Advanced Bash File Operations

December 17, 2023 | by Arround The Web | No comments

Advanced Bash File Operations

Files are used to store any content permanently in the system that can be used later. Bash has no open() function like other standard programming languages to create, access, and modify a file. But a file can be created or accessed in multiple ways using the Bash commands and Bash script. Many commands exist in Bash to get the details about a file, search and replace the content of the file using a regex pattern, and delete the file. Different advanced-level file operations in Bash are shown in this tutorial.

List of Contents:

Create a New File

Multiple ways exist in Bash to create a new file in Bash. New files can be created from the terminal using different types of text and GUI editors. The method of creating a new file from the terminal using the Bash “cat” and “echo” commands is shown in this part of the tutorial. Then, the file creation is shown using the nano editor.

Run the following command to create a text file named “courses.txt”:

$ cat > courses.txt

Type the following content in the terminal:

Code Name

CSE-106 Object-oriented programming

CSE-208 Visual programming

CSE-303 Advanced DBMS

CSE-407 Unix programming

Press “Ctrl+d” to stop writing and go to the command prompt.

Now, run the following command to check the content of the file:

$ cat courses.txt

Run the following command to create a file of a simple text:

$ echo "It is a testing file." > test.txt

Next, run the “cat” command to check the content of the “text.txt” file.

The following output appears after executing the previous commands:

Run the following command to open the nano editor to create a text file named “course-teacher.txt”:

$ nano course-teacher.txt

Type the following content in the editor:

Code Teacher

CSE-208 Saima Akter

CSE-303 Mahmuda Ferdous

CSE-407 Sabrina Sultana

Press “Ctrl+x” to save and exit from the editor.

Run the following command to check the content of the file:

$ cat course-teacher.txt

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Check the Existence of a File

The “-f” option is used to check the existence of the regular file and the “-s” option is used to check the existence of the non-empty file. If the file exists and contains a data, a message is printed to inform that the file exists and contains a data. Otherwise, a message is printed to inform that the file is empty or does not exist.

#!/bin/bash

read -p "Enter an existing filename: " fn

#Check whether the file exists with data or not

if [ -f "$fn" ]

then

if [ -s "$fn" ]

then

#Print a message if the file is not empty

echo "$fn file exists and contains data."

else

#Print a message if the file is empty

echo "$fn file is empty."

else

#Print a message if the file does not exist

echo "$fn file does not exist."

The “courses.txt” file is created earlier that contains the data. So, the following output appears if the script is executed with the “courses.txt” filename:

According to the following output, the “t.txt” file exists and the file is empty:

According to the following output, the “course.txt” file does not exist in the current location:

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Read a File Using the “For” Loop

Any existing file can be read using different loops in Bash. The following script shows the use of the “for” loop to read the content of a file where the filename is given as the command-line argument value. The IFS variable is used in the script to detect the new line of the file. The “cat” command is used inside the “for” loop to read the content of the file line by line. The “$counter” variable is used to print the line number with each line of the file.

#!/bin/bash

#Initialize the filename from the command-line argument

fn=$1

#Initialize counter

counter=1

#Set the internal field separator

IFS=$'\n'

#Read the file line by line

for val in $(cat "$fn")

#Print each line of the file with the line number

echo "Line-$counter: $val"

#Increment the counter

((counter++))

done

According to the output, the “cat” command is used to check the original content of the “courses.txt” file before executing the script. Then, the script is executed by giving the “courses.txt” as the first command-line argument value.

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Read a File Using the “While” Loop

The following script shows the use of a “while” loop to read the content of a file where the filename is given as the command-line argument value. The “$counter” variable is used in this script also to print the line number with each line of the file.

#!/bin/bash

#Initialize the counter

counter=1

#iterate the loop to read the file line by line

while read -r line

#Print each line with the line number

echo "Line-$counter: $line"

#Increment the counter

((counter++))

#Read the filename from the command-line argument

done < "$1"

The output of the provided script is similar to the previous example.

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Calculate the Size of a File

The size of the file can be calculated in different ways using a Bash script. In the following script, the “stat” command with the “-c” option is used to calculate the size of the file in bytes. The filename is taken from the user. The size of the file is printed if the file exists and is non-empty. Otherwise, an error message is printed.

#!/bin/bash

#Take the filename from the user

read -p "Enter an existing filename: " fn

if [ -s $fn ]

then

#Count the size of the file

size=$(stat -c %s $fn)

#Print the file size in bytes

echo "The size of the $file file is $size bytes."

else

echo "File does not exist or empty."

The following output appears after executing the previous script. Here, the size of the “courses.txt” file is 153 bytes:

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Append the Content into a File

The “>>” operator is used in Bash to append the content of the file. Suppose you have to add a new line in the “courses.txt” file using the “echo” command. Run the “cat” command before and after executing the “echo” command that adds a new line at the end of the “courses.txt” file.

$ cat courses.txt

$ echo "CSE-202 Data Structure" >> courses.txt

$ cat courses.txt

The following output appears after executing the previous commands. A new line is added at the end of the “courses.txt” file:

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Make a Copy of a File

The “cp” command is used to create a new file from an existing file. The following script creates a new file named “courses_copy.txt” by copying the content of the “courses.txt” file:

#!/bin/bash

#Define the source and destination filename

source_file="courses.txt"

destination_file="courses_copy.txt"

if [ -f $source_file ]

then

#Copy the file if exists

cp $source_file $destination_file

echo "File is copied successfully."

else

echo "Source file does not exist."

The following output appears if the “copy.txt” file exists in the current location. Next, the “cat” command is executed to check whether the content of the file is copied properly or not:

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Move the File Location

The “mv” command is used to move any file from one location to another location. Run the following commands to check the content of the “courses_copy.txt” file, move the file to the “temp” location, and check whether the file is moved successfully or not:

$ cat courses_copy.txt

$ mv courses_copy.txt temp

$ cat temp/courses_copy.txt

The following output shows that the “courses_copy.txt” file is moved successfully to the “temp” folder:

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Count the Total Lines of the File

Multiple commands exist in Bash to count the total lines. In the following script, the “wc”, “sed”, and “awk” commands are used to count the total lines of the file that will be taken from the user. If the filename that is taken from the user does not exist, an error message is printed.

#!/bin/bash

#Take the filename from the user

read -p "Enter an existing filename: " fn

if [ -s $fn ]

then

#Count total lines of the file using `wc`, `sed`, and `awk` commands

wc_lines=$(wc -l < $fn)

sed_lines=$(sed -n '$=' $fn)

awk_lines=$(awk 'END { print NR }' $fn)

#Print the output of different commands

echo "Total lines (wc): $wc_lines"

echo "Total lines (sed): $sed_lines"

echo "Total lines (awk): $awk_lines"

else

#Print a message if the file does not exist or is empty

echo "File does not exist or empty."

The following output shows that the output of three commands that are used in the script are the same and the “courses.txt” file contains six lines:

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Search the Content in a File

The “grep” command is used to search any content in a file using the search words or the regular expression pattern. The method of searching the content in a file using a particular searching word in the “grep” command with the “-i” option is shown in the following script. Here, the “-i” option indicates the ignore case. So, the searching word is searched in the file in a case-insensitive way.

#!/bin/bash

#Take the filename from the user

read -p "Enter an existing filename: " fn

#Take the search word from the user

read -p "Enter the search word: " src

if [ -s $fn ]

then

#Search the particular word in the file

output=$(grep -i $src $fn)

if [ "$output" != "" ]

then

#Print the output of the `grep` command

echo "$output"

else

#Print a message if the file does not exist

echo "File does not exist or empty."

The following output appears after executing the script with the “courses.txt” filename, and the CSE-208 searching word. The second line of the file contains the “CSE208” word:

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Search and Replace the Content of a File Using Regex

The “sed” command is one of the ways to search and replace the content of a file in Bash. In the following script, the method of searching and replacing the content of a file using the “sed” command with the “-i” option is shown. The filename, searching word, and replacing word are taken from the user.

#Take the filename

read -p "Enter an existing filename: " fn

#Take the search word

read -p "Enter the search word: " src

#Take the replace word

read -p "Enter the replace word: " rep

if [ -s $fn ]

then

#Search and replace the particular word in the file

sed -i "s/$src/$rep/" $fn

else

#Print a message if the file does not exist

echo "File does not exist or empty."

Run the following output to check the content of the “courses.txt” file before and after executing the script:

$ cat courses.txt

$ bash file11.bash

$ cat courses.txt

The following output shows that the CSE-303 search word exists in the “courses.txt” file and it is replaced by the CSE-307 replacing word:

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Delete a File

The “rm” command is used to delete a file in Bash. This command is used with the “-i” option in the following script to remove a file if it exists. Here, the filename is taken from the user.

#!/bin/bash

# Take the filename

read -p 'Enter the filename: ' fn

# Check the existence of the file

if [ -f $fn ]; then

# Remove the file

rm -i "$fn"

# Check whether the file is removed or not

if [ -e fn ]; then

echo "File is not removed."

else

echo "File is removed."

else

echo "File does not exist."

According to the output, the “t.txt” filename is located in the current location and the file is removed after the confirmation.

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Conclusion

Different types of file operations in Bash are explained in this tutorial. The scripts of this tutorial will help the Bash user to know the various file-related tasks such as creating the files, appending the files, counting the file size, searching the content, replacing the content, deleting the files, etc.

Source: linuxhint.com