Bash File

Bash Read File

Bash check if file Exists

Bash Check if Variable is Set

Bash Alias

There are many ways that we can use to read a file in Bash Shell Scripting. Some of the important methods are given below (Assuming, name of the file that we are reading is 'read_file.txt'):

Reading File Using 'cat fileName'
We can use the following syntax to take a print of the contents of the file to a terminal.

value=`cat file_name`  

Example

#!/bin/bash  

value=`cat read_file.txt`  
echo "$value" 

Reading File Using '$()
Following is the syntax to read the content of the file using '$'

value=$(file_name) 

Example

#!/bin/bash  

value=$(<read_file.txt)  
echo "$value"  

Reading File Content from Command-line
If we want to read a file line by line from commands-line without using the 'cat' command, we can run the following command to perform a task:

Command

while read line; do Command; done < input.file  

Output

Here, while loop will reach each line of the file and store the content of the line in $line variable which will be printed later.

Reading File Content Using Script
To read the file content using the script, we need to create a bash file and add the following code:

Bash Script

#!/bin/bash  

file='read_file.txt'  

i=1  
while read line; do  

#Reading each line  
echo "Line No. $i : $line"  
i=$((i+1))  
done < $file 

Output

Here, an existing filename is stored in $file variable, and $i variable is used to keep the value of the line number of that line.

Passing filename from Command line and reading the File
Create a bash and add the following script which will pass filename from the command line and read the file line by line. The first argument value is read by the variable $1, which will include the filename for reading. If the file is available in the specified location then while loop will read the file line by line and print the file content.

Bash Script

#!/bin/bash  

file=$1  
while read line; do  

#Readind each line in sequence  
echo $line  
done <read_file.txt  

Output

Here, the filename is used as an argument value. The output will provide the content of 'read_file.txt' with no extra spaces between words.

Reading file by omitting Backslash Escape
If we want to read each line of a file line by line by omitting backslash-escape then we are required to use the '-r' option with a 'read' command in 'while' loop, e.g.:

Bash Script

#!/bin/bash  

while read -r line; do  

#Reading each line by omitting backslash escape  
echo $line  
done < read_file.txt 

Output

Bash check if file Exists

Most of the time, we may find a situation where we may need to perform an action that will check whether a file exists or not.

In Bash, we can use a 'test command' to check whether a file exists and determine the type of a file.

Following are the syntaxes of the test command, and we can use any of these commands:

test expression  
[ expression ]  
[[ expression ]]  

We are required to use a single bracket '[' command to make our script portable for all POSIX shells. The upgraded version of the test command contains double brackets '[[' which is supported on most of the modern systems using Bash, Zsh, and Ksh as a default shell.

Check If File Exists
While checking if a file exists, the most commonly used file operators are -e and -f. The '-e' option is used to check whether a file exists regardless of the type, while the '-f' option is used to return true value only if the file is a regular file (not a directory or a device).

The most common option to check if the file exists or not is to use the test command with the 'if conditional statement'.

Following are the examples to check whether the 'read_file.txt' file exists:

Method 1

#!/bin/bash  

File=read_file.txt  
if test -f "$File"; then  
echo "$File exist "  
fi  

Method 2

#!/bin/bash  

File=read_file.txt  
if [ -f "$File" ]; then  
echo "$File exist "  
fi  

Method 3

#!/bin/bash  

File=read_file.txt  
if [[ -f "$File" ]]; then  
echo "$File exist "  
fi  

Output

Output for all the three methods will be as below because we have a file (read_file.txt) present in the directory:

read_file.txt exist

If we want to perform an action which will provide a result based on whether the file exists or not, we can use the if/then construct in the following way:

Example

#!/bin/bash  

File=read_file.txt  
if [ -f "$File" ]; then  
echo "$File exist"  
else  
echo "$File does not exist"  
fi  

Output

read_file.txt exist

We can also use the test command without the if statement. We can use any of the following methods:

Method 1

#!/bin/bash  

File=read_file.txt  
test -f read_file.txt && echo "$File exist"  

Method 2

#!/bin/bash  

File=read_file.txt  
[ -f read_file.txt ] && echo "$File exist" 

Method 3

#!/bin/bash  

File=read_file.txt  
[[ -f read_file.txt ]] && echo "$File exist"

Output

Output for all the three methods will be as below because we have a file (read_file.txt) present in the directory:

read_file.txt exist

If there are several commands to be run after the && operator, then enclose the commands in curly brackets separated by semicolon(;) or AND (&&), i.e.:

Example

#!/bin/bash  

File=read_file.txt  
[ -f read_file.txt ] && { echo "$File exist"; echo "Task Completed"; }  

Unlike &&, the statement after the || operator is executed only if the exit status of the test command is 'false'.

Example

#!/bin/bash  

File=read_file.txt  

[ -f read_file.txt ] && echo "$File exist" || echo "$File does not exist"  

Output

read_file.txt exist

These are the commonly used methods in Bash to check whether the file exists or not.

Check If Directory Exists
The operator '-d' allows us to test whether a file is a directory or not.

Following are the methods to check whether the 'Javatpoint' directory exists:

Method 1

#!/bin/bash  

File=Javatpoint  
if [ -d "$File" ]; then  
echo "$File is a directory"  
fi  

Method 2

#!/bin/bash  

File=Javatpoint  
[ -d "$File" ] && echo "$File is a directory"  
Note: We can also use double brackets '[[' instead of a single bracket '['.

Output

Output for both the above methods will be as below as we have a directory (named Javatpoint) present in the specified location:

Javatpoint is a directory
Check IF File does not Exist
The test expression can be negated by using the exclamation mark (! -logical NOT operator). Check out the following example:

Example

#!/bin/bash  

File=missing_read_file.txt  
if [ ! -f "$File" ]; then  
echo "$File does not exist"  
fi  

Above script can also be written as below:

#!/bin/bash  

File=missing_read_file.txt  
[ ! -f "$File" ] && echo "$File unavailable"  

Output

missing_read_file.txt unavailable