22 comment

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

   To demonstrate, use:

   echo ":$line:"

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

   1. 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:

2. 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.

3. 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.

   Thanks

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

   #!/bin/sh
   #!/bin/bash
   num=1
   echo -n "server name    "
   echo "port number"
   echo "-----------------------------------------"
   while read line
   do
   echo -n "$line        "
   i=$(($num % 2 ))
   if [ $i -eq 0 ]
   then
   echo "" #### print the new line ###
   fi
   num=`expr $num + 1`
   done< serverlist.txt
   

   Thanks

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

      cat list.txt | paste – – –

5. 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.

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

   while :
   do
     read server
     read port
     read ipaddr
     : do whatever
   done < "$file"
   

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

   while :
   do
     read server || break
     read port
     read ipaddr
     : do whatever
   done < "$file"
   

8. 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

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

      1. 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 ‘{print $1,$2,$3}’ 4 > 5

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

9. echo “$line” # <— will fail if "$line" is "-n", it won't print anything at all, even an empty line.

   You should use printf builtin to work around this behavior:
   printf "%s\n" "$line"

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

11. 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:

    247   an-IP   Name1
    37   another-IP   Name1
    52    more-IPs   Name2
    1562   etc   Name2
    29   etc   name3
    4   etc   name3
    The report has:
    284 Name1
    1591 Name2

    Code with troubleshooting echos and most comments deleted:

    #control vars
    GT=0
    name-Name
    cat  | sort -k3 | while read -r Count IP Name
    do
    # Handle the first line
       if [ "Name" != "$name" -a $GT -eq 0 ]
       then
          name=$Name
          GT=$Count
    # The read name is different than the last line (new category)
       elif [ "$Name != $name" -a $GT -gt 0 ]
       then
    # Write previous name and count to the report
          echo " ${name} - ${GT}" >> 
    # Assign new categories values to the vars
          GT=$Count
          name=$Name
    # New line but same category found
       else [ "$Name" == "$name"  ]
    # Increment counts of the category
          GT=`expr $GT + $Count`
       fi
    done
    

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

       elif [ "$Name != $name" -a $GT -gt 0 ]
       then
    # Write previous name and count to the report
          echo " ${name} - ${GT}" >>
    # Assign new categories values to the vars
          GT=$Count
          name=$Name
    # New line but same category found
       else [ "$Name" == "$name"  ]
    # Increment counts of the category
          GT=`expr $GT + $Count`
       fi
    done
    REPLY
    

13. UUOC

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

    is the same as

    sort -k3 | while read -r Count IP Name

14. 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

       1,2,3,4
       A,B,C
    ==>
       4,3,2,1
       C,B,A
    

    Code
    

    while IFS="," read -a A ; do 
       SEP=""  
       for ((i=${#A[@]}-1;i>=0;i--)) ; do 
         echo -n "$SEP${A[i]}"  
         SEP="," 
       done 
       echo 
    done < input.txt
    

15. 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

16. 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,

17. 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

    #!/bin/ksh
    file="/home/mysqladm/server_list/server_list.txt"
    # while loop
    while IFS= read -r a
    do
            # display line or do somthing on $line
            output=`ssh $a df -h | grep mysqlshare`
    #echo $a,$output
    echo $a ,$output >>/home/mysqladm/server_list/output.txt
    done <"$file"
    

    Can someone help me out..

18. Thanks man!

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