Core utilities: Difference between revisions
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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 {{Pkg|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 page]]s, [[GNU]] commands tend to be documented in {{man|1|info}} pages, some [[shell]]s provide a {{ic|help}} command for [[shell]]-builtin commands. Additionally most commands print their usage when run with the {{ic|--help}} argument. | |||
== File management == | == File management == |
Revision as of 12:31, 9 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.
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;
- the first character is the entry type, see
- 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 ascat 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
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
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.
mktemp -d
mv
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
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
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.
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.
- GNU wdiff — A wordwise implementation of GNU diff, does not support colors.
- cwdiff — A GNU wdiff wrapper that colorizes the output.
Text streams
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
cat file | grep pattern
is replaceable withgrep pattern file
- There are grep alternatives optimized for VCS source code, such as ripgrep, the_silver_searcher, and ack.
- 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
command 2>&1 | grep args
or (for Bash 4) command |& grep args
. See also I/O Redirection.For color support, see Color output in console#grep.
See grep(1) for more details.
sed
sed is stream editor for filtering and transforming text.
Here is a handy list of sed one-liners examples.
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).
- mawk — A very fast AWK implementation.
- 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.
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.
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 theFF 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.
status=progress
option to the command.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
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.
wipefs
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
- POSIX Utilities
- GNU Coreutils online documentation
- A sampling of coreutils on Reddit , part 2 , part 3 - Overview of commands in coreutils
- Learn the DD command