Difference between revisions of "Core utilities"
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=== dd ===
=== dd ===
[[dd]] is a utility for Unix and Unix-like operating systems whose primary purpose is to convert and copy a file.
=== iconv ===
=== iconv ===
Revision as of 08:45, 31 August 2018
This article deals with so-called core utilities on a GNU/Linux system, such as less, ls, and grep. The scope of this article includes, but is not limited to, those utilities included with the GNUpackage. What follows are various tips and tricks and other helpful information related to these utilities.
Most command-line interfaces are documented in man pages, utilities by the GNU Project are documented in Info manuals, some shells provide a
help command for shell builtin commands. Additionally most commands print their usage when run with the
|cd||Change directory (shell built-in command)||cd /etc/pacman.d|
|mkdir||Create a directory||mkdir ~/newfolder|
|rmdir||Remove empty directory||rmdir ~/emptyfolder|
|rm||Remove a file||rm ~/file.txt|
|rm -r||Remove directory and contents||rm -r ~/.cache|
|ls||List files||ls *.mkv|
|ls -a||List hidden files||ls -a /home/archie|
|ls -al||List hidden files and file properties|
|mv||Move a file||mv ~/compressed.zip ~/archive/compressed2.zip|
|cp||Copy a file||cp ~/.bashrc ~/.bashrc.bak|
|chmod +x||Make a file executable||chmod +x ~/.local/bin/myscript.sh|
|cat||Show file contents||cat /etc/hostname|
|find||Search for a file||find ~ -name myfile|
ls lists directory contents.
exa is a modern, and more user friendly alternative to
tree, that has more features, such as displaying Git modifications along with filenames, colouring differently each columnn in
--long mode, or displaying
--long mode metadata along with a
tree view. exa is available as the package.
By default, file and directory names that contain spaces are displayed surrounded by single quotes. To change this behavior use the
--quoting-style=literal options. Alternatively, set the
QUOTING_STYLE environment variable to
cat is a standard Unix utility that concatenates files to standard output.
If you need to list file lines in reverse order, there is a coreutil command called tac (cat reversed).
less is a terminal pager program used to view the contents of a text file one screen at a time. Whilst similar to other pagers such as more and the deprecated pg, less offers a more advanced interface and complete feature-set.
See List of applications#Terminal pagers for alternatives.
mkdir makes directories.
To create a directory and its whole hierarchy, the
-p switch is used, otherwise an error is printed.
Changing mode of a just created directory using chmod is not necessary as the
-m option lets you define the access permissions.
mv moves and renames files and directories.
To limit potential damage caused by the command, use an alias:
alias mv='mv -iv'
This alias asks for confirmation before overwriting any existing files and lists the operations in progress.
rm removes files or directories.
To limit potential damage caused by the command, use an alias:
alias rm='rm -Iv --one-file-system'
This alias asks confirmation to delete three or more files, lists the operations in progress, does not involve more than one file systems. Substitute
-i if you prefer to confirm even for one file.
Zsh users may want to prefix
noglob to avoid implicit expansions.
To remove directories believed to be empty, use rmdir as it fails if there are files inside the target.
find is part of thepackage, which belongs to the package group.
findthat provides more sensible defaults (e.g. ignores hidden files, directories and
fd PATTERNinstead of
find -iname '*PATTERN*'). It features colorized output (similar to
ls), Unicode awareness, regular expressions and more.
One would probably expect a find command to take as argument a file name and search the filesystem for files matching that name. For a program that does exactly that see #locate below.
Instead, find takes a set of directories and matches each file under them against a set of expressions. This design allows for some very powerful "one-liners" that would not be possible using the "intuitive" design described above. See GregsWiki:UsingFind for usage details.
Install the package. The package contains an
updatedb.timer unit, which invokes a database update each day. The timer is enabled right after installation, start it manually if you want to use it before reboot. You can also manually run updatedb as root at any time. By default, paths such as
/mnt are ignored, so locate may not discover files on external devices. See for details.
The locate command is a common Unix tool for quickly finding files by name. It offers speed improvements over the find tool by searching a pre-constructed database file, rather than the filesystem directly. The downside of this approach is that changes made since the construction of the database file cannot be detected by locate.
Before locate can be used, the database will need to be created. To do this, execute
updatedb as root.
diff compares files line by line. Its output can be saved to a so-called patch file, which can be applied using theutility. The default Arch Linux diff is from the GNU , which also provides cmp to compare files byte by byte.
A similar command, which lets you compare two sorted files line by line is comm, see.
When comparing text files a word per word diff is often more desirable:
git diffcan do a word diff with
--no-indexit can also be used for files outside of Git working trees.
- dwdiff — A word diff front-end for the diff program; supports colors.
- GNU wdiff — A wordwise implementation of GNU diff; does not support colors.
- cwdiff — A GNU wdiff wrapper that colorizes the output.
- https://github.com/junghans/cwdiff || AUR, AUR
grep is a command line text search utility originally written for Unix. The grep command searches files or standard input for lines matching a given regular expression, and prints these lines to standard output.
- To include file line numbers in the output, use the
- grep can also be used for hexadecimal search in a binary file, to look for let say the
A1 F2sequence in a file, the command line is:
$ LANG=C grep --text --perl-regexp "\xA1\xF2" /path/to/file
For color support, see Color output in console#grep.
Seefor more details.
sed is stream editor for filtering and transforming text.
Here is a handy list of sed one-liners examples.
AWK is a pattern scanning and processing language. There are multiple implementations:
- gawk — GNU version of awk, see .
- https://www.gnu.org/software/gawk/ || (part of )
- nawk — The one, true implementation of AWK, see .
- mawk — A very fast AWK implementation.
- BusyBox also includes an AWK implementation.
|mount||Mount a partition||mount /dev/sdc1 /media/usb|
|df -h||Show remaining space on all partitions|
|ps -A||Show all running processes|
|killall||Kill all running instances of a process|
|ss -at||Display a list of open TCP sockets|
which shows the full path of shell commands. In the following example the full path of
ssh is used as an argument for
# journalctl $(which sshd)
block devices along with their partitioning schemes, for example:will show all available
$ lsblk -f
NAME FSTYPE LABEL UUID MOUNTPOINT sda ├─sda1 vfat C4DA-2C4D /boot ├─sda2 swap 5b1564b2-2e2c-452c-bcfa-d1f572ae99f2 [SWAP] └─sda3 ext4 56adc99b-a61e-46af-aab7-a6d07e504652 /
The beginning of the device name specifies the type of block device. Most modern storage devices (e.g. hard disks, SSDs and USB flash drives) are recognised as SCSI disks (
sd). The type is followed by a lower-case letter starting from
a for the first device (
b for the second device (
sdb), and so on. Existing partitions on each device will be listed with a number starting from
1 for the first partition (
2 for the second (
sda2), and so on. In the example above, only one device is available (
sda), and that device has three partitions (
sda3), each with a different file system.
ip allows you to show information about network devices, IP addresses, routing tables, and other objects in the Linux IP software stack. By appending various commands, you can also manipulate or configure most of these objects.
|ip addr||protocol address management|
|ip addrlabel||protocol address label management|
|ip l2tp||tunnel Ethernet over IP (L2TPv3)|
|ip link||network device configuration|
|ip maddr||multicast addresses management|
|ip monitor||watch for netlink messages|
|ip mroute||multicast routing cache management|
|ip mrule||rule in multicast routing policy db|
|ip neigh||neighbour/ARP tables management|
|ip netns||process network namespace management|
|ip ntable||neighbour table configuration|
|ip route||routing table management|
|ip rule||routing policy database management|
|ip tcp_metrics||management for TCP Metrics|
|ip tunnel||tunnel configuration|
|ip tuntap||manage TUN/TAP devices|
|ip xfrm||manage IPsec policies|
help command is available for all objects. For example, typing
ip addr help will show you the command syntax available for the address object. For advanced usage see the iproute2 documentation.
The Network configuration article shows how the ip command is used in practice for various common tasks.
|strings||Show printable characters in binary files||strings /usr/bin/free|
dd is a utility for Unix and Unix-like operating systems whose primary purpose is to convert and copy a file.
iconv converts the encoding of characters from one codeset to another.
The following command will convert the file
foo from ISO-8859-15 to UTF-8, saving it to
$ iconv -f ISO-8859-15 -t UTF-8 foo > foo.utf
Seefor more details.
The od (octal dump) command is useful for visualizing data that is not in a human-readable format, like the executable code of a program, or the contents of an unformatted device. See the manual for more information.
seq prints a sequence of numbers. Shell built-in alternatives are available, so it is good practice to use them as explained on Wikipedia.
As an early Unix archiving format, .tar files—known as "tarballs"—are widely used for packaging in Unix-like operating systems. Both pacman and AUR packages are compressed tarballs, and Arch uses GNU's tar program by default.
For .tar archives, tar by default will extract the file according to its extension:
$ tar xvf file.EXTENSION
Forcing a given format:
|File Type||Extraction Command|
The construction of some of these tar arguments may be considered legacy, but they are still useful when performing specific operations. Seefor details.
See also Archiving and compression.
Seefor more information.
For example, to erase all signatures from the device
/dev/sdb and create a signature backup
~/wipefs-sdb-offset.bak file for each signature:
# wipefs --all --backup /dev/sdb