How do I write a for loop in bash


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I'm looking for the basic loop like:

for(int i = 0; i < MAX; i++) {
  doSomething(i);
}

but for bash.


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    From this site:

    for i in $(seq 1 10);
    do
        echo $i
    done
    

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    for ((i = 0 ; i < max ; i++ )); do echo "$i"; done
    

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    The bash for consists on a variable (the iterator) and a list of words where the iterator will, well, iterate.

    So, if you have a limited list of words, just put them in the following syntax:

    for w in word1 word2 word3
    do
      doSomething($w)
    done
    

    Probably you want to iterate along some numbers, so you can use the seq command to generate a list of numbers for you: (from 1 to 100 for example)

    seq 1 100
    

    and use it in the FOR loop:

    for n in $(seq 1 100)
    do
      doSomething($n)
    done
    

    Note the $(...) syntax. It's a bash behaviour, it allows you to pass the output from one command (in our case from seq) to another (the for)

    This is really useful when you have to iterate over all directories in some path, for example:

    for d in $(find $somepath -type d)
    do
      doSomething($d)
    done
    

    The possibilities are infinite to generate the lists.


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    Bash 3.0+ can use this syntax:

    for i in {1..10} ; do ... ; done
    

    ..which avoids spawning an external program to expand the sequence (such as seq 1 10).

    Of course, this has the same problem as the for(()) solution, being tied to bash and even a particular version (if this matters to you).


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    Try the bash built-in help:

    
    $ help for
    
    for: for NAME [in WORDS ... ;] do COMMANDS; done
        The `for' loop executes a sequence of commands for each member in a
        list of items.  If `in WORDS ...;' is not present, then `in "$@"' is
        assumed.  For each element in WORDS, NAME is set to that element, and
        the COMMANDS are executed.
    for ((: for (( exp1; exp2; exp3 )); do COMMANDS; done
        Equivalent to
            (( EXP1 ))
            while (( EXP2 )); do
                COMMANDS
                (( EXP3 ))
            done
        EXP1, EXP2, and EXP3 are arithmetic expressions.  If any expression is
        omitted, it behaves as if it evaluates to 1.
    
    
    

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    I commonly like to use a slight variant on the standard for loop. I often use this to run a command on a series of remote hosts. I take advantage of bash's brace expansion to create for loops that allow me to create non-numerical for-loops.

    Example:

    I want to run the uptime command on frontend hosts 1-5 and backend hosts 1-3:

    % for host in {frontend{1..5},backend{1..3}}.mycompany.com
        do ssh $host "echo -n $host; uptime"
      done
    

    I typically run this as a single-line command with semicolons on the ends of the lines instead of the more readable version above. The key usage consideration are that braces allow you to specify multiple values to be inserted into a string (e.g. pre{foo,bar}post results in prefoopost, prebarpost) and allow counting/sequences by using the double periods (you can use a..z etc.). However, the double period syntax is a new feature of bash 3.0; earlier versions will not support this.


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    #! /bin/bash
    
    function do_something {
       echo value=${1}
    }
    
    MAX=4
    for (( i=0; i<MAX; i++ )) ; {
       do_something ${i}
    }
    

    Here's an example that can also work in older shells, while still being efficient for large counts:

    Z=$(date) awk 'BEGIN { for ( i=0; i<4; i++ ) { print i,"hello",ENVIRON["Z"]; } }'
    

    But good luck doing useful things inside of awk: How do I use shell variables in an awk script?


  • Harley Lee
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    if you're intereased only in bash the "for(( ... ))" solution presented above is the best, but if you want something POSIX SH compliant that will work on all unices you'll have to use "expr" and "while", and that's because "(())" or "seq" or "i=i+1" are not that portable among various shells


  • Miles Lee
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    I use variations of this all the time to process files...

    for files in *.log; do echo "Do stuff with: $files"; echo "Do more stuff with: $files"; done;

    If processing lists of files is what you're interested in, look into the -execdir option for files.