How to understand special character for line reading in bash shell?


 
Thread Tools Search this Thread
Top Forums Shell Programming and Scripting How to understand special character for line reading in bash shell?
# 1  
Old 09-21-2019
How to understand special character for line reading in bash shell?

I am still learning shell scripting. Recently I see a function for read configuration. But some of special character make me confused. I checked online to find answer. It was not successful. I post the code here to consult with expert or guru to get better understanding on these special characters such as:^[^#]*=, line%%=*, line#*=. how to understand the usage of these special character here. Thanks.

Code:
read_cfg()
{”‹
  while read line; do”‹
     if [[ "$line" =~ ^[^#]*= ]]; then”‹
        l_parameter=`echo ${line%%=*} | tr -d ' '` l_value=`echo ${line#*=} | tr -d ' '`”‹
”‹
        case "$l_parameter" in ”‹
           "EMAIL_LIST" ) ”‹      #email list provided in command line overwrites the one in configuration file”‹
               if [ -z "$g_bkp_email" ] ; then”‹
                  g_bkp_email=$l_value”‹
               fi”‹
               ;;”‹

# 2  
Old 09-21-2019
Not sure I'm an expert nor a guru, but this is what I'd do: refer to the respective man pages.
man bash:
Quote:
Parameter Expansion

${parameter#word}
${parameter##word}

Remove matching prefix pattern.


${parameter%word}
${parameter%%word}

Remove matching suffix pattern.



CONDITIONAL EXPRESSIONS.



An additional binary operator, =~, is available, with the same precedence as == and !=. When it is used, the string to the right of the operator is considered a POSIX extended regular expression and matched accordingly (as in regex(3))
Of course you need to learn the difference between shell's pattern matching and regex matching.

man regex:
Quote:
An atom is a regular expression ... '^' (matching the null string at the beginning of a line)

A bracket expression is a list of characters enclosed in "[]". It normally matches any single character from the list (but see below). If the list begins with '^', it matches any single character (but see below) not from the rest of the list.
Admittedly the ambiguous use of the caret is something you need to accustom to.
These 2 Users Gave Thanks to RudiC For This Post:
# 3  
Old 09-21-2019
^[^#]*= is an extended regular expression and it means 0 or more non-# characters starting at the beginning of the line followed by an equal sign..

${line%%=*} is parameter expansion and it returns the characters to the left of the first = sign
${line#*=} is parameter expansion and it returns the characters to the right of the rightmost = sign
These 3 Users Gave Thanks to Scrutinizer For This Post:
# 4  
Old 09-21-2019
RudiC, Scrutinizer:

Thanks a lot to both of you. The explanation is clear and make me better understanding on these special characters. I will learn more.
Login or Register to Ask a Question

Previous Thread | Next Thread

10 More Discussions You Might Find Interesting

1. UNIX for Beginners Questions & Answers

Escape bash-special character in a bash string

Hi, I am new in bash scripting. In my work, I provide support to several users and when I connect to their computers I use the same admin and password, so I am trying to create a script that will only ask me for the IP address and then connect to the computer without having me to type the user... (5 Replies)
Discussion started by: arcoa05
5 Replies

2. Shell Programming and Scripting

Shell script question in special character

when I execute the unix commands its works normally in the 1st part. When I the tried the same in shell scripting the directory is not displayed in 2nd part example. please let me know what needs to be done. Unix : client=~zsvdbs echo $client /shome/zsvhome/zsvdbs Using... (3 Replies)
Discussion started by: keerthi2016
3 Replies

3. UNIX for Dummies Questions & Answers

Reading character by character - BASH

Hello every one and thanks in advance for the time you will take to think about my problem. I would like to know if it's possible (in BASH) to read a text file character after character. Exactly this is what I would like to do : Txt file : ATGCAGTTCATTGCCAAA...... (~2.5 millions... (3 Replies)
Discussion started by: sluvah
3 Replies

4. Shell Programming and Scripting

parse special character in the line

Hi all, I have a file with some module names as below. Font::AFM Data::Grove ---> libxml-perl Net::LDAP ---> perl-ldap DBI XML .... ... .... and so on ... The file has some lines with the character " -->" . Now how can I cut only the last column of the line wherever "-->" is... (4 Replies)
Discussion started by: vijaya2006
4 Replies

5. Shell Programming and Scripting

Printing special character in bash

I am using this character as a delimiter 'þ' Currently, I set it straight: DELIMITER='þ' However, while copying the file, this character often gets mangled. Is there a bash way (perhaps using tr or printf) of generating this character. It corresponds to "chr(0xfe)" if using perl. (I've... (6 Replies)
Discussion started by: sentinel
6 Replies

6. Shell Programming and Scripting

I need to understand the differences between the bash shell and the Bourne shell

I do not claim to be an expert, but I have done things with scripts that whole teams of folks have said can not be done. Of course they should have said we do not have the intestinal fortitude to git-r-done. I have been using UNIX actually HPUX since 1992. Unfortunately my old computer died and... (7 Replies)
Discussion started by: awk_sed_hello
7 Replies

7. Shell Programming and Scripting

How to insert a character in line in Special condition?

Hi, I have a log file generated by a tool which has the following look : /tmp/releases/directory/datefilename1_release_date.zip /tmp/releases/directory/datefilename2_release_date.zip /tmp/releases/directory/datefilename3_release_date.zip... (8 Replies)
Discussion started by: bhaskar_m
8 Replies

8. Shell Programming and Scripting

Adding a special character at the end of the line

I used following to add * at the end of the line in file1. It adds * at the end but has a space before it for some lines but some other lines it adds exactly after the last character. How do I take out the space ? sed 's/$/*/' file1 > file2 example: contents of file1 : ... (2 Replies)
Discussion started by: pitagi
2 Replies

9. Shell Programming and Scripting

bash shell: 'exec', 'eval', 'source' - looking for help to understand

Hi, experts. Whould anybody clear explay me difference and usage of these 3 commands (particulary in bash) : exec eval source I've tryed to read the manual pages but did not get much. Also could not get something useful from Google search - just so much and so not exactly, that is... (3 Replies)
Discussion started by: alex_5161
3 Replies

10. UNIX for Dummies Questions & Answers

[OpenServer 5]Line Printing and special character (é @)

Hello, On Sco OpenServer 5, i want to print using the lpr command, no CUPS installed. I print on an HP LaserJet 4050 on LAN (IP 192.168.x.x) the printer is installed by HP Network Printer service. it works fine, but Specials characters, like é, @ or ° print bad characters. Is there... (5 Replies)
Discussion started by: tankd
5 Replies
Login or Register to Ask a Question