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Bash

Revision as of 11:18, 14 November 2014 by Freephile (talk | contribs)

This article is a cheat sheet for things you learn to do in Bash (The Bourne Again Shell).

For Loops

The bash one-liner for doing a for loop looks something like this: 

for FILE in $(ls); do [COMMAND]; done

Here is a real example that will lowercase the names of all files in the current directory: 

for FILE in $(ls); do mv $FILE $(echo $FILE | tr [A-Z] [a-z]); done

To do simple range looping, use the seq command: 

for i in `seq 1 10`; do echo "$i, "; done

If construct

The then can go on the same line as the if as long as you use a semi-colon to terminate the if clause. Alternately, you can put the then on it's own line 

if EXPR; then
  # do stuff
fi

is equivalent to 

if EXPR
then
  # do stuff
fi

Adding an else clause 

if EXPR; then
  # do stuff
else
  # do other stuff
fi

Adding multiple else clauses with elif; then 

if EXPR; then
  # do stuff
elif EXPR; then
  # do other stuff
else
  # final else
fi

Note: sometimes you want to comment out a section of an if/else block, or maybe it does nothing at all. In this case, you'll get an error. To avoid the error, you can use the bash built-in : (colon command) 

 : [arguments]

Do nothing beyond expanding arguments and performing redirections. The return status is zero. 

if [ -f "/tmp/Non-existing-file.txt" ] ; then
  echo "I found the non-existing file"
else
  : # the colon command prevents an error if there are no other statements in this block
fi

Using Find

The find command in Linux is very powerful, and thus somewhat complex to learn all the syntax and options. Suffice to say that you can read the manpage and info pages to answer your questions. However, in case you are trying to figure out the prune option so that you can efficiently scan a directory for something while also ignoring .svn metadata, here is an example: 

find ./ -name .svn -prune -o -name "*html*"

Or, a more complex example: 'wcgrep', from the contrib section of the svn repo: 

#!/bin/bash

# Copyright 2004 Ben Reser <ben@reser.org>
# Licensed under the terms subversion ships under or GPLv2.

# Useful for greping in a subversion working copy.
# Essentially it behaves the same way your grep command does (in fact it
# ultimately calls the grep command on your path) with a few exceptions.
# Ignores the subversion admin directories (.svn) and vi(m) backup files.
# Recursive is always on with or without -r.
# Always print filename and line numbers.
# Ignores binary files.
# If no path is given the current working directory is searched not stdin.
# Other than that it will take any parameter or pattern your standard grep
# does.
#
# This script requires GNU findutils and by default GNU grep (though that
# can be changed with environment variables).
#
# There are three environment variables you can set that modify the default
# behavior:
#
# WCGREP_GREP      Controls what command is used for the grep command.
#                  If unset or null wcgrep will use the command named grep.
# WCGREP_GREPARGS  Controls what arguments are always passed to the grep
#                  command before the arguments given on the command line.
#                  If unset or null it defaults to -HnI (always print file
#                  names, line numbers and ignore binary files).  If you wish
#                  to set no default args set the variable to a space (" ").
# WCGREP_IGNORE    Controls what files are ignored by the grep command.
#                  This is a regex that is passed to the find command with
#                  -regex so see find's man page for details.  If unset or
#                  null defaults to '.*~$\|.*/\.svn\(/\|$\)', which will
#                  ignore vim backup files and subversion admin dirs.


arg_count=$#
for (( i=1; i <= $arg_count; i++ )); do
arg="$1"
shift 1
if [ -z "$pattern" ]; then
if [ "$arg" == "--" ]; then
grepargs="$grepargs $arg"
pattern="$1"
shift 1
((i++))
elif [ "${arg:0:1}" != "-" ]; then
pattern="$arg"
else
grepargs="$grepargs $arg"
fi
else
pathargs="$pathargs $arg"
fi
done

find $pathargs -regex ${WCGREP_IGNORE:-'.*~$\|.*/\.svn\(/\|$\)'} -prune -o \
-type f -print0 | xargs -r0 ${WCGREP_GREP:-grep} ${WCGREP_GREPARGS:--HnI} \
$grepargs "$pattern"

Examples

This script sets the svn:executable property on a number of files 

#!/bin/bash

DIRECTORIES="./example.org/htdocs/
./www.example.org/htdocs/
./security.example.org/htdocs/
./insurance.example.org/htdocs/
./hr.example.org/htdocs/"

for DIRECTORY in $DIRECTORIES
do
echo
echo "working on $DIRECTORY";
for FILE in $(find $DIRECTORY -name .svn -prune -o -type f -regex '.*s?html$'|\
grep -v .svn | xargs grep -l '\-\-\#include');
do svn propset svn:executable ON $FILE;
# do ls $FILE;
done
echo "$DIRECTORY processing complete"
echo
done

echo
echo "Finished fixing websites"
echo

Sometimes when using find, you end up with "Permission denied" errors that add noise to your output. There are a couple solutions for this. Use the prune option to skip entire trees that you should avoid (e.g. /proc). Use shell redirection to ignore remaining error messages (e.g. 2>/dev/null).

The following example searches all of my hard drive starting at / but skips over the backups directory I have in my external disk drive and also skips over the "process" directory. Any errors like files in /var that I do not have permission to see are discarded by redirecting STDERR to the bitbucket. 

  find / -path /media/disk/backups -prune -o -path /proc -prune -o -type d -name soffice.cfg 2>/dev/null
/home/greg/.openoffice.org2/user/config/soffice.cfg
/home/greg/.openoffice/1.1.1/user/config/soffice.cfg
/home/greg/.openoffice.org/3/user/config/soffice.cfg
/home/greg/spidey2/.openoffice.org2/user/config/soffice.cfg
/home/greg/liberty/greg/.openoffice.org2/user/config/soffice.cfg
/home/greg/liberty/greg/.openoffice/1.1.1/user/config/soffice.cfg
/opt/openoffice.org/basis3.0/share/config/soffice.cfg
/usr/lib/openoffice/basis3.0/share/config/soffice.cfg
/proc
/media/disk/backups

Resources

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