There are several hundred lines of the script between the cds, but this is the minimal configuration to reproduce the error. Note that the anomalous output binds to var the output of the current directory, not dir_with_text_files.
EDIT II: I've been looking at the outputs in more detail. The ls output doesn't appear alone, it's along with with two shuffled files (between their contents, or after or before them, intact). But it gets better; let me set up the stage to talk about particular directories.
[~/projects/upload] ls -1
checked // dir
lines // dir, the files to shuffle are here
pages // also dir
proxycheck
singlepost
uploader
indexrefresh
t
tester
So far, I've seen the output of ls ran from upload, but now I saw the output of ls */* (also ran from upload). It was in the form of "someMangledText ls moreMangledText ls */* finalBatchOfText". Is it possible that the sequence ls that undoubtedly was generated was somehow executed?
No problems here either.
I would also rewrite the above to this:
sed 's:\(.\):\1\n:g' < <(shuf -e * | head -2 | xargs cat) | shuf | tr -d '\n'
Do not use ls to list a directory's content, use *.
Moreover, do some debugging. Use a shebang followed by:
set -e
set -o pipefail
and run the script like this:
/bin/bash -x /path/to/script
and do inspect the output.
Instead of debugging the whole script, you can surround just the part that seems to be problematic with -x
set -x
...code that may have problems...
set +x
so that the output focuses on that part of the code.
Also, use the pipefail option.
Some definitions:
-e : Exit immediately if a simple command exits with a non-zero status, unless the command that fails is part of the command list immediately following a while or until keyword, part of the test in an if statement, part of a && or || list, or if the command's return status is being inverted using !. A trap on ERR, if set, is executed before the shell exits
-x : Print a trace of simple commands, for commands, case commands, select commands, and arithmetic for commands and their arguments or associated word lists after they are expanded and before they are executed. The value of the PS4 variable is expanded and the resultant value is printed before the command and its expanded arguments
pipefail : If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully
For debugging purposes you may also clear the environment using env -i and filter out non-printable characters:
#!/usr/bin/env -i /bin/bash --
set -ef
set -o pipefail
unset IFS PATH LC_ALL
IFS=$' \t\n'
PATH="$(PATH=/bin:/usr/bin getconf PATH)"
LC_ALL=C
export IFS PATH LC_ALL
#var="$((find . -type f -maxdepth 1 -print0 | shuf -z -n 2 | xargs -0 cat) | sed -e 's/\(.\)/\1\n/g' | shuf | tr -d '\n')"
var="$((find . -type f -maxdepth 1 -print0 | shuf -z -n 2 | xargs -0 cat) | tr -cd '[[:print:]]' | grep -o '.' | shuf | tr -d '\n')"
Before running the script you may also disable the GNU readline library and ! style history expansion:
bash --noediting
set +H
Based on what you say wrt to your failure rates, and given the success of the other tests performed by the posters above, it sounds like a problem that could be caused by an occasional directory-change failure. Is the directory you're accessing in this script mounted from a remote machine by chance? If so, it might just be a small and temporary network-related failure that's not being handled properly. (Just a guess.)
Suppose I have some output from a command (such as ls -1):
a
b
c
d
e
...
I want to apply a command (say echo) to each one, in turn. E.g.
echo a
echo b
echo c
echo d
echo e
...
What's the easiest way to do that in bash?
It's probably easiest to use xargs. In your case:
ls -1 | xargs -L1 echo
The -L flag ensures the input is read properly. From the man page of xargs:
-L number
Call utility for every number non-empty lines read.
A line ending with a space continues to the next non-empty line. [...]
You can use a basic prepend operation on each line:
ls -1 | while read line ; do echo $line ; done
Or you can pipe the output to sed for more complex operations:
ls -1 | sed 's/^\(.*\)$/echo \1/'
for s in `cmd`; do echo $s; done
If cmd has a large output:
cmd | xargs -L1 echo
You can use a for loop:
for file in * ; do
echo "$file"
done
Note that if the command in question accepts multiple arguments, then using xargs is almost always more efficient as it only has to spawn the utility in question once instead of multiple times.
You actually can use sed to do it, provided it is GNU sed.
... | sed 's/match/command \0/e'
How it works:
Substitute match with command match
On substitution execute command
Replace substituted line with command output.
A solution that works with filenames that have spaces in them, is:
ls -1 | xargs -I %s echo %s
The following is equivalent, but has a clearer divide between the precursor and what you actually want to do:
ls -1 | xargs -I %s -- echo %s
Where echo is whatever it is you want to run, and the subsequent %s is the filename.
Thanks to Chris Jester-Young's answer on a duplicate question.
xargs fails with with backslashes, quotes. It needs to be something like
ls -1 |tr \\n \\0 |xargs -0 -iTHIS echo "THIS is a file."
xargs -0 option:
-0, --null
Input items are terminated by a null character instead of by whitespace, and the quotes and backslash are
not special (every character is taken literally). Disables the end of file string, which is treated like
any other argument. Useful when input items might contain white space, quote marks, or backslashes. The
GNU find -print0 option produces input suitable for this mode.
ls -1 terminates the items with newline characters, so tr translates them into null characters.
This approach is about 50 times slower than iterating manually with for ... (see Michael Aaron Safyans answer) (3.55s vs. 0.066s). But for other input commands like locate, find, reading from a file (tr \\n \\0 <file) or similar, you have to work with xargs like this.
i like to use gawk for running multiple commands on a list, for instance
ls -l | gawk '{system("/path/to/cmd.sh "$1)}'
however the escaping of the escapable characters can get a little hairy.
Better result for me:
ls -1 | xargs -L1 -d "\n" CMD
I'm well aware of the source (aka .) utility, which will take the contents from a file and execute them within the current shell.
Now, I'm transforming some text into shell commands, and then running them, as follows:
$ ls | sed ... | sh
ls is just a random example, the original text can be anything. sed too, just an example for transforming text. The interesting bit is sh. I pipe whatever I got to sh and it runs it.
My problem is, that means starting a new sub shell. I'd rather have the commands run within my current shell. Like I would be able to do with source some-file, if I had the commands in a text file.
I don't want to create a temp file because feels dirty.
Alternatively, I'd like to start my sub shell with the exact same characteristics as my current shell.
update
Ok, the solutions using backtick certainly work, but I often need to do this while I'm checking and changing the output, so I'd much prefer if there was a way to pipe the result into something in the end.
sad update
Ah, the /dev/stdin thing looked so pretty, but, in a more complex case, it didn't work.
So, I have this:
find . -type f -iname '*.doc' | ack -v '\.doc$' | perl -pe 's/^((.*)\.doc)$/git mv -f $1 $2.doc/i' | source /dev/stdin
Which ensures all .doc files have their extension lowercased.
And which incidentally, can be handled with xargs, but that's besides the point.
find . -type f -iname '*.doc' | ack -v '\.doc$' | perl -pe 's/^((.*)\.doc)$/$1 $2.doc/i' | xargs -L1 git mv
So, when I run the former, it'll exit right away, nothing happens.
The eval command exists for this very purpose.
eval "$( ls | sed... )"
More from the bash manual:
eval
eval [arguments]
The arguments are concatenated together
into a single command, which
is then read and executed, and its
exit status returned as the exit
status of eval. If there are no
arguments or only empty arguments, the
return status is zero.
$ ls | sed ... | source /dev/stdin
UPDATE: This works in bash 4.0, as well as tcsh, and dash (if you change source to .). Apparently this was buggy in bash 3.2. From the bash 4.0 release notes:
Fixed a bug that caused `.' to fail to read and execute commands from non-regular files such as devices or named pipes.
Try using process substitution, which replaces output of a command with a temporary file which can then be sourced:
source <(echo id)
Wow, I know this is an old question, but I've found myself with the same exact problem recently (that's how I got here).
Anyway - I don't like the source /dev/stdin answer, but I think I found a better one. It's deceptively simple actually:
echo ls -la | xargs xargs
Nice, right? Actually, this still doesn't do what you want, because if you have multiple lines it will concat them into a single command instead of running each command separately. So the solution I found is:
ls | ... | xargs -L 1 xargs
the -L 1 option means you use (at most) 1 line per command execution. Note: if your line ends with a trailing space, it will be concatenated with the next line! So make sure each line ends with a non-space.
Finally, you can do
ls | ... | xargs -L 1 xargs -t
to see what commands are executed (-t is verbose).
Hope someone reads this!
`ls | sed ...`
I sort of feel like ls | sed ... | source - would be prettier, but unfortunately source doesn't understand - to mean stdin.
I believe this is "the right answer" to the question:
ls | sed ... | while read line; do $line; done
That is, one can pipe into a while loop; the read command command takes one line from its stdin and assigns it to the variable $line. $line then becomes the command executed within the loop; and it continues until there are no further lines in its input.
This still won't work with some control structures (like another loop), but it fits the bill in this case.
To use the mark4o's solution on bash 3.2 (macos) a here string can be used instead of pipelines like in this example:
. /dev/stdin <<< "$(grep '^alias' ~/.profile)"
I think your solution is command substitution with backticks: http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_04.html
See section 3.4.5
Why not use source then?
$ ls | sed ... > out.sh ; source out.sh