Linux Commands Examples

A great documentation place for Linux commands

cat

concatenate files and print on the standard output


see also : tac

Synopsis

cat [OPTION]... [FILE]...


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examples

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cat $2
1

cat f - g

Output f’s contents, then standard input, then g’s contents.

cat

Copy standard input to standard output.


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What does  cat $1 mean?



$1 is the variable that contains the value of the first argument when calling a script.



Here is a practical example :



$  echo 'cat $1' > test_script #put the content 'cat $1' in the file test_script

$  chmod u+x ./test_script #make the script executable by the user, aka you

$  echo 'asdfaab asfa' > foo #add the line 'asdfaab asfa' in the file foo

$  ./test_script foo bar beta  # call the script test_script with three arguments: foo, bar and beta



When calling the script, the first argument will be foo. Thus, the script will execute the command 'cat foo', meaning : outputing the file foo .
example added by LeBerger
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cat $0
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Why is cat not changing the access time?

http://en.wikipedia.org/wiki/Stat_(system_call)

Criticism of atime

Writing to a file changes its mtime and ctime, while reading a file changes its atime. As a result, on a POSIX-compliant system, reading a file causes a write, which has been criticized. This behaviour can usually be disabled by adding a mount option in /etc/fstab.

However, turning off atime updating breaks POSIX compliance, and some applications, notably the mutt mail reader (in some configurations), and some file usage watching utilities, notably tmpwatch. In the worst case, not updating atime can cause some backup programs to fail to back up a file.

Linux kernel developer Ingo Molnár called atime "perhaps the most stupid Unix design idea of all times," adding: "[T]hink about this a bit: 'For every file that is read from the disk, lets do a ... write to the disk! And, for every file that is already cached and which we read from the cache ... do a write to the disk!'" He further emphasized the performance impact thus:

Atime updates are by far the biggest IO performance deficiency that Linux has today. Getting rid of atime updates would give us more everyday Linux performance than all the pagecache speedups of the past 10 years, combined.

how to know if noatime or relatime is default mount option in kernel?

man mount
....
   relatime
          Update inode access times relative to  modify  or  change  time.
          Access time is only updated if the previous access time was ear?
          lier than the current modify or change time. (Similar  to  noat?
          ime,  but  doesn't break mutt or other applications that need to
          know if a file has been read since the last time  it  was  modi?
          fied.)

          Since Linux 2.6.30, the kernel defaults to the behavior provided
          by this option (unless noatime was  specified), and the stricta?
          time  option  is  required  to  obtain traditional semantics. In
          addition, since Linux 2.6.30, the file's  last  access  time  is
          always  updated  if  it  is more than 1 day old.
....

Which is how that particular partition was mounted and why cat does not update the access time as I expected.

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Linux shellscript combine all files without for loop

If there aren't too many files:

cat * > /some/new/file

Otherwise:

find . -exec cat {} + > /some/new/file
find . -exec cat {} \; > /some/new/file

0
cat /proc/version

## What does it do ?

Linux version and other info

## Output

Linux version 3.2.0-4-amd64 (debian-kernel@lists.debian.org) (gcc version 4.6.3 (Debian 4.6.3-14) ) #1 SMP Debian 3.2.54-2
example added by LeBerger
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cat README
cat README
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cat $1
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cat .descript
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cat catarrhini
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Are there any options to let cat output with color?

No, cat has no syntax highlighting abilities. If you'd like to view source code with syntax highlighting, pop it into vim or your editor of choice (that has syntax highlighting). This way, you can even page through the output if it's a long file using Ctrl + F (forward) and Ctrl + B (backwards).

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Is it wasteful to call cat?

Seven. But seriously, it's hard enough to know how long a disk will last while idling let alone under heavy load. There's no answer other than to say it will probably wear the disk faster.

The better argument against this is it would generally be quite slow. Why would you need to hammer the disk like that?

If you're looking to find out when something changes, perhaps look at inotify which is a kernel-based file event system that can call some code when something happens, negating the need to hammer the disk.

There are wrappers like pyinotify to make things easier.

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Concatenate files over FTP

Unless you've got remote desktop access to the FTP server and can open a session thru Remote Desktop and launch a concatenation program on the server, then the answer is no : the ftp servers I know don't allow remote execution and don't do concatenation.
I'm afraid you'll need to re-upload the unsplit file.

I would also like to add that uploading thru multiple connections doesn't improve the upload time, which stays always limited by your bandwidth. For example, if your upload bandwidth is 20k, then one connection will upload at the speed of 20k, while two connections will upload at the speed of 2X10k=20k. Total gain is then zero. When uploading a large file, it's important to use an FTP client that supports resumes, so in case of disconnection the data already uploaded is not lost and you can later restart from where you stopped.

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Showing the line count of a specific file

You can use this command:

wc -l <file>

This will return the total line number count in the provided file.

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Is there a way to reassemble files split without enumerating all of the parts explicitly?

This is what wildcards and brace expansion are for. See if echo file.bz2.part-* returns the filenames in the desired order, and use cat file.bz2.part-* > file.bz2 if it does. Otherwise, figure out some other more complex expansion that does.

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List only the device names of all available network interfaces

to just print the first column:

netstat -a | awk '{print $1}'

you can incorporate other rules in awk to add or remove entries as needed.

EDIT: same goes with ifconfig (like Doug pointed out)

ifconfig | awk '{print $1}'

This is an example excluding the 'lo' interface

ifconfig | awk '{if ($1 != lo) print $1}'

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Linux cat example

Because it opens and truncates the file before reading the data — it being shell, the redirections are processed by shell before even starting cat.

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Cat command output varies unusually in regards to * character

Remember that the command line is parsed and wildcards are replaced BEFORE it is executed.

If you specify the username, then the sudo ... command can see the file because it's accessing it as root.

When you type '*', your USER shell expands that to the paths it can see, well... the paths which allow your current user to search the subdirectories.

You might try enclosing the path string in SINGLE quotes to prevent expansion, I'm not sure that'll work since most unix programs expect the shell to expand wildcards and thus don't perform the globbing action themselves.

EDIT: A thought about how to accomplish your goal of reading the authorized keys files for all the users:

sudo find /home -name "authorized_keys" -exec cat "{}" \; > all_the_data

Maybe crude, but it'll work since you're executing the file search as root, not your user.

I suppose another way:

sudo 'find /home -name "authorized_keys" -print0 | xargs -0 cat' > all_the_data

That one's nicer on the process count, since it accumulates all the filenames and then 'cat's them... but I like the first one, no special quoting required.

The quotes (single or otherwise) around the entire command are required because of the pipe command. both the find and xargs command MUST be executed as root. Yeah, more complicated than the first one.

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Can cat be used to clone a partition?

In principle, you could use either. There are few important differences, but none that apply here.

  • When you use > redirection, the target file is opened, and truncated. Only then it is written to. However this does not apply to block devices — they have a fixed size, so “truncation†doesn't do anything to them.

  • With cat you can not easily tell it to only copy the first n bytes or skip/seek. This is what dd is useful for.

  • cat does not let you specify a block size. This won't matter today when block sizes are masked by the file systems being used, but it used to make a difference where devices would be read from with specific block sizes (tapes).

  • For hard disks, cat may be slightly faster (better even than dd with a well-chosen block size, let alone the default which slows things down).

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How to recursively find a .doc file that contains a specific word?

Use find for recursive searches:

find -name '*.doc' -exec catdoc {} + | grep "specificword"

This will also output the file name:

find -name '*.doc' | while read -r file; do
    catdoc "$file" | grep -H --label="$file" "specificword"
done

(Normally I would use find ... -print0 | while read -rd "" file, but there's maybe a .0001% chance that it would be necessary, so I stopped caring.)

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Utility to cat/gunzip a list of files

less does that.

less /var/log/messages*

If you pipe the output of less to another program, less will behave like cat or gunzip -c and not page the output.

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Can I use pipe output as a shell script argument?

Command substitution.

./Myscript.sh "$(cat text.txt)"

description

Concatenate FILE(s), or standard input, to standard output.
-A
, --show-all

equivalent to -vET

-b, --number-nonblank

number nonempty output lines, overrides -n

-e

equivalent to -vE

-E, --show-ends

display $ at end of each line

-n, --number

number all output lines

-s, --squeeze-blank

suppress repeated empty output lines

-t

equivalent to -vT

-T, --show-tabs

display TAB characters as ^I

-u

(ignored)

-v, --show-nonprinting

use ^ and M- notation, except for LFD and TAB

--help

display this help and exit

--version

output version information and exit

With no FILE, or when FILE is -, read standard input.

copyright

Copyright © 2012 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.

reporting bugs

Report cat bugs to bug-coreutils[:at:]gnu[:dot:]org
GNU coreutils home page: <http://www.gnu.org/software/coreutils/>
General help using GNU software: <http://www.gnu.org/gethelp/>
Report cat translation bugs to <http://translationproject.org/team/>


see also

tac

The full documentation for cat is maintained as a Texinfo manual. If the info and cat programs are properly installed at your site, the command

info coreutils 'cat invocation'

should give you access to the complete manual.


author

Written by Torbjorn Granlund and Richard M. Stallman.

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