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.
cat (catenate) is a standard Unix utility that concatenates and lists files.
- As cat is not a shell built-in, on many occasions you may find more convenient to use a redirection, for example in scripts, or if you care a lot about performance. In fact
< filedoes the same of
- To append multiple lines to a file a construct like this one is used:
$ cat << EOF >> path/file first line ... last line EOF
- If you need to list file lines in reverse order, there is a utility called tac (cat reversed).
cron is a time-based job scheduler in Unix-like computer operating systems.
See the main article.
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 patternis replaceable with
grep pattern file
- If you have to grep VCS source code, there is an optimized utility written in Perl and called official web site. . See the
Beyond aesthetics, grep's color output is immensely useful for learning regexp and grep's functionality.
To use the default colors for grep, write the following entry to your shell configuration file, e.g. if using Bash:
alias grep='grep --color=auto'
To include file line numbers in the output, add
alias grep='grep -n --color=auto'
The environment variable
GREP_COLORS may be used to specify different colors than the defaults.
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
man iconv for more details.
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-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.
The Network Configuration article shows how the ip command is used in practice for various common tasks.
less is a terminal pager program used to view the contents of a text file one screen at a time. Whilst similar to other pages such as more and pg, less offers a more advanced interface and complete feature-set.
Colored output through environment variables
Add the following lines to your shell configuration file:
export LESS=-R export LESS_TERMCAP_me=$(printf '\e[0m') export LESS_TERMCAP_se=$(printf '\e[0m') export LESS_TERMCAP_ue=$(printf '\e[0m') export LESS_TERMCAP_mb=$(printf '\e[1;32m') export LESS_TERMCAP_md=$(printf '\e[1;34m') export LESS_TERMCAP_us=$(printf '\e[1;32m') export LESS_TERMCAP_so=$(printf '\e[1;44;1m')
Change values as you like. References: ANSI escape code.
Colored output through wrappers
You can enable code syntax coloring in less. First, install, then add these lines to your shell configuration file:
export LESSOPEN="| /usr/bin/source-highlight-esc.sh %s" export LESS='-R '
Frequent users of the command line interface might want to install.
Users may now list the compressed files inside of an archive using their pager:
$ less compressed_file.tar.gz
==> use tar_file:contained_file to view a file in the archive -rw------- username/group 695 2008-01-04 19:24 compressed_file/content1 -rw------- username/group 43 2007-11-07 11:17 compressed_file/content2 compressed_file.tar.gz (END)
lesspipe also grants less the ability of interfacing with files other than archives, serving as an alternative for the specific command associated for that file-type (such as viewing HTML via).
Re-login after installing lesspipe in order to activate it, or source
Vim as alternative pager
Vim (visual editor improved) has 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:
There is also an alternative to less.sh macro, which may work as the
PAGER environment variable. Install AUR and add the following to your shell configuration file:
export PAGER='vimpager' alias less=$PAGER
Now programs that use the
PAGER environment variable, like git, will use vim as pager.
locate serves to find files on filesystems. It searches through a prebuilt database of files generated by updatedb or by a daemon and compressed using incremental encoding. It operates significantly faster than find, but requires regular updating of the database.
See the main article.
ls (list) is a command to list files in Unix and Unix-like operating systems.
- ls can list file permissions.
- Colored output can be enabled with a simple alias. File
~/.bashrcshould already have the following entry copied from
alias ls='ls --color=auto'
- The next step will further enhance the colored ls output; for example, broken (orphan) symlinks will start showing in a red hue. Add the following to your shell configuration file:
eval $(dircolors -b)
man (manual page) is a form of online software documentation usually found on a Unix or Unix-like operating system. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts. See Man Pages.
mkdir (make directory) is a command to create directories.
- To create a directory and its whole hierarchy,
-pswitch is used, if not a error is printed. As users are supposed to know what they want,
-pswitch may be used as a default.
alias mkdir='mkdir -p -v'
-vswitch make it verbose.
- Changing mode of a just created directory using chmod is not necessary as the
-moption lets you define the access permissions.
mv (move) is a command to move and rename files and directories. It can be very dangerous so it is prudent to limit its scope:
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.
rm (remove) is a command to delete files and directories.
- It can be very dangerous so it is prudent to limit its scope:
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
-iif you prefer to confirm even for one file.
- Zsh users may want to put
timeoutto avoid implicit expansions.
- To remove directories known to be empty, use rmdir as it fails in case of files inside the target.
sed (stream editor) is a Unix utility that parses and transforms text.
Here is a handy list of sed one-liners examples.
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.
shred is a command to securely delete files and directories. It can be very dangerous so it is prudent to limit its scope:
alias shred=' timeout 3 shred -v'
This alias suspends shred after three seconds, 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.
Zsh users may want to put
timeout to avoid implicit expansions.
Sudo (as superuser do) is a program for Unix-like computer operating systems that allows users to run programs with the security privileges of another user (normally the superuser, or root). See Sudo.
- chmod (change mode) is the name of a Unix shell command and a system call, which both change the access permissions to file system objects (including files and directories), as well as specifying special flags.
- chown (change owner) is used on Unix-like systems to change the owner of a file.
- chattr (change attributes) is a command in the Linux operating system that allows a user to set certain attributes on a file residing on many Linux filesystems.
- lsattr (list attributes) is a command-line program for listing the attributes on a Linux extended file system.
ls -llists files attributes.