Linux read commands from file

Linux/UNIX: Bash Read a File Line By Line

Syntax: Read file line by line on a Bash Unix & Linux shell:

Tutorial details
Difficulty level	Easy
Root privileges	No
Requirements	Bash/ksh/zsh/tcsh
Est. reading time	10m

1. The syntax is as follows for bash, ksh, zsh, and all other shells to read a file line by line
2. while read -r line; do COMMAND; done
3. The -r option passed to read command prevents backslash escapes from being interpreted.
4. Add IFS= option before read command to prevent leading/trailing whitespace from being trimmed —
5. while IFS= read -r line; do COMMAND_on $line; done

How to Read a File Line By Line in Bash

Here is more human readable syntax for you:

The input file ( $input ) is the name of the file you need use by the read command. The read command reads the file line by line, assigning each line to the $line bash shell variable. Once all lines are read from the file the bash while loop will stop. The internal field separator (IFS) is set to the empty string to preserve whitespace issues. This is a fail-safe feature.

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How to use command/process substitution to read a file line by line

Process or command substitution means nothing more but to run a shell command and store its output to a variable or pass it to another command. The syntax is:

Using a here strings

How to file descriptor with read command

The syntax is simple:

Here is a sample script:

Examples that shows how to read file line by line

Here are some examples:

The same example using bash shell:

You can also read field wise:

Fig.01: Bash shell scripting- read file line by line demo outputs

Bash Scripting: Read text file line-by-line to create pdf files

My input file is as follows (faq.txt):

Read from bash shell variable

Let us say you want a list of all installed php packages on a Debian or Ubuntu Linux, enter:

You can now read from $list and install the package:

Conclusion

This page explained how to read file line by line in bash shell script.

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Comments on this entry are closed.

The bash example is correct for all shells. The ksh example is not correct; it will strip leading and trailing whitespace.

To demonstrate, use:

on a file with leading and/or trailing whitespace on one or more .

The BASH password file parsing example does not work for my centos 7 box: all the colons are stripped from each line and the whole line without colons is stored in f1
my output reads:
Username: root/root/bin/bash, Home directory: , Shell:

Reading a file line by line with a single loop is fine in 90% of the cases.

But if things get more complicated then you may be better off opening the file as a file descriptor.

Situations such as:

* the read of the file is only a minor aspect and not the main task, but you want to avoid opening an closing the file multiple times. For example preserving where you have read up to or avoiding file closure (network, named pipes, co-processing)

* the file being read requires different methods of parsing, that is better done in simpler but different loops. Reading emails (header and body) is an example of this, but so is reading files with separate operational and data blocks.

* you are reading from multiple files at the same time, and switching between different files, in a pseudo-random way. That is you never quite know whch file you will need to read from next.

I have a scenario like this. I have a file which has servername, port and ipaddress
svr1
port1
ip1
svr2
port2
ip2
…
…

I tried couple of ways reading these records and assigning them a field and print the output to a file like below but nothing is helpful

servername port ipaddress
svr1 port1 ip1
scr2 port2 ip2
and so on

can someone please help me.

I figured it out. May be not professional but it is working for me.

if the file is 100% predictable in each host,ip,port coming in groups of 3 you can use “paste”

cat list.txt | paste – – –

HOW DO YOU SPLIT INPUT INTO FIELDS FOR PROCESSING?

I have a question concerning how you’d actually process individual fields of input.
For example, let’s say you’re doing the usual

while read LINE; do
process the statements
done firstbyteinq and looking at firstbyteinq with a read, but 1). the cut doesn’t seem to be doing it just to the record being read, it does it to ALL the records all at one time; 2). the read of the firstbyteinq then advances the other file one record.

It seems to me there ought to be some way to “declare” the positions from a to b as fields and THEN use the while read, but I’m not sure how to do that.
I’ll try anything. Thanks VERY much in advance.

princess anu, there’s no need to use arithmetic; just read the file in groups of three lines:

A correction; you need to check that the read succeeded; checking the first should be enough:

I have scenario :

One file which contain values like:
ABS
SVS
SGS
SGS

another file is template which has to use values from 1st file (One value at a time, line by line )

ARGS=ABS >> save in one file

New file:
ARGS=SVS >> save in another file

Can you please help me to achive this

u can use paste command if the fields are same …..and then output the records to the other file and print another.

rm 3 4 5
first=1
seco=2
cat 1 | while read l
do
echo “$l =” >> 3
done
paste 3 2 | while read line
do
echo “$line” >> 4
done
awk ‘’ 4 > 5

now u can move the records line by line to another file

echo “$line” # Chris F.A. Johnson Jul 3, 2013 @ 11:40

Some shells shells will output -n; that is what POSIX requires.

My while/read is having an issue with the data not being printed for the last line read.
The info is read and summed but is not being printed to the report.
The script is reading a file with tab delimited fields, sorting on a field then reading each line while assigning the fields to var names.
If statements determine whether the numbers are summed or if a new category is found with the previous categories data being written to the report. All categories except the last are complete in the report. The last category is left out.
I added trouble shooting echo statements to check the workflow and they showed that the last category is being read and summed but it doesnt get printed.
It’s as if the read gets to the last line, does the work and then doesnt know what to do with the output.
The sorted data looks something like:

Code with troubleshooting echos and most comments deleted:

The read name is different than the last line (new category)

cat | sort -k3 | while read -r Count IP Name

sort -k3 | while read -r Count IP Name

It is also possible to store the elements in a bash array using the -a option

The following example reads some comma separated values and print them in reverse order:
input.txt

Hi guys. I’m just getting started with scripting and I am needing some help reading a file into the script. This shell script below works and does what it needs to do but I have to manually input the file name for “read list”. How do I modify this script to automatically read the file. Inside the file is just a list of aix server names.

#!/usr/bin/sh
echo ” Please input the master list you know to use”
read list
for i in `cat $list`
do
ssh $i hostname
ssh $i df
done

Hi all,
I need to write a script to look for user in /etc/passwd file. The program takes user input from command line and check to see if the user that the person entered exists. If user doesn’t exist, the script sends error message. I use grep to print the user which exist. But not sure what command to use and look and if the user doesn’t exist, send error message?
Thanks,

Hi stone,
me trying to get the list of server have /mysqlshare file system in it but i am unable to get it..
for now trying get only 2 server details from txt file ie(text.txt) which have
server1_name
server2_name
when i try this script its only reading 1st server details and getting exit from script

Can someone help me out..

can anyone help me to include the line number while read line by line ,

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bash execute read command from file

After some manipulations, I got the file with commands that I would be glad to be able to execute.

Input file (name of file is inp2.txt):

EDIT: sorry, I omitted important part of code that I thought is not important for understanding. Full code:

It seems that command is proper, but something is wrong and I can’t figure out what’s wrong exactly.

5 Answers 5

The script, data file and output that you posted are inconsistent. Neither the script not the data file contain mv , yet your screenshot does. Also, your screenshot mentions a line 28, which the script you posted doesn’t have. It’s difficult to pinpoint your problem when you give us inconsistent information.

That said, you’re trying to do one of two things, neither of which can work the way you’re trying.

If the input file contains lines like

then it’s really a shell script. Instead of reading it line by line, execute it as a shell script. Make sure that you can trust this file, since you’ll be executing whatever is in there, including rm -rf

If the input file contains lines like

then the way you’re reading it doesn’t work. read LINE does the following:

• read one line;
• if that line ends with a backslash, remove the backslash and read another line (repeat until a line that doesn’t end with a \ has been read);
• replace all backslash+character sequences by the second character only;
• set LINE to the concatenation of the lines read, minus the newlines.

When the shell executes the command $LINE , it does what it always does when it sees a variable substitution outside quotes, which is:

• split the value of the variable into a list of words at every place where it contains whitespace (assuming the default value of IFS );
• treat each word as a glob pattern, and expand it if it matches at least one file.

Sounds useless? It is. And note that there’s nothing about quotes in here: quotes are part of the shell syntax, they aren’t part of the shell expansion rules.

What you probably should to is have inp2.txt contain a list of file names, one per line. See Why is `while IFS= read` used so often, instead of `IFS=; while read..`? for how to read a list of lines from a file. You’ll be wanting something like

Just for completeness, I’ll mention another possibility, but I don’t recommend it because it’s fiddly and it won’t let you do what you seem to be doing. A file like

then it can be read by the xargs command. The input to xargs is a whitespace-delimited list of elements, which can be either a literal (possibly containing whitespace) surrounded by single quotes, a literal (possibly containing whitespace) surrounded by double quotes, or an unquoted literal which may contain backslash escapes ( \ quotes the next character). Note that the xargs syntax is unlike anything the shell might recognize.

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