Command Operators

Command Operators

Command-chaining operators are special characters used to write miniature shell scripts in the command line. This is incredibly useful in automating tasks.

• References: 10 Linux Command-Line Operators and What They Do

&

The Ampersand Operator. Executes that Linux command in the background.

gedit &

;

The Semicolon Operator. Executes commands in a defined, sequential order.

note

The shell does not check if each command terminates successfully.

pwd ; mkdir test ; cd test ; touch file

||

The OR Operator. Executes the command that follows only if the preceding command fails.

bad_command || ls

In this example, only if the bad_command fails will the ls command be executed.

|

The Pipe Operator. Directs the output of the preceding command as input to the succeeding command. It is most commonly used to filter data with the grep command.

cat test | grep -i "makeuseof"

This command example sends the output of the cat command as input to the grep command, which then filters the output against a specified string.

&&

The AND Operator. Executes commands only if the preceding command was successfully executed.

pwd && mkdir test && cd test && bad_command && ls

Since bad_command fails to run, the shell will return an error and skip the ls command.

!

The NOT Operator. Similar ways to an except statement in programming.

rm -r !(*.txt)

This sample command will recursively remove all files in a directory except for files that have a .txt extension.

(..)

The Precedence Operator. Sets groups and precedence to ensure that the execution sequence meets your expectations.

(ls *.txt > txt-files.list && cp *.tx ~) && (ls *.deb > deb-packages.list && cp *.deb ~) || echo "Precedence Test!"

In this sample syntax, both groups of commands must return an exit code 0 to ensure the successful execution of the last command. This example requires that both commands in the first set () exit with 0 for the second set () to run.

{..}

The Combination Operator. Groups commands. Whichever commands you want to group you can place them inside curly brackets, and they will be executed depending upon the exit code of the first command.

test -f /etc/passwd && {pwd ; date} && echo $0 ; echo "Hello"

The sample syntax will test if the /etc/passwd file is present, print the current working directory, date, shell name, and echo "Hello".

\

Concatenation or the Escape Operator. You can either use it to concatenate two commands or as an escape character when working with strings in the shell.

mkdir test0 test1 \ test2
echo "Hello! from the \nother side"

The first command will make 3 directories named test0, test1, and test2, and the second command will print the string separated by a new line.

In other words, if a command is too long, you can use \ and Enter to split it into different lines without executing them immediately. For example:

pwd && \
ls

>, >>, <

The Redirection Operators. Redirects output or input to a file either by re-writing the file or by appending to it.

• To re-write a file, use the single angle bracket > syntax.
• To append to a file, use the double angle bracket syntax >>.

echo "dsd" > test ; echo "bssss" >> test

In the sample syntax, the first command will overwrite the test file with the provided string. In the second command, the string provided will be appended to the test file.