Symbols and Operators in shell script

Last updated at: September 09, 2024

Written by: Abdul

Shell scripting relies on a variety of symbols and operators that control the flow of commands, handle input/output redirection, and perform logical operations. Understanding these symbols is crucial for both beginners and advanced users to automate tasks effectively. Symbols like -d and -f check for directories and files, while comparison operators such as -eq, -lt, and -gt help manage conditional logic. Redirection symbols like >, >>, and | allow you to control how data flows between commands and files, offering powerful ways to process and manipulate data.

In addition, shell scripts use special symbols such as $$ to access the current process ID and $? to check the status of the last executed command, aiding in error handling and debugging. Logical operators like && and || are essential for chaining commands based on success or failure. By mastering these common shell script symbols and their use cases, users can build efficient and flexible scripts to automate a wide range of tasks, from system administration to software deployment.

Symbol	Description
`-d`	Checks if a directory exists
`if [ -d /path/to/directory ]; then echo "Directory exists" fi`
`-f`	Checks if a file exists
`if [ -f /path/to/file ]; then echo "File exists" fi`
`-eq`, `-ne`, `-lt`, `-le`, `-gt`, `-ge`	Numerical comparison operators: `-eq` (equal), `-ne` (not equal), `-lt` (less than), `-le` (less than or equal), `-gt` (greater than), `-ge` (greater than or equal)
`if [ "$a" -eq "$b" ]; then echo "Equal" fi`
`$?`	Returns the exit status of the last command (0 for success, non-zero for failure)
`command; echo $?`
`$#`	Returns the number of arguments passed to a script
`echo "Number of arguments: $#"`
`$$`	Returns the process ID of the current shell
`echo "PID of this shell: $$"`
`$!`	Returns the process ID of the last background process
`sleep 10 &; echo $!`
`&&`	Logical AND: Executes the next command only if the previous one was successful
`command1 && command2`
`\|\|`	Logical OR: Executes the next command only if the previous one failed
`command1 \|\| command2`
`>`	Redirects output to a file (overwrites)
`command > output.txt`
`>>`	Appends output to a file
`command >> output.txt`
`<`	Redirects input from a file
`command < input.txt`
`<<`	Here document: Reads input from the current script until a given delimiter
`cat << EOF ... EOF`
`\|`	Pipes the output of one command to another command
`command1 \| command2`
`*`	Matches all files or acts as a wildcard in patterns
`ls *`
`?`	Matches any single character in a filename
`ls file?.txt`
`;`	Separates multiple commands on a single line
`command1; command2`
`#`	Marks a comment line
`# This is a comment`