Difference between revisions of "Core utilities"

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The construction of some of these ''tar'' arguments may be considered legacy, but they are still useful when performing specific operations. See {{man|1|tar}} for details.
 
The construction of some of these ''tar'' arguments may be considered legacy, but they are still useful when performing specific operations. See {{man|1|tar}} for details.
  
{{Note|Although GNU's ''tar'' is installed as the default ''tar'' program, official Arch Linux projects like [[pacman]] and [[mkinitcpio]] use ''bsdtar'' from the {{Pkg|libarchive}} package.}}
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{{Note|GNU's ''tar'' is installed as the default ''tar'' program, however official Arch Linux projects like [[pacman]] and [[mkinitcpio]] use ''bsdtar'' from the {{Pkg|libarchive}} package.}}
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{{Tip|''Bsdtar'' supports more [https://github.com/libarchive/libarchive/wiki/LibarchiveFormats archive formats] than GNU tar and auto-detects the file type.}}
  
 
=== wipefs ===
 
=== wipefs ===

Revision as of 22:30, 12 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 GNU coreutils package. What follows are various tips and tricks and other helpful information related to these utilities.

Most command-line interfaces are documented in man pages, GNU commands tend to be documented in info(1) pages, some shells provide a help command for shell-builtin commands. Additionally most commands print their usage when run with the --help argument.

File management

Command Description Manual page Example
cd Change directory (shell built-in command) cd(1p) cd /etc/pacman.d
mkdir Create a directory mkdir(1) mkdir ~/newfolder
rmdir Remove empty directory rmdir(1) rmdir ~/emptyfolder
rm Remove a file rm(1) rm ~/file.txt
rm -r Remove directory and contents rm -r ~/.cache
ls List files ls(1) ls *.mkv
ls -a List hidden files ls -a /home/archie
ls -al List hidden files and file properties
mv Move a file mv(1) mv ~/compressed.zip ~/archive/compressed2.zip
cp Copy a file cp(1) cp ~/.bashrc ~/.bashrc.bak
chmod +x Make a file executable chmod(1) chmod +x ~/.local/bin/myscript.sh
cat Show file contents cat(1) cat /etc/hostname
find Search for a file find(1) find ~ -name myfile

ls

ls lists directory contents.

See info ls or the online manual for more information.

exa is a modern, and more user friendly alternative to ls and 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

Long format

The -l option displays some metadata, for example:

$ ls -l /path/to/directory
total 128
drwxr-xr-x 2 archie users  4096 Jul  5 21:03 Desktop
drwxr-xr-x 6 archie users  4096 Jul  5 17:37 Documents
drwxr-xr-x 2 archie users  4096 Jul  5 13:45 Downloads
-rw-rw-r-- 1 archie users  5120 Jun 27 08:28 customers.ods
-rw-r--r-- 1 archie users  3339 Jun 27 08:28 todo
-rwxr-xr-x 1 archie users  2048 Jul  6 12:56 myscript.sh

The total value represents the total disk allocation for the files in the directory, by default in number of blocks.

Tango-edit-cut.pngThis section is being considered for removal.Tango-edit-cut.png

Reason: Unnecessary detail found in man pages (Discuss in Talk:Core utilities#)

Below, each file and subdirectory is represented by a line divided into 7 metadata fields, in the following order:

  • type and permissions:
    • the first character is the entry type, see info ls -n "What information is listed" for an explanation of all the possible types; for example:
      • - denotes a normal file;
      • d denotes a directory, i.e. a folder containing other files or folders;
      • p denotes a named pipe (aka FIFO);
      • l denotes a symbolic link;
    • the remaining characters are the entry's permissions;
  • number of hard links for the entity; files will have at least 1, i.e. the showed reference itself; folders will have at least 2: the showed reference, the self-referencing . entry, and then a .. entry in each of its subfolders;
  • owner user name;
  • group name;
  • size;
  • last modification timestamp;
  • entity name.

File names containing spaces enclosed in quotes

By default, file and directory names that contain spaces are displayed surrounded by single quotes. To change this behavior use the -N or --quoting-style=literal options. Alternatively, set the QUOTING_STYLE environment variable to literal. [1]

cat

cat is a standard Unix utility that concatenates files to standard output.

  • Because cat is not built into the shell, on many occasions you may find it more convenient to use a redirection, for example in scripts, or if you care a lot about performance. In fact < file does the same as cat file.
  • cat can work with multiple lines:
$ cat << EOF >> path/file
first line
...
last line
EOF

Alternatively, using printf:

$ printf '%s\n' 'first line' ... 'last line'
  • If you need to list file lines in reverse order, there is a coreutil command called tac (cat reversed).

less

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: less is a complex beast, and this section should explain some of the basic less commands (Discuss in Talk:Core utilities#)
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.

Vim as alternative pager

Tango-go-next.pngThis article or section is a candidate for moving to Vim.Tango-go-next.png

Notes: Discusses a vim specifity (Discuss in Talk:Core utilities#)
Vim includes a script to view the content of text files, compressed files, binaries and directories. Add the following line to your shell configuration file to use it as a pager:
~/.bashrc
alias less='/usr/share/vim/vim80/macros/less.sh'

There is also an alternative to the less.sh macro, which may work as the PAGER environment variable. Install vimpager and add the following to your shell configuration file:

~/.bashrc
export PAGER='vimpager'
alias less=$PAGER

Now programs that use the PAGER environment variable, like git, will use vim as pager.

mkdir

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.

Tip: If you just want a temporary directory, a better alternative may be mktemp: mktemp -d

mv

mv moves and renames files and directories.

Note: "Security aliases" are dangerous because you get used to them, resulting in potential data loss when you use another system / user that does not have these aliases.

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

rm removes files or directories.

Note: "Security aliases" are dangerous because you get used to them, resulting in potential data loss when you use another system / user that does not have these aliases.

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 with -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.

chmod

See File permissions and attributes#Changing permissions.

chown

See File permissions and attributes#Changing ownership.

find

find is part of the findutils package, which belongs to the base package group.

Tip: fd is a simple, fast and user-friendly alternative to find that provides more sensible defaults (e.g. ignores hidden files, directories and .gitignore'd files, fd PATTERN instead 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.

locate

Install the mlocate 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 /media and /mnt are ignored, so locate may not discover files on external devices. See updatedb(8) 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.

See also How locate works and rewrite it in one minute.

diff

diff compares files line by line. The default Arch Linux diff is from the GNU diffutils, which also provides cmp to compare files byte by byte.

When comparing text files a word per word diff is often more desirable:

  • git's git diff can do a word diff with --color-words, using --no-index it can be used for files outside of Git working trees.
  • dwdiff — A word diff front-end for the diff program, supports colors.
https://os.ghalkes.nl/dwdiff.html || dwdiff
  • GNU wdiff — A wordwise implementation of GNU diff, does not support colors.
https://www.gnu.org/software/wdiff/ || wdiffAUR
  • cwdiff — A GNU wdiff wrapper that colorizes the output.
https://github.com/junghans/cwdiff || cwdiffAUR, cwdiff-gitAUR

Text streams

Shell pipelines operate on stdout by default. To operate on stderr(3) you can redirect stderr to stdout with command 2>&1 | othercommand or, for Bash 4, command |& othercommand. See also I/O Redirection.

grep

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.

  • Remember that grep handles files, so a construct like grep pattern < file is replaceable with grep pattern file.
  • To include file line numbers in the output, use the -n option.
  • grep can also be used for hexadecimal search in a binary file, to look for let say the A1 F2 sequence 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.

See grep(1) for more details.

Tip: There are grep alternatives optimized for VCS source code, such as ripgrep, the_silver_searcher, and ack.

sed

sed is stream editor for filtering and transforming text.

Here is a handy list of sed one-liners examples.

Tip: More powerful alternatives are awk and the Perl language.

awk

AWK is a pattern scanning and processing language. There are multiple implementations:

  • gawk — GNU version of awk, see gawk(1).
https://www.gnu.org/software/gawk/ || gawk (part of base)
  • nawk — The one, true implementation of AWK, see nawk(1).
https://www.cs.princeton.edu/~bwk/btl.mirror/ || nawk
  • mawk — A very fast AWK implementation.
http://invisible-island.net/mawk/ || mawkAUR
  • BusyBox also includes an AWK implementation.

System administration

Command Description Manual page Example
mount Mount a partition mount(8) mount /dev/sdc1 /media/usb
df -h Show remaining space on all partitions df(1)
ps -A Show all running processes ps(1)
killall Kill all running instances of a process killall(1)
ss -at Display a list of open TCP sockets ss(8)

sudo

See Sudo.

which

which shows the full path of shell commands. In the following example the full path of ssh is used as an argument for journalctl:

# journalctl $(which sshd)

lsblk

lsblk(8) will show all available block devices along with their partitioning schemes, for example:

$ 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 (sda), 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 (sda1), 2 for the second (sda2), and so on. In the example above, only one device is available (sda), and that device has three partitions (sda1 to sda3), each with a different file system.

Other common block device types include for example mmcblk for memory cards and nvme for NVMe devices. Unknown types can be searched in the kernel documentation.

ip

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.

Note: The ip utility is provided by the iproute2 package, which is included in the base group.
Object Purpose Manual page
ip addr protocol address management ip-address(8)
ip addrlabel protocol address label management ip-addrlabel(8)
ip l2tp tunnel Ethernet over IP (L2TPv3) ip-l2tp(8)
ip link network device configuration ip-link(8)
ip maddr multicast addresses management ip-maddress(8)
ip monitor watch for netlink messages ip-monitor(8)
ip mroute multicast routing cache management ip-mroute(8)
ip mrule rule in multicast routing policy db
ip neigh neighbour/ARP tables management ip-neighbour(8)
ip netns process network namespace management ip-netns(8)
ip ntable neighbour table configuration ip-ntable(8)
ip route routing table management ip-route(8)
ip rule routing policy database management ip-rule(8)
ip tcp_metrics management for TCP Metrics ip-tcp_metrics(8)
ip tunnel tunnel configuration ip-tunnel(8)
ip tuntap manage TUN/TAP devices
ip xfrm manage IPsec policies ip-xfrm(8)

The 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.

Note: You might be familiar with the ifconfig command, which was used in older Linux systems. It is deprecated in Arch Linux; use ip instead.

ss

ss is a utility to investigate network ports and is part of the iproute2 package in the base group. It has a similar functionality to the deprecated netstat utility.

Common usage includes:

Display all TCP Sockets with service names:

$ ss -at

Display all TCP Sockets with port numbers:

$ ss -atn

Display all UDP Sockets:

$ ss -au

For more information see ss(8) or ss.html from the iproute2 package.

Miscellaneous

Command Description Manual page Example
strings Show printable characters in binary files strings(1) strings /usr/bin/free

dd

dd is a utility for Unix and Unix-like operating systems whose primary purpose is to convert and copy a file.

Similarly to cp, by default dd makes a bit-to-bit copy of the file, but with lower-level I/O flow control features.

Some notable applications of dd are:

  • Binary file patching: let say one wants to replace offset 0x123AB of a file with the FF C0 14 hexadecimal sequence, this can be done with the command line:
    # printf '\xff\xc0\x14' | dd seek=$((0x123AB)) conv=notrunc bs=1 of=/path/to/file

For more information see dd(1) or the full documentation.

Tip: By default, dd outputs nothing until the task has finished. To monitor the progress of the operation, add the status=progress option to the command.
Warning: One should be extremely cautious using dd, as with any command of this kind it can destroy data irreversibly.

iconv

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 foo.utf:

$ iconv -f ISO-8859-15 -t UTF-8 foo > foo.utf

See iconv(1) for more details.

Convert a file in place

Tip: You can use recode instead of iconv if you do not want to touch the mtime.

Unlike sed, iconv does not provide an option to convert a file in place. However, sponge from the moreutils package can help:

$ iconv -f WINDOWS-1251 -t UTF-8 foobar.txt | sponge foobar.txt

See sponge(1) for details.

od

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

seq prints a sequence of numbers. Shell built-in alternatives are available, so it is good practice to use them as explained on Wikipedia.

tar

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
file.tar tar xvf file.tar
file.tgz tar xvzf file.tgz
file.tar.gz tar xvzf file.tar.gz
file.tar.bz bzip -cd file.bz | tar xvf -
file.tar.bz2 tar xvjf file.tar.bz2
bzip2 -cd file.bz2 | tar xvf -
file.tar.xz tar xvJf file.tar.xz
xz -cd file.xz | tar xvf -
file.tar.zst tar -I zstd xvf file.tar.zst

The construction of some of these tar arguments may be considered legacy, but they are still useful when performing specific operations. See tar(1) for details.

Note: GNU's tar is installed as the default tar program, however official Arch Linux projects like pacman and mkinitcpio use bsdtar from the libarchive package.
Tip: Bsdtar supports more archive formats than GNU tar and auto-detects the file type.

wipefs

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: Why would you want to erase magic strings? (Discuss in Talk:Core utilities#)

wipefs can list or erase file system, RAID or partition-table signatures (magic strings) from the specified device. It does not erase the file systems themselves nor any other data from the device.

See wipefs(8) for 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

See also