Core utilities - ArchWiki

# Core utilities

Related articles

- Core utilities/Tips and tricks
- Bash
- Zsh
- General recommendations
- GNU Project
- sudo
- cron
- man page
- Securely wipe disk#shred
- File permissions and attributes

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.

## Basic commands

The following table lists basic shell commands every Linux user should be familiar with. Commands in bold are part of the shell, others are separate programs called from the shell. See the below sections and Related articles for details.

Command | Description | Example
--- | --- | ---
man | Show manual page for a command | man ed
cd | Change directory | 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
strings | Show printable characters in binary files | strings /usr/bin/free
find | Search for a file | find ~ -name myfile
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 |

## cat

cat (catenate) is a standard Unix utility that concatenates and lists files.

- Because cat is not a built-in 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 is able to work with multiple lines, although this is sometimes regarded as bad practice:

    $ cat << EOF >> path/file
    first line
    ...
    last line
    EOF

  A better alternative is the echo command:

    $ echo "\
    first line
    ...
    last line" \
    >> path/file

- If you need to list file lines in reverse order, there is a utility called tac (cat reversed).

## dd

This article or section is a candidate for merging with Disk cloning.

Notes: The cp <-> dd note could go to #cp, which should be created (Discuss in Talk:Core utilities#)
dd is a command on Unix and Unix-like operating systems whose primary purpose is to convert and copy a file.

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. See the manual for more information.

Note: cp does the same as dd without any operands but is not designed for more versatile disk wiping procedures.

## grep

grep (from ed's g/re/p, global/regular expression/print) is a command line text search utility originally written for Unix. The grep command searches files or standard input globally for lines matching a given regular expression, and prints them to the program's standard output.

- Remember that grep handles files, so a construct like `cat file | grep pattern` is replaceable with `grep pattern file`

- grep alternatives optimized for VCS source code do exist, such as the_silver_searcher and ack.

### Standard error

Some commands send their output to standard error, and grep has no apparent effect. In this case, redirect standard error next to standard out:

    $ command 2>&1 | grep args

or Bash 4 shorthand:

    $ command |& grep args

See also I/O Redirection.

## find

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

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 UsingFind for usage details.

## locate

Install the mlocate package. When `mlocate` is installed, a script is automatically scheduled to run daily via `systemd`, to update the database. 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 `man updatedb.conf` for details.

`locate` 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`. This problem is minimised by regular, typically scheduled use of the `updatedb` command, which (as the name suggests) updates the database.

Before `locate` can be used, the database will need to be created. To do this, simply run `updatedb` as root.

See also How locate works and rewrite it in one minute

## 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 `man iconv` 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` can be used to handle it, it comes with moreutils.

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

See `man sponge` for details.

## 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 Name
--- | --- | ---
ip addr | protocol address management | ip-address
ip addrlabel | protocol address label management | ip-addrlabel
ip l2tp | tunnel Ethernet over IP (L2TPv3) | ip-l2tp
ip link | network device configuration | ip-link
ip maddr | multicast addresses management | ip-maddress
ip monitor | watch for netlink messages | ip-monitor
ip mroute | multicast routing cache management | ip-mroute
ip mrule | rule in multicast routing policy db |
ip neigh | neighbour/ARP tables management | ip-neighbour
ip netns | process network namespace management | ip-netns
ip ntable | neighbour table configuration | ip-ntable
ip route | routing table management | ip-route
ip rule | routing policy database management | ip-rule
ip tcp_metrics | management for TCP Metrics | ip-tcp_metrics
ip tunnel | tunnel configuration | ip-tunnel
ip tuntap | manage TUN/TAP devices |
ip xfrm | manage IPsec policies | ip-xfrm

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 versions of Linux for interface configuration. It is now deprecated in Arch Linux; you should use ip instead.

## less

This article or section needs expansion.

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 pg, less offers a more advanced interface and complete feature-set.

See List of applications#Terminal pagers for alternatives.

## ls

ls (list) is a command to list files in Unix and Unix-like operating systems.

See `info ls` or the online manual for more information.

### 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;
- 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]

## mkdir

mkdir (make directory) is a command to create directories.

To create a directory and its whole hierarchy, the `-p` switch is used, otherwise an error is printed. As users are supposed to know what they want, `-p` switch may be used as a default:

    alias mkdir='mkdir -p -v

The `-v` switch make it verbose.

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 (make temporary): `mktemp -p`.

## mv

mv (move) is a command to move and rename files and directories.

To limit potential damage caused by the command, use an alias:

    alias mv='timeout 8 mv -iv'

This alias suspends mv after eight seconds, asks confirmation to delete three or more files, lists the operations in progress and does not store itself in the shell history file if the shell is configured to ignore space starting commands.

## 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.

## pv

This article or section needs language, wiki syntax or style improvements.

Reason: Is this article really the place to report bugs? (Discuss in Talk:Core utilities#)

You can use pv (pipe viewer) to monitor the progress of data through a pipeline, for example:

    # dd if=/source/filestream | pv -monitor_options -s size_of_file | dd of=/destination/filestream

In most cases `pv` functions as a drop-in replacement for `cat`, however there are undocumented differences. For example, under both Zsh and Bash, the following command hangs forever:

    # cat <(pv /usr/share/dict/words)

Use of strace shows that `pv` is stopped with `SIGTTOU`.

## rm

rm (remove) is a command to delete files and directories.

To limit potential damage caused by the command, use an alias:

    alias rm='timeout 3 rm -Iv --one-file-system'

This alias suspends rm after three seconds, asks confirmation to delete three or more files, lists the operations in progress, does not involve more than one file systems and does not store itself in the shell history file if the shell is configured to ignore space starting commands. Substitute `-I` with `-i` if you prefer to confirm even for one file.

Zsh users may want to put `noglob` before `timeout` to avoid implicit expansions.

To remove directories known to be empty, use rmdir as it fails in case of files inside the target.

## sed

sed (stream editor) is a Unix utility that parses and transforms text.

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

Tip: More powerful alternatives are AWK and even Perl language.

## seq

seq (sequence) is a utility for generating 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 compression format, `tar` files (known as tarballs) are widely used for packaging in Unix-like operating systems. Both pacman and AUR packages are 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 -`

The construction of some of these `tar` arguments may be considered legacy, but they are still useful when performing specific operations. The Compatibility section of `tar`'s man page shows how they work in detail.

## which

The which command is useful to determine the path to an executable, for example:

    # journalctl $(which sshd)

## 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

- A sampling of coreutils , part 2 , part 3 - Overview of commands in coreutils
- GNU Coreutils online documentation
- Learn the DD command

Categories:

- System administration
- Command shells

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