How to Read a File Line by Line in a Shell Script ~ Bash Shell Scripting by Examples. There are many ways to handle any task on a Unix platform, but some. CPU time. Most of the wasted time is spent in unnecessary variable assignment and. Using a. pipe also has a negative impact on the timing. In this article I will explain various techniques for parsing a file. In this example, we place the while loop statement after another statement. However, Perl evaluates the statements from right to left, which means Perl evaluates the. Perl file handling tutorial: learn to open, read, write and close files with Perl. I am using the following code to read line by line from a file, however my problem is that $value=0 at the end of the loop possibly because bash has created a. Some techniques are very fast and some make you wait for. The techniques used in this article are measurable, and I. I don't explain in depth every thing, but if you know basic shell. I hope you can understand easily. I extracted last five lines from my /etc/passwd file, and stored in a. Project Admin: /home/project: /bin/bashking: x: 5. I use this file whenever a sample file required. Method 1: PIPED while- read loop. SCRIPT: method. 1. PURPOSE: Process a file line by line with PIPED while- read loop. FILENAME=$1count=0cat $FILENAME | while read LINEdolet count++echo "$count $LINE"doneecho - e "\n. Total $count Lines read". With catting a file and piping the file output to a while read loop a. LINE on each loop. This continuous loop will run until all of the lines in the. Bash can sometimes start a subshell in a PIPED "while- read" loop. So. the variable set within the loop will be lost (unset) outside of the. Therefore, $count would return 0, the initialized value outside. Project Admin: /home/project: /bin/bash. We still use the while read LINE syntax, but this time we feed the loop from the bottom (using file redirection) instead of using a pipe. You will find that this is. To read the next line of the file with newline included, you use the following syntax. Total 0 Lines read. Method 2: Redirected "while- read" loop. SCRIPT: method. 2. PURPOSE: Process a file line by line with redirected while- read loop. FILENAME=$1count=0while read LINEdolet count++echo "$count $LINE"done < $FILENAMEecho - e "\n. Total $count Lines read". We still use the while read LINE syntax, but this time we feed the. You will find that this is one of the fastest ways to process each. The first time you see this it looks a little unusual. Unlike method 1, with method 2 you will get total number of lines out. Project Admin: /home/project: /bin/bash. Total 5 Lines read. Note: In some older shell scripting languages, the redirected loop. Method 3: while read LINE Using File Descriptors. A file descriptor is simply a number that the operating system assigns. Consider it a simplified version. It is analogous to a file handle in C. There are always three default "files" open, stdin (the keyboard). These, and any other open files, can be redirected. Redirection simply. Each open file gets assigned a file descriptor. The file descriptors. For opening. additional files, there remain descriptors 3 to 9 (may be vary depend- . OS). It is sometimes useful to assign one of these additional. This simplifies restoration to normal after complex redirection. There are two steps in the method we are going to use. The first step. is to close file descriptor 0 by redirecting everything to our new file. We use the following syntax for this step. Now all of the keyboard and mouse input is going to our new file des- . The second step is to send our input file, specified by the. FILENAME, into file descriptor 0 (zero), which is standard. This second step is done using the following syntax. FILENAME. At this point any command requiring input will receive the input from. FILENAME file. Now is a good time for an example. SCRIPT: method. 3. PURPOSE: Process a file line by line with while read LINE Using#File Descriptors. FILENAME=$1count. FILENAMEwhile read LINEdolet count++echo "$count $LINE"doneexec 0< & 3echo - e "\n. Total $count Lines read". But the beginning of this. The first exec comm- . The second exec command red- . FILENAME file into stdin, which is file descriptor 0. Now. the while loop can just execute without our having to worry about how. LINE variable. When the while loop. In other words we set it back to the system’s default value. Project Admin: /home/project: /bin/bash. Total 5 Lines read. Method 4: Process file line by line using awkawk is pattern scanning and text processing language. It is useful. for manipulation of data files, text retrieval and processing. Good. for manipulating and/or extracting fields (columns) in structured. Its name comes from the surnames of its authors: Alfred Aho, Peter. Weinberger, and Brian Kernighan. I am not going to explain everything here. To know more about awk just. At the command line, enter the following command. You should see the contents of your /etc/passwd file appear before. Now, for an explanation of what awk did. When we called awk. When we executed awk, it. All. output is sent to stdout, and we get a result identical to catting. Now, for an explanation of the { print } code block. In. awk, curly braces are used to group blocks of code together, similar. C. Inside our block of code,we have a single print command. In awk. when a print command appears by itself, the full contents of the curr- . Here is another awk example that does exactly the same thing. In awk, the $0 variable represents the entire current line, so print. Now is a good time for an. SCRIPT: method. 4. PURPOSE: Process a file line by line with awk. FILENAME=$1awk '{kount++; print kount, $0}END{print "\n. Total " kount " lines read"}' $FILENAMEOutput. Project Admin: /home/project: /bin/bash. Total 5 lines read. Awk is really good at handling text that has been broken into multiple. The following script will print. F": " '{ print $1 "\t " $3 }' /etc/passwd. Above, when we called awk, we use the - F option to specify ": " as the. By default white space (blank line) act as filed sep- . You can set new filed separator with - F option. When awk proc- . esses the print $1 "\t " $3 command, it will print out the first and. Method 5: Little tricky with head and tail. SCRIPT: method. 5. PURPOSE: Process a file line by line with head and tail commands. FILENAME=$1. Lines=`wc - l < $FILENAME`count=0while [ $count - lt $Lines ]dolet count++LINE=`head - n $count $FILENAME | tail - 1`echo "$count $LINE"doneecho - e "\n. Total $count lines read". On each iteration head command extracts top $count lines, then tail. A very stupid method. Project Admin: /home/project: /bin/bash. Total 5 lines read. Time Comparison for the Five Methods. Now take a long breath, we are going test each technique. Before you. get into test each method of parsing a file line by line create a large. Use bigfile. sh script to create a large file. I don't know exactly how much time it. This file is extremely large to parse a file line by line. I needed a large file to get the timing data greater than zero. M bigfile. 4. 22. I waited more than half day, still i didn't get result, then I createda 1. Method 4 came in first place,it has taken very less time 2. Method 4 with other methods, because awk is not. Method 2 and method 3 are tied for second place, they produce mostly. Method 1. came in third at 6 minutes and 2. Method 5 has taken more than half a day. Note: If file contain escape characters, use read - r instead of read,then Backslash does not act as an escape character. The back- slash isconsidered to be part of the line. In particular, a backslash- newlinepair may not be used as a line continuation.
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