Core Utilities
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.
Contents
1 Basic commands
The following table lists basic shell commands every GNU/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 | Manual page | Example |
|---|---|---|---|
| man | Show manual page for a command | man 7 man | man ed |
| cd | Change directory | man 1 cd | cd /etc/pacman.d |
| mkdir | Create a directory | man 1 mkdir | mkdir ~/newfolder |
| rmdir | Remove empty directory | man 1 rmdir | rmdir ~/emptyfolder |
| rm | Remove a file | man 1 rm | rm ~/file.txt |
| rm -r | Remove directory and contents | rm -r ~/.cache | |
| ls | List files | man 1 ls | ls *.mkv |
| ls -a | List hidden files | ls -a /home/archie | |
| ls -al | List hidden files and file properties | ||
| mv | Move a file | man 1 mv | mv ~/compressed.zip ~/archive/compressed2.zip |
| cp | Copy a file | man 1 cp | cp ~/.bashrc ~/.bashrc.bak |
| chmod +x | Make a file executable | man 1 chmod | chmod +x ~/.local/bin/myscript.sh |
| cat | Show file contents | man 1 cat | cat /etc/hostname |
| strings | Show printable characters in binary files | man 1 strings | strings /usr/bin/free |
| find | Search for a file | man 1 find | find ~ -name myfile |
| mount | Mount a partition | man 8 mount | mount /dev/sdc1 /media/usb |
| df -h | Show remaining space on all partitions | man 1 df | |
| ps -A | Show all running processes | man 1 ps | |
| killall | Kill all running instances of a process | man 1 killall | |
| ss -at | Display a list of open TCP sockets | man 8 ss |
2 cat
cat 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:
$ 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 utility called tac (cat reversed).
3 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.
'dd' can be used to:
- drive-related tasks:
- Create an image.
- Write an image.
- Clone whole drive, or partition.
- Wipe drive or partition.
- Erase partition table or boot sector.
- Backup boot sector.
- Restore system.
- Get stream from device, dd /dev/random, or some input device.
- Create load on CPU (example: dd if=/dev/zero of=/dev/null).
- Create load on disk (example: dd if=/dev/zero of=/path/testfile bs=number_ofG count=times oflag=fdatasync).
- Create I/O load on disk (many rapid read/writes) (example above with little block size (bs) and big amount of cycles (count) and needed oflag, based on what and how you want to test).
- As a backup utility or part of solution.
- Convert a file to upper/lower case.
For more information see man 1 dd or the full documentation.
4 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 for lines matching a given regular expression, and prints these lines 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
- There are grep alternatives optimized for VCS source code, such as the_silver_searcher and ack.
- To include file line numbers in the output, use the -n option.
For color support, see Color output in console#grep.
5 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.
6 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.
6.1 Convert a file in place
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.
7 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 | man 8 ip-address |
| ip addrlabel | protocol address label management | man 8 ip-addrlabel |
| ip l2tp | tunnel Ethernet over IP (L2TPv3) | man 8 ip-l2tp |
| ip link | network device configuration | man 8 ip-link |
| ip maddr | multicast addresses management | man 8 ip-maddress |
| ip monitor | watch for netlink messages | man 8 ip-monitor |
| ip mroute | multicast routing cache management | man 8 ip-mroute |
| ip mrule | rule in multicast routing policy db | |
| ip neigh | neighbour/ARP tables management | man 8 ip-neighbour |
| ip netns | process network namespace management | man 8 ip-netns |
| ip ntable | neighbour table configuration | man 8 ip-ntable |
| ip route | routing table management | man 8 ip-route |
| ip rule | routing policy database management | man 8 ip-rule |
| ip tcp_metrics | management for TCP Metrics | man 8 ip-tcp_metrics |
| ip tunnel | tunnel configuration | man 8 ip-tunnel |
| ip tuntap | manage TUN/TAP devices | |
| ip xfrm | manage IPsec policies | man 8 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.
The Network configuration article shows how the ip command is used in practice for various common tasks.
8 locate
Install the mlocate package. After installation a script is automatically scheduled to run a daily task to update its 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 1 updatedb 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. 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, execute updatedb as root.
9 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 pg, less offers a more advanced interface and complete feature-set.
See List of applications#Terminal pagers for alternatives.
9.1 Vim as alternative pager
Vim includes a script to view the content of text files, compressed files, binaries, directories. Add the following line to your shell configuration file to use it as a pager:
~/.bashrc
alias less='/usr/share/vim/vim74/macros/less.sh'
There is also an alternative to 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.
10 ls
ls lists directory contents.
See info ls or the online manual for more information.
10.1 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 info ls -n "What information is listed" for an explanation of all the possible types; for example:
- 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.
10.2 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.
11 lsblk
lsblk 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.
12 mkdir
mkdir makes 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.
13 mv
mv moves and renames 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.
14 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.
15 pv
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.
16 rm
rm removes files or 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.
17 sed
sed is stream editor for filtering and transforming text.
Here is a handy list of sed one-liners examples.
18 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.
19 ss
ss is a utility to investigate network ports and is part of the iproute2 package in the base group.
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 man 8 ss.
20 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 packages are compressed tarballs, and Parabola 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. See its man page with man tar for details.
21 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)
22 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 man 8 wipefs 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
