Linux commands

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Revision as of 14:34, 1 September 2015 by Nad (talk | contribs) (Clone a device)
Info.svg This is the start of a list of useful Linux shell commands (generally "one liners") we use a lot


Ubuntu package management

Searching for an installed package

Use dpkg and grep;

dpkg -l | grep java

will list all packages with java in the name

Searching for installable packages

apt-cache performs a variety of operations on APT´s package cache

apt-cache search java

lists all packages in the cache with java in the name that are installable

SSH & SCP

tar pipe

## From a local to remote machine
tar -zcvf - directory | ssh remote 'cd somewhere; tar -zxvf -'
## From remote machine to local machine
ssh remote 'cd somewhere; tar -zcvf - directory' | tar -zxf -
## remote tar and pipe to tgz
ssh remote 'cd somewhere; tar -zcf - directory' | cat - > directory.tar.gz

Example of sending a file to a remote server:

scp -P 2222 /my/local/file.txt USER@example.com:/home/USER

Example of retrieving a file from a remote server:

scp -P 2222 USER@example.com:/home/USER/file.txt ./

Note using port forwarding for commonly accessed sites you can define non standard ports in your .ssh/config so you never need to explicitly state them above.

Recovering from an interrupted transfer

This ability is included natively in Linux and used to be very useful for splitting large backups up so they could fit onto small media such as floppy disks. But there's one time when it's very useful nowadays too which is when a large file transfer gets interupted and there's no option for continuation such as when using SCP.

When this happens, go onto the target machine, check how many bytes were transferred of the target file and rename it to "xaa". Then on the local machine, do the following command to split the source file into two parts, the first being of the size of the number of bytes that have already been transferred (in this example 1234567 bytes).

split -b 1234567 foo.tar.gz

The resulting files are called "xaa" and "xab", and the latter is the containing the remaining content that needs to be transferred to the target server. Once you've transferred it across, you can then join the two files (remember you renamed the first part to "xaa" so there's no need to transfer that) together using cat as follows, and then remove all the xa* files from noth source and target servers.

cat xa* > foo.tar.gz

Files & Devices

List all the storage devices and partitions attached to the system (even if they're unformatted or unmounted)

cat /proc/partitions

To check what filesystem a device has on it use file, e.g.

file -s /dev/sda1

Get the size of a directory and its contents

du -sh /home/foo


Use this more specific version to find the size of a users Maildir folder:

du -sh /home/*/Maildir|sed 's|/home/||'|sed 's|/Maildir||'

Search for file content recursively

Here's an example looking for a phrase within a specific file type recursively through a directory structure, and printing the file names and line numbers of the resulting matches.

grep -rn "Foo" *.php

There are other tips at stackoverflow.com.

Search for file content recursively and tar

## Find and tar
find . -name "*.R" -print0 | xargs -0 tar -cvf Rfiles.tar
## check contents
tar -tvf Rfiles.tar

## Find and tar.gz
find . -name "*.R" -print0 | xargs -0 tar -zcvf Rfiles.tar.gz
## check contents
tar -ztvf Rfiles.tar.gz

Search for file by name recursively

find . -name "*.R"

Count occurrences of a word in a file or files

grep -roh "WORD" file*.txt | wc -

Search and replace content in files

You could also use find and sed, but I find that this little line of perl works nicely.

perl -pi -w -e 's/SEARCH/REPLACE/g;' *.php
  • -e means execute the following line of code.
  • -i means edit in-place
  • -w write warnings
  • -p loop

EXTS="7z t7z"

Mass renaming

mmv is a mass move/copy/renaming tool that uses standard wildcards to perform its functions. According to the manual the “;” wildcard is useful for matching files at any depth in the directory tree (ie it will go below the current directory, recursively).

Example
mmv \*.JPG \#1.jpg

The first pattern matches anything with a “.JPG” and renames each file (the “#1” matches the first wildcard) to “.jpg”. Each time you use a \(wildcard) you can use a #x to get that wildcard. Where x is a positive number starting at 1. Copied off: http://tldp.org/LDP/GNU-Linux-Tools-Summary/html/mass-rename.html

Compare two directory structures for differences

This is very handy if you need to know if two directory structures are the same or not including file content. It uses the diff command with the r switch to work recursively and the x switch to allow skipping of the .svn directories.

diff -qrx .svn DIR1 DIR2

Mount a .iso

See this HOWTO

Mount a USB stick

If you don't know the device name of the stick, plug it in and find it with the following:

dmesg |grep SCSI

Then mount the first partition,

mkdir -p ~/memstick
mount -t vfat -o rw,users /dev/sdX1 ~/memstick

Clone a device

Remember that this is a very low-level operation and so if you're cloning a live partition you may end up with partially written files or other corruption issues on the destination. Stop as many services as you can first, do an fsck afterwards and manually copy any directories that had issues.

dd if=/dev/sda of=/dev/sdb bs=32M

A good way to find more information about the devices first, for example if you have a number of unformatted devices of the same size and need more specific information about the hardware, is to use the 'lsscsi command which gives the following sample output:

[0:0:0:0]    disk    AMCC     9650SE-2LP DISK  4.10  /dev/sdb 
[1:0:0:0]    disk    ATA      Hitachi HUA72201 JP4O  /dev/sda 
[2:0:0:0]    disk    ATA      TOSHIBA MG03ACA1 FL1A  /dev/sdc

The sdb and sdc devices look identical with commands such as lsblk or fdisk -l, but one of the is a RAID pair and the other just a normal drive.

Image manipulation

Resizing JPG's and changing quality setting

The first line shows how to reduce and image to 25% and quality to 50% adding "_resized" to the results filename. The second command uses Perl to apply this same command to all JPG's in the current directory.

convert foo.jpg -resize 25% -quality 50% foo_resized.jpg

perl -e 'for (glob "*.jpg") { $img = $_; s/(....)$/_resized$1/; qx "convert \'$img\' -resize 25% -quality 50% \'$_\'"; }'

Apply an opaque background of a specified colour to a directory of transparent PNG's

  • This command requires ImageMagick to be installed
  • It loops through all PNG's in the CWD and puts them in a directory called processed which must exist
perl -e 'qx "convert $_ -background #ff00ff -flatten foo/$_" for glob "*.png"'

Audio conversion

The following converts a .wav file to an mp3:

ffmpeg -i foo.wav foo.mp3

Here's a more complicated example which converts mp4 video into mp3 audio at a lower quality with only one channel at 11KHz and 32kbps.

ffmpeg -i foo.mp4 -acodec libmp3lame -ab 16k -ar 11025 -ac 1 foo.mp3

To do a whole directory you could do this:

perl -e 'qx "ffmpeg -i $_ $_.mp3" for glob "*.wav"'

Or a little more complex; a whole directory with a proper name change accounting for names with spaces in them. This

perl -e 'for (glob "*.mp4") { $i=$_; s/.mp3$/.converted.wav/; qx "ffmpeg -i \"$i\" \"$_\"" }'

Video manipulation

Use the following commands to extract a small snippet out of a video (the -ar switch is only needed for outputting to flv I think).

ffmpeg -i "foo.avi" -ss 00:10:10 -t 00:00:05 -ar 22050 "foo.flv"

See also

System monitoring

List the top 10 memory consuming processes

ps -auxf | sort -nr -k 4 | head -10


List the top 10 CPU consuming processes

ps -auxf | sort -nr -k 3 | head -10

Network commands

Restart the network after changing configuration

/etc/init.d/networking restart

List all the listening sockets and their ports and programs

netstat -lp

Get current default gateway

netstat -nr

The default gateway is on the last line, it should have the U and G flags set

Release DHCP lease

dhclient -r

Obtain a new DHCP lease

dhclient

Scan a local subnet for active IP addresses

nmap -sP 192.168.1.0/24

Get the MAC address and hostname of an IP on the current subnet

arp -a 192.168.1.1

Internet

Download a copy of an entire site

This will download an entire site, if the site is already downloaded, then only newer files are transferred.

wget -m http://foo.bar

Port forwarding

Port forwarding allows a remote client to gain access to a network so an intranet can be accessed.

ssh -fN -L[PORT]:appserver:[PORT] username@sshdserver

Then point your webbrowsers proxy server to appserver:[PORT] and access the intranet etc.

To subvert a firewalled environment where outgoing ssh is allowed.

$ ssh -D 9000 username@remotehost

Then point your web browser to a SOCKS proxy @ localhost:9000

Further it's possible to get ssh through a web proxy using corkscrew.

ssh -oProxyCommand='corkscrew local_web_proxy proxy_port %h %p' username@remotehost

General

Add and remove startup items

update-rc.d ITEM defaults
update-rc.d -f ITEM remove

See also